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100% Solar Home: First Impressions

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Build Smart with SimonFerguson Drywall IncEnhanced Living, Inc.
House 12-10-20

After seven years of talking, planning and building we moved in last month – FINALLY!  We’re still not finished (cabinets, trim, painting, etc.), but after living here a month I have some interesting tid-bits to share.

Amazing Sun! Last Tuesday was a very clear and cold night with outside temperatures dipping to minus nine. All night our space heater struggled to keep up, and by 7 am the temperature inside had dipped two degrees to 65 (see note at bottom regarding temperatures). It was five below outside as the sun broke above the horizon at 7 am. By 11 am the space heater was off and we were inside basking in sun and 72 degrees. What’s more, the concrete floor began to heat as well. Using an infrared thermometer, we watched the floor temperature go from 65, to 66, to 67, to 68, to 69. This heat will come back to us later after the sun sets. AND let’s not forget, the sun heated the entire Capital Region to a sultry 6 degrees by 11 am. Yes, the amount of energy delivered by the sun is truly amazing! And its free for the taking