Building a house on Lakeside Court

Information about construction, energy, power, and conservation. Also including a time log of progress on the construction. (See what's new to track changes.) These pages started as a guide for ourselves, our friends, but most importantly our architects and contractors. These pages became a history of our house building project as well as a record of our plans and our questions that we needed to resolve as we move forward. The energy and construction pages summarize what we have learned about how to build a house that is as carbon neutral as we can make it. They are meant as a resource to others who share our concerns about the need to reduce our nation's reliance on fossil fuels and are interested in building energy efficient houses. Please visit the post construction web site for a more user friendly discussion of what we have learned.



Because greenhouse induced global warming is a major threat to the world's environment, we all need to concerned about how to reduce our emissions of CO2. To do this requires reducing energy consumption throughout the world and particularly in the westernized part of it. This will require everyone to be concerned about how to reduce their energy consumption. This is both an environmentally and an economically sound policy. (Note that this disagrees with the energy "policy" of the current administration that emphasizes increasing production rather than reducing consumption.) For a very thoughtful analysis of the need for renewable energy and the oxymoronic nature of "sustainable growth", see the essays of Al Bartlett of the University of Colorado. Except for the current administration of the US, governments around the world have recognized the need to reduce the production of C02 and other greenhouse gases. To do so will require the concerted actions of citizens and governments at all levels, from local to state, national and global. (Note that other gases, e.g., N2O is 310 times as potent as CO2, have even more effects per ton on global warming than CO2, but that the largest man made component affecting climate change is CO2.

It is interesting to examine the tables provided by EcoWorld comparing C02 production by country (the US is far away the largest producer), per capita (the US is 6th, the top three being Qatar, the UAE, and Bahrain), per BTU (the US is very efficient and thus quite low in the list at 119, and Tons of C02 per $ GNP (the US is much more effecient than less developed countries, but much less efficient than most of Western Europe.)

The construction and power utilization of our house is our way of meeting the challenge of reduced C02 consumption. By using better building techniques and taking advantage of developments in harvesting renewable sources of energy, we hope to make a house that can be seen as an example of an environmentally responsible home. This and the related pages are designed to help others understand the process of building such a home.

Energy issues to consider are twofold: sources and uses. To minimize the energy effects of a house, we can attempt to take advantage of renewable resources such as solar energy and also attempt to minimize the use of energy through efficient appliances and design. In the UK, for example, roughly 50% of the total energy budget of the country is associated with residential and office buildings (rather than in manufacturing or transportation). Steps towards improving the energy efficiency of the house are therefore most important. Building a house involves using energy in terms of the construction materials as well as the efficiency of the operation of the house.

Several relevant references on design of energy efficient houses are

  • Thomas, Randall, 1999 (Ed) Environmental Design: An introduction for architects and engineers (2nd Edition). Spon Press. New York
  • Thomas, Randall (2001)) Photovoltaics and Architecture
  • Roaf, Sue; Fuentes, Manuel; Thomas, Stephanie (2001) Ecohouse: A design guide. Architectural Press, Oxford.
  • Web resources abound, but the need for reducing our energy impact is well summarized on a page from the Energy Saving Trust of the UK or various publications of the Department of Energy in the US.
  • A rather "cutsey" set of pages by the Alliance to Save Energy tries to convince consumers that saving energy is good for personal as well as societal reasons.
  • A much more serious set of pages has been developed by the Rocky Mountain Institute who discuss energy as well as broader environmental issues.
  • Green Building News is an up to date compendium of news about energy and building issues.
  • The Renewable Energy Policy Project/Center for Renewable Energy and Sustainable Technology (REPP/CREST) maintains several very useful list serves with relatively low activity and a high quality of suggestions. The members of the Greenbuilding list and the Solar Power List at REPP/CREST have been particularly useful as we worked on our house. REPP/CREST also maintains a collection of informative pages on Solar (as well as other renewable) energy.
  • A list of the "top 50" solar websites in Germany (auf Deutsch) provides links to producers and consumers of solar power.

To minimize the long term environmental impact of a house is to make the house "carbon neutral" in terms of its energy budget. Although becoming carbon neutral is probably an impossible goal, it does drive much of our thinking. To do this requires taking advantage of renewable sources of energy as much as possible as well as designing the house to require as little energy as feasible (see the section on house construction). Given the constraints of living in Evanston (wind farming is out), the obvious renewable source is solar energy. (Although harvesting wave energy is conceptually possible given our location on the lake, the technology is not yet well enough developed to be practical. Experimental work is being done on harvesting wave energy on the north coast of California and off of the UK and Denmark.)

A thoughtful report from the EPA analyzing photovoltaic production in the US points out that much of it is for export because the cost/watt of grid provided power is currently less than the cost of photovoltaic power. Much of the growth in photovoltaic usage (e.g., in Germany and Japan) has been because of government subsidies. Usage in the US is primarily off grid or by "green" consumers. The economics of various renewable energy sources including wave and solar are reviewed in a collection of pages by the European Union. PV prices will continue their historic decline (and thus become more competitive) as more PV is installed. However, this requires some people to use it now. That is, in order to make solar more affordable in the US, there is a need for early adapters to install the technology as role models and demonstrations of feasibility.

Solar Energy

There are at least three ways to use solar energy in homes: Passive solar design takes into account solar heating and cooling by designing the house to maximize the effects of sunlight in the winter and minimize it in the summer; Active solar heating of hot water; and conversion of sunlight to electricity using PhotoVoltaic cells. What is not well understood by most lay people is that solar heating and Photovoltaic energy production are practical even at latitudes such as Chicago (42 degrees north) or northern Europe. Maps of solar flux throughout the US suggest that roughly 5 KWH/sq meter/day is received in Nothern Illinois.

The average solar flux at high noon when the sun is directly overhead is about 1,000W/sq m (1 KW/sq m) or about 93- W/sq ft. The useful amount of solar energy is a fraction of this due to solar elevation (less in winter), angle (azimuth), and weather. (Note that although most scientific publications will talk about W/sq m, most commercial pages talk about W/sq ft or number of BTUs/day. W/sq meter is the instantaneous energy flux, when this is integrated over the day it becomes Joules/sq m/day or BTUs/sq ft/day.) On a clear day, the amount of power received over the day is roughly 2,000 BTUs/sq ft or 23 MegaJoules/sq m/day. In Phoenix or Albuquerque, clear days predominate and the 2,000 figure is appropriate. In Chicago, the average is about 1,300 BTUs/day for a collector at 45 degrees. (This is from tables provided by Solar Rating and Certification Council) Relevant conversion factors are 1 sq meter 10.8 sq ft, and 1 kw for an hour (1 KwH) = 3,412 BTU).

A midwest oriented publication, Conscious Choice, has run articles on Illinois' drive to become the "greenest" state and the various initiatives sponsored by Chicago and the State of Illinois. The Environmental Law and Policy Center is oriented towards the mid west and lists many of the local dealers for solar heating and photovoltaic products. They provide a copy of the grant application form that may be submitted to the Department of Commerce and Community Affairs for partial support of photovoltaic energy installations (up to 60% of the installed cost with $6/watt and $300,000 maximum) as well as the rebate program for solar heating.

Solar heating is most important in the winter and requires that the plates be at moderately steep angles (in Evanston this is 57 degrees = latitude + 15 degrees) to catch the winter sun. Photovoltaics are able to produce more electricity in the summer when there is more sun and require relatively shallow angles (32 degrees = latitude - 10 degrees). As pointed out by a tutorial on solar radiation from NREL, being off by 15 degrees doesn't change the output very much. An excellent review of design issues when building a passive solar house is available from Dennis Holloway, an architect in New Mexico.

Solar Heating

Solar panels made of glass enclosing pipes containing water or antifreeze can supply about 1000 BTU/day/sq ft with maximal solar input (Solar Rating and Certification Council). Assuming that this is based upon an irradiance of 2,000 BTU/sq ft/day, this implies a conversion efficiency of about 50%. According to a local dealer, solar panels provide 50% of this amount in winter, 75% in fall and spring, and 95% in summer.

An excellent discussion of the technology is found on the DOE's EREN page. To achieve optimal heating in the winter months, panels need to be tilted roughly 57 degrees from the horizontal. (This is the latitude + 15 degrees.) Optimal in December is not the same as optimal across all of the heating months. Somewhere between latitude+10 to latitude +15 seems optimal depending upon how long the heating system is.) From the NREL page, the average expected output of a flat panel tilted at 57 degrees is between 4 and 5 KwH/day/sq meter. That is, roughly 1,400 BTU/day/sq ft. (Note that this does not agree with the solar rating listings.) In December a good estimate is about 1,000 BTU/day/sq ft. Solar panels come in varying sizes, but we were quoted 4x 8 feet at a cost of roughly $1200/panel. (Note that a 4*8 panel is 32 sq feet and is roughly 3 sq meters.) Thus, with 7 panels, or 224sq. ft, we should produce about 224,000 BTU/day during the winter with higher output in spring and summer. Averaged over the year, we should be able to produce about 320,000 BTUs/day or somewhere between 1,100-1,200 therms/year.

A Therm (about 100 CF of gas) is 100,000 BTU. (This is the equivalent of 93KWh/day which is consistent with the 6 times higher efficiency of heating over PV.) Based upon data from our current house, the cost/therm averaged over the year is $.69 (ranging from $.20 in the summer to $.96 in the winter, this average is found by weighting monthly prices by the number of therms consumed). For an investment of about $8,400 for the panels and $10,000 for installation and related materials (storage tanks, pipes, etc.), with a $5,000 energy rebate, we have a carrying cost of $1.00/therm if we produce around three therms a day. (Our estimated energy use over the year is 1 therm/day for hot water and cooking and about 1.5 for heating. Unfortunately, the peak demand in winter for heating is of the order of 12-14 therms/day.)

A leading installer and advocate for solar heating in the Chicago area is Brandon Leavitt of Solar Services. Solar Services is in the process of developing a web page. They recently (Spring, 2003) installed solar heating on the "world's largest laundromat". One distributor of solar heating equipment is Solar Depot. Another, Solar Dynamics in Nova Scotia, offers a somewhat more technical page. The amount of energy available from solar heating as a function of tilt, month, and location is available in graphic form from NREL. These data may be summarized graphically, although the units of energy produced need to be rescaled. Unfortunately, getting the data in BTU/sq ft or in efficiency as a percentage of available energy seems difficult.


Just as solar energy can be converted into heat by hot water collectors, it may be converted into electricity by the photovoltaic collectors taking advantage of the photovoltaic effect. Although this effect has been known for more than 100 years, it is has been very expensive (greater than $100/watt) until recently and was used for such applications as energy production for satellites and space probes. The price has now dropped to the point where it is affordable for off-net applications and where energy is expensive (see the report by the EPA.)

Basic data on solar energy can be obtained from (among others) the American Solar Energy Society. It is possible to calculate the amount of solar power produced by a house from tables developed at BP and elsewhere. Information on photovoltaic houses come from a variety of sources: BP has a page that allows one to do calculations of costs and power output in any location, e.g., Evanston. This seems to be just an application of the NREL PVWatts calculator.

Relevant CAD software for designing solar systems helps figure out appropriate angles, etc. See also the design tools from the UCLA Architecture Department.

Some of the calculations can be done without software just by looking at various tables of solar altitudes and azimuths as a function of time of year and hour of the day and doing some simple trig. Optimal roof angles seem to be somewhat controversial in that according to some, the optimal roof angle year round is the latitude of the house. In Evanston, this is 42 degrees. Thomas, however, suggests that the optimal angle is roughly the latitude - 20 degrees. The difference seems to be due to maximizing the roof angle for winter when a steeper roof is appropriate, but when there is very little sunlight, and summer when a flatter roof is appropriate, and there is more energy. Thomas also points out that the "sweet spot" for pitch and direction is fairly robust.

More detailed information for the sceptic is available from the National Center for Photovoltaics, part of the National Renewable Energy Laboratory, which has many useful resources including ones that address roof angles and orientations for the US. Among their many pages are ones that discuss roof angles in degrees as well as roofer terms (e.g., 7/12, etc.) and roof orientations in terms of N, S, E, W, etc. Based upon observed data from weather stations they have produced two estimation tools to estimate the PV output as a function of location, angle and pitch. Supposedly, one is precise for any locations but I can not get it to load. A somewhat simpler one based upon data from Chicago confirms Randall's claim that the "sweet spot" is very large. Using the NREL calculator suggests that the optimal pitch in Evanston is about 31 degrees or the latitude minus 11 degree. It also suggests that output is robust to variations in both pitch and orientation. For a range of orientations ranging from 150 to 210 degrees with pitches of 21, 31, and 41 degrees, the expected annual output for a 4 KW system ranges from 5,740 to 5,991 with a maximum at 180 degrees and 31 degrees. Although pitch differences of 20 degrees lead to a 20% difference of 232 versus 269 KW production in December and a 10% difference in July (694 vs. 629), on a yearly basis there is only a 2% difference between the optimal pitch (31) and a lesser pitch (21). (These analyses are summarized in a more detailed discussion of pitch and orientation.)

Part of the question of optimal angle roof angle is addressed in the supplement on estimating the effects of shading of nearby trees where it is obvious that for the winter months the sun is so low in the sky as to be hard to capture with anything other than a very steep roof. However, the conclusion from all of this analysis suggests that we do not need to do anything fancy such as sawtoothing the roof to adjust the orientation to be exactly southerly.

Which PV to use?

Although we had originally thought that we would use Solar Shingles from Uni-Solar, we have decided that a much better alternative are Solar Slates from Atlantis Energy. A web search suggests that Solar Slate are in use in Europe and the US (primarily California). The producer (Atlantis Energy) has a recently revised web page that supplies much more information and examples than were previousl available. The solar sail is an interesting example of what can be done with their technology(EcoWorld ran an interesting article describing the company's history and plans.) According to this article, Atlantis was spun off from a Swiss company in 1998 and has 45 employees in Virginia assembling the slates and 7 in marketing in California. Although it is hard to find detailed information on their web page, they have been very helpful in phone calls and emails. SolarSlates require a minimum pitch of 4 in 12. In further conversation with Atlantis Energy, these slates offer 10 W/sq ft and seem to be compelling alternative to solar shingles. It will be possible to install 264 slates on the 2nd floor and Belvedere roofs with an output of 10 W/sq ft/slate (and each slate is 1.33 sq ft ). Thus, it is possible to achieve significant PV production (10 *1.33* 264 = 3,432 W) without having to go to drastic steps such as moving the tree potentially shading the garage roof. Moreover, by using the garage roof as well, we will have a total of 480 SunSlates that are rated at 13.3W each. Thus, the total theoretical output can by 6,384 W (6.4KW) or an expected annual production of roughly 9,600 KWH or an average of 26KWH/day. This amount far exceeds our current energy usage of about 20KWH/day. (Note added in March, 2003: I have corrected the previous estimates to reflect more recent data from Atlantis. See the specific output information we have collected from January, 2003.)

Yet another alternative is solar tiles produced by PowerLight. These Powerguard tiles provide insulation as well as 10W/sq ft. They are only good up to a 3/12 pitch, so they would be useful over the garage. (Complementing the solar slates that require steeper pitches.) However, they are currently specializing in commercial rather than residential applications.

Atlantis Energy is a California based subsidiary of a Swiss company. They emphasize producing for the California residential market but are interested in penetrating the mid west. Uni-Solar is a merger of Michigan based Energy Conversion Devices and European based Bekaert. Powerlight is a California based corporation specializing in commercial applications.

The pitches and orientations of our roofs differ from each other (and the optimal) but still seem satisfactory. The following table was developed using the NREL calculator for pitches and orientations of a 4KW system and then converting them to the areas and pitches we have. Table 1 considers the output of 480 Solar Slates based upon angle and pitch with the nominal output of 10 w/sq ft (13.3 watt/slate) and the expected annual output.

Roof Pitch Orientation output of a 4 KW system Number of Slatesrated output (watts) expected annual output (KWH)
Garage 18 180 5751 216 2,873 4,130
2nd floor 30 150 5846 120 1,5962,333
Belvedere 33 180 5994 144 1,9152,870
Total 480 6,384 watts 9,333 annual kwh

The nomimal output using SolarSlates is roughly twice that of UniSolar at roughly the same quoted cost. However, this is an upper bound. Earlier estimates from Atlantis Energy had (I thought) discussed slates with an output of 17W/slate which is the expected output of a 1.33 sq ft slate times 13W/sq ft. Their most recent estimates are 10 watts/sq foot or 13.3/slate. At $100/slate this is $7.50/watt.

An article from the Environmental Defense newsletter discusses Solar Shingles. Uni-Solar produces Standing Seam Roofs as well as Solar Shingles. SSR come in 9.5' x 16" and 18.3' x 16" configurations. Solar Shingles come in 86.4" by 12" modules rated at 17W at 9 v. One panel exposes 3 sq ft. which implies that each 12" wide panel overlaps the others by 7" and exposes 5" (for figuring number of panels to order). One source quotes these at $175/shingle or about $10/W. Another sourceprovides information about installation and estimates costs at $10,000 / KWp = $10/W (but they have discounts for their members). Finally, JadeMountain quotes $160/solar shingle which is $9.4/rated watt. Yet another article discusses both shingles and slates and gives even lower estimates. A preliminary quote from Old Country Roofing for Solar Slates is $96/slate or $7.4/Watt. This is almost identical to a quote on Solar Shingles at $129/shingle or $7.5/Watt.

In addition to the cost of shingles or slates or collectors of any kind, it is important to consider the cost of converters (from DC to AC), battery backups for critical circuits, and miscellaneous wiring. This is presumably a relatively fixed cost insensitive to minor variations in system capacity. Rough estimates of these costs are about $7,000 for inverters and associated equipment and $5,000 for battery storage of 8 12V 225AmpH batteries. (Since deep discharge batteries are rated at running for about 20 hours, this produces roughly 1 KW/H for up 16-20 hours. The deeper and faster the discharge, the less the battery lasts. Output data on selected batteries is available from various companies.)

In all of this cost per shingle or cost per slate and cost per watt it is useful to remember that a rated Watt of PV produces about 4 Watts/day and thus about 1.5KWh/year. (Calculations based upon NREL tables for Northern Illinois. Our observed output from January 28 to May 3 suggests that we have been averaging 38W/slate/day or 2.8 W/day/rated watt during late winter and early spring. ) Thus, the capital cost per KWh of production over a year is about $5.0/KWH ($96/(13W * 4 H/d * 365 h/year)) or a carrying cost of .35/kwh on a 7% mortgage. This is compared to the metered rate of about $.086/KWH. One also needs to consider the current subsides of 60% available in Illinois and the deductibility of mortgage interest costs. This gets the cost down to about $.20/kwh. Still twice the going rate, but well within the feasible. Assuming increases in energy costs in the future there is in fact a positive return on one's investment in about 10 years or so. A number of references also point out that one does not compare as carefully the relative costs of many other design features of houses such as flooring or kitchen counters. What we should be asking is whether the additional cost of $100/month for clean energy is worthwhile. We believe it is.

On October 5, 2002 there was a tour of Evanston homes that have solar systems. This was sponsored by an Illinois group for renewable energy, a group that provides interesting information. Most of the houses on the tour tend to use solar hot water heating at least one has photovoltaic supplies. The tour demonstrates some interest among Evanstonians and our neighbors. An Illinois wide conference on renewable resources was held in Rockford in July with representatives from the major solar providers, Illinois energy companies (e.g., Commonwealth Edison), the Illinois Department of Commerce and Consumer Affairs, as well as about 300 very interested end users.

An enormous list of Solar sites is found at (alphabetical, but not well annotated.) This list was taken from PV Power and is a good beginning of information. This one is worth poking around.

A helpful FAQ on solar energy comes from UNI-Solar of Michigan. They suggest that in a northerly home, one gets about 1.5 KWH/year per installed Watt (see also the NREL tables). They also suggest that an average building requires about 1 Watt/sq ft or 1 KW/person (but this seems high for a house). Using the former estimate, and the size of our house, we should plan on about 6000KWH/year capacity. The installed cost is estimated between $8-12/installed Watt. Thus, for a 6000KWH house, we need about 4000 installed Watts or about $32-48K in installation. (See the rough energy budget based upon current and expected usage. A recurring point in all discussions of solar power is that it is easier and cheaper to reduce usage by various modifications of existing lights and appliances than it is to provide solar power for that amount of usage.) This estimate matches that of the BP calculator program. BP points out that there are subsidies to reduce this cost substantially. Furthermore, they estimate about 5 watts/sq feet or 200 sq ft/kw. Thus we need about 800 square feet of solar paneling for a 6000 KWH/year house. However, if we use SolarSlates at 10 W/sq ft we could achieve the same amount of power with less area or alternatively, produce more power. The Unisolar site also links to a library of reference material. Another major company that provides estimates and product is Siemens.

A well organized resource on energy issues in general and on solar in particular may be found at JadeMountain which has a comparison of various producers of solar panels as well as the cost per watt which seems to range from $4-8+$8+ per watt (For the panels themselves, not counting installation or conversion equipment.) UniSolar (see above) produces solar shingles and solar seamless roofing panels. They provide a useful FAQ about Standing Seam Roofing installations. Pictures of houses with solar shingles are available at JadeMountain. One advantage of solar shingles and seamless panels is that they replace part of the overall roofing cost. "The SSR product is a structural product that is installed over purlins, or over a roof deck."

Financial incentives for solar energy. A table showing various programs (also available directly at DSIRE) points out that in Illinois there is a 5,000 limit on the 60% PV subsidy. But there is also a competive grants program for installations of more than 2 KW capacity. These grants will pay up to 60% of the costs. See also the IREC (Interstate Renewable Energy Council web site on photovoltaics, with a particular link to Illinois. Commonwealth Edison has started a 5 year experiment on reverse metering and "capturing the power". They supply contact information to enroll in the program as well as links to the State of Illinois for financial assistance.

When thinking about installing PV, it is useful to consider efficiency, convenience, and attractiveness. The three main ways of installing PV are shingles (Uni-solar) versus PV panels versus PV slates (Atlantis). Shingles and slates seem not as well tested in the US but also seem to have many advantages in terms of installation in that they replace roofing.

Installation instructions for standing seam PV roofing suggest that the roofs may be installed using standard professional roofers working in cooperation with electricians. Wire race-ways need to be installed under the ridge cap. Similar installation instructions are available from Atlantis. Atlantis requires that roofers be trained by them.

Spire and BP Solar have teamed up to develop a Photovoltaic plant on a "Brightfield" in Chicago. The purpose of this is to develop PV production and skills in the Chicago metro area. Supposedly, this is located at the Midwest Center for Green Technology (formerly known as the Sacromento Stone Building). SpireChicago will provide turnkey installation. The Environmental Law and Policy Center offers seminars for architects and lawyers on energy issues. They also provide links to PV companies in Chicago. Nekolux is one of these, as is Spire.

Ground Source (Geothermal) Heat Pumps

According to the DOE, geothermal heat pumps are one of the most efficient ways to heat and cool a house. They seem to have the additional advantage that they do not require external (to the house) heat exchangers and operate very quietly. The DOE page provides several case studies of cost of installation and operation. The Center for Renewalable Energy and Sustainable Technology (CREST) says that geothermal heat pumps are worth exploring for the airconditioning and heating, particularly as an alternative heat exchange unit for air conditioning. "GSHP's are 50-70% more efficient at heating and 20-40% more efficient at cooling [7] and can reduce electricity use by 25%-60% compared to traditional electric heating and cooling systems. [8] According to the U.S. Department of Energy, GSHP's provide water heating free in summer, use about half the water heating energy in winter, have a payback time about 2 to 10 years and, and reduce emissions up to 72% when compared to an electric resistance heating and standard air conditioning systems." Basically, this approach uses the earth as a part of the heat exchange unit. Although discussed as possible sources of heat as well, other literature suggests that that electrically driven Heat Pumps are not as energy efficient as gas. However, that seems to be the case for air exchange rather than geothermal heatpumps. In any case, for the occasional air conditioning, using geothermal seems very useful. A review article from the LBLabs suggest that they are competitive if electricity costs are >.07/kwh (it is >.08 in Illinois). As a main source of heat, more research is needed but it seems promising from what I have read. However, we decided not to install geothermal heating.

As might be expected, there are associations of geothermal heating (i.e., the geothermal heatpump consortium and the International ground source heat pump association) that provides useful information. Installers for this technology exist in Illinois, the closest being in Glenview. An very informative information survival kit for prospective geothermal owners is available as an html and pdf document from the Oregon Institute of Technology. As part of this FAQ, they discuss how it is possible to combine geothermal with radiant hot water heat. Such a heat pump could also be used to heat a swimming pool (particularly in summertime.) A company that produces geothermal heat pumps, WaterFurnace has a web page with a useful Q&A about the concept and some interesting pdf encoded case studies. They produce a system for radiant heating and duct driven air conditioning. Although it is clear that heat pumps are effecient uses of electricity, what is missing from all of this discussion is relative efficiency when you consider the efficiency of actually making the electricity. Nor have I yet found a discussion of how much power is actually used by such a system.

Tentative Energy Budget

We currently use two sources of energy, gas and electrical. We want to reduce the amount used of both, and to supplement the amount of energy supplied by carbon based fuels with solar based energy. To do this requires understanding our current demands and ways of replacing these with alternative (renewable) resources.


Probably the fundamental concept in heating is that heat may be thought of as a fluid. Heat flows from high to low. Thus, it flows out in the winter and in in the summer. Heat flows are reduced by insulation (the R factors is a measure of the resistance of a particular material to the heat flow.) To heat a house means to provide enough heat to make up for the flow (leakage) of heat to the outside. Heat loss (and thus the amount of heat required to maintain a fixed temperature) is a linear function of how much colder it is outside than inside. The heating (and cooling, for it is the same problem, but in reverse) industry uses the concept of the "degree day" or the number of degrees (F) below 65 degrees. Heating demands are a function of "degree days", or the number of degrees (F) below 65 degrees. For our current house, the correlation between degree days and number of therms of demand is .99 with a regression equation of therms/day = .35 * degree days. (100 cubic feet of natural gas produces approximately 1 therm = 100,000 BTU -- the conversion factor changes on a monthly basis, but this is close.) The fit is particular good, and reflects the fact that Nicor estimates many measurements. Degree days in Chicago are available from the National Weather Service. Note that they differ from the three recording stations with the highest winter demand at Ohare and the lowest at the University of Chicao. Unfortunately, I have been unable to find such data for Evanston. However, from October 98-October 2000, NICOR reported 6236 degree days/year in their bill for our current house. This agrees with the historical averages of Ohare and Midway: Annual degree days at O'hare are 6600, and about 6200 at Midway. Heating demands are estimated in the section on insulation in the construction page. The basic conclusion there is that a 40-50K BTU boiler is probably adequate. After long discussions with our energy contractor, we are installing a larger boiler than this. His point was that I had not considered air infiltration, open doors, or speed of recovery. If we are no more efficient than our current house, then a very generous overestimate of heating use should be 1600 therms a year or 4/day. Base load (cooking and hotwater) throughout the year is about 1.5 therms/day with the remainder going into heating. Based upon my estimate of 588 BTU/degree/hour (see the construction page) and "rounding" up to 670 BTU/hour, leads to a predicted yearly heating use of 1,000 therms or 2.7/day. These two different ways of estimating gas demand suggest that a lower bound is about 4 therms/day with an upper bound of 6. Assuming that roughly 2 therms/day can be produced by solar, this leads to a daily load (primarily in the winter) of 2-4 therms or 730-1400 therms.

Gas furnaces

Socal gas has a rating table of the efficiency of various central gas heaters and discusses facts to consider. They also have discussion of programmable thermostats (clearly an energy saving requirement.) Although geared for the Southern California market, this is still a useful page. Their interactive guide to home energy efficiency, while geared to gas, is useful. Also see the discussion (above) of groundsource (geothermal) heating and cooling. In any case, we need to provide electrical backup to the gas furnaces because current boilers have electronic ignition and a powerfailure should not result in a loss of heating.


(An expanded version of this section, with more detail about fireplaces has been added and updated. )

Esthetically, a fireplace is a traditional part of most houses. Conventional fireplaces are, however, notoriously inefficient and may actually remove heat from a house rather than produce any. National Resources Canada, the Canadian equivalent of the EPA, has a very detailed discussion of fireplaces in which they review the problems with conventional fireplaces. In particular, they recommend advanced combustion fireplaces which have efficiences of 50-70% (and are thus more than 10 times efficient than a conventional fireplace.) Another discussion of energy efficient fireplaces comes from Home Energy.

Magnum, one maker of advanced combustion fireplaces, offers a comparison table of the top 7 brands. An advanced combustion fireplace from FireplaceXtraordinair comes either in wood fired or gas fired options. In particular, the Elite 44 is said to produce 2.5 grams of emissions/hour and burn with an efficiency of 72%. The BIS II from Security is yet another advanced combustion zero clearance fireplace. Majestic Fireplaces seem to be moderately efficient, combining glass doors, outside air, and doing some circulation of internal air. It does not seem to be an "advanced combustion" and is thus less efficient.

Fireplaces, as do all combustion devices, produce a number of biproducts that can prove fatal (e.g., CO, CO2) or harmful. The EPA offers advice for how to deal with combustion devices in the home. Other advice for the operation of catalytic wood stoves (which the advanced combustion fireplaces functionally are) comes from the Hearth Products Association who also offer general advice on how to choose and operate fireplaces and stoves.

Electrical lighting and power

To evaluate how much energy we are currently using and how much we should plan on using, it has been helpful to use a "Watts Up" to measure particular appliances. To estimate usage of such things as lights, it is more practical to simply observe wattages and estimate hours. Two conclusions come from this: a great deal of energy is used in normal lighting and 2) improving the efficiency of appliances (e.g. refrigerators, dishwashers) is important, but perhaps less important than switching to more efficient lighting. This has been possible for some of our appliances, but not all. In addition, I have estimated hours of use of current lights and current wattages. We are experimenting with Compact Florescent Bulbs which use roughly 1/4 of the energy for the same of amount of light output. Unfortunately, the lighting industry has not started to produce many attractive lights that use CFLs. We have been told by people at lightology that this is changing. We hope so.

A coming attraction in low power light will be Light Emitting Diodes (LEDs). Although typically associated with dim red light, they are now available in arrays that produce white light with low power consumption. To be explored. Randall Thomas discusses the spectrum of incandescents, compact flourescents, etc. Direct comparisons of lights needs to be made. Because incandescents produce roughly 70% of their energy in the infrared part of the spectrum, they are much less efficient than lights that produce primarily visible light.

It is also clear that one can become an addict in analyzing power consumption and tracking down "vampires". Little drains add up to lots of power, but the typical 100 watt incandescent bulb is the big hit.

The following table was developed in 2001/2002 to predict our demand. It is kept here for historical purposes. Our current empirical estimates are that we are using about 16kwh/day.

Current Energy Consumption
Appliance KW/H KWH/day % of Total
Computers (WR) .07 1.8 .11
Computers (ER) .02 .48 .03
Refrigerator 1 (est) .07 1.7 .10
Refrigerator 2 .07 1.7 .10
Washing Machine
Lighting (estimated total) .42 10.3 .52
Kitchen .15 3.5 .18
Dining room .03 .6 .03
Study 1 .07 1.6 .08
Study 2 .07 1.6 .08
Bedroom E/W .03 .6 .03
Bedroom DJ trivial
Bedroom DR trivial
Total Accounted for.68 16.4 .83

What should be clear from this table is how much power is used by conventional incandescent light fixtures. We are experimenting with various Compact Florescent Bulbs. For many uses, they seem fine. The biggest issue is the "warmth" of the light. "Traditional incandescent lamps are known for a warm, yellowish cast. Though fluorescent has been known for less-than-lovely color and a cool tone, today's technology has changed. Colors are now rendered more accurately, and a wide range of temperatures are available. CFLs for residential use often are labeled "warm color," though more precise labels include the actual color temperature.

(Once we found lights with suitable colors,we switched over to CFLs almost completly. The effect at the old house was a reduction in power consumption by almost 30%.)
Color temperature is measured in degrees Kelvin (K) with the higher numbers being cooler (more bluish) in appearance. The incandescent lamp is rated at 2700 degrees K and so are the residential model CFLs typically found in retail stores; however, the CFLs are also available in neutral 3000-3500 degrees K, and cool white 4100, 5100 K and even higher. Lamps in a shared space will look best if all the lamps are of the same color temperature." (From's FAQ about lighting.

These observations are consistent with the observed energy usage of about 20-21 KWH/day or 7200-7500 /year, although they suggest that we are not measuring about .15 - .20 KW/H. Using this component as an estimate of a fixed error, it is possible to estimate the savings by using CFB to replace all incandescent bulbs as roughly 2200 kwhr/year. This leads to an annual energy budget of 5040KWH or 420 KHW/month. A more reasonable estimate is probably 5400 KWH/year or 15 KWH/day (a 25% reduction from current usage). [As an interesting note, our electrical consumption has fallen roughly 15% over the past year as we have become more sensitive to "vampires" and switched to using more CFLs. This is without using a more energy efficient refrigerator or doing a drastic switch of lighting.]

The following is a very tentative energy budget. A more complete estimate is in progress. See Glen Hunter's page on the progress (and process) of building a solar home in Ontario for an excellent example of issues to consider when thinking about off grid loads.

Appliance KWH/day KWH/year% of total
Refrigerator 1.4 510
Refrigerator2 1.1 400
Dishwasher .96 400
Clothes Washer .7 250
Computers (WR) 1.8 656
Computers (ER) .5175
Lighting (current) 103650
As yet unspecified 2720
Total 5675

Energy and C02 equivalents

We are powering and heating our house with a mixture of active solar techniques (solar hot water heating and photovoltaic power), passive solar techniques (site design, high levels of insulation, efficient windows), and external sources of energy (natural gas, grid tie electricity, firewood). These differ in their energy efficiency and their C02 production. It is helpful to try to compare these various contributions to the total picture.

An interesting web page that allows one to estimate C02 equivalents of various sources and uses of power allows us to calculate the C02 equivalent of our house. This conversion program is found at the Forests Absorbing Carbon dioxide Emision (FACE) foundation . This will analyze C02 production equivalents of a variety of activities. Using this calculator, it estimates .63 kg/KHW for electricity production (in the Netherlands). Similarly, 100 therms of natural gas is 500 kg or 5 kg/therm. Since 1 therm is approximately 100,000 BTU = 29.3 KWh, this means that electricity is roughly 3.7 times as C02 intensive as natural gas for the same amount of energy. These are, obviously, tentative estimates and need to be confirmed elsewhere. Another conversion table from the deltaland trust gives comparable values (expressed in pounds of C02 per activity): 11.7 pounds/therm and 1.3 lbs/KWH. (More appropriately, this is 5kg/therm and .6 kg/kwh). These ratios are consistent with the observation that heat production is roughly 3.3 times more efficient than electrical production.

The amount of CO2 per KwH is primarily a function of how the power is produced. Our power company, Commonwealth Edison, uses more nuclear fired generators than most other midwestern states. Thus, they report that they produce .7 lbs/kwh (.3Kg/kwh) while other midwestern states produce 1.9 lbs or .88kg. Nuclear power, although not producing any CO2 or particulate pollution, does produce high and lowlevel nuclear waste. However, realistically, they are part of the national grid and the power they sell reflects national averages as well. The EPA has a calculator that allows one to calculate emissions as a function of consumption and energy provider.

So, assuming that we can produce 10,000 KwH of PV electricity and 300,000 BTU/day (3 therms), this is the equivalent of 3.5 tons of C02 for the PV and 5.49 for the solar heating. According to this calculator, this is slightly less than the equivalent of 1.0 hectare of forest.

Now, in reality, this is what we are estimating as production, but we are also going to consuming more than that in natural gas for cooking and some heating. Assuming 1 therm a day for that use, (i.e., 5 kg of C02/day) we would need to generate 6000 KWH of surplus power (or an internal use of 4,000 KWH) annually to be carbon neutral. A very difficult challenge. If we were living in Iowa, using coal fired electrical generation, we would need to generate only 3,000 KWH/year to be carbon neutral, a more doable challenge.

Our goal has been to Carbon Neutral. This is almost achieveable. However, this is a far cry from being a Zero Net Energy House. To be a ZNE house would require reducing our use of gas to .25 therms/day and using 6900 kwh of electricity/year or 18kwh/day.

To put this in a more realistic perspective, 1 round trip to Australia from Europe per year is the energy equivalent of 3.2 tons of C02, and 50,000 airmiles/year is 10 tons of C02 so although our house might be C02 neutral, our lifestyle will not be.

version of October 12, 2002 - partially updated September,2003
As is true of all web pages, this is part of a constantly growing set of pages. If working off of a printed copy, it is useful to look at the date of the last version. As changes are added to the various pages on, the "What's New" page will track changes.
Prepared by William Revelle. Comments to W. Revelle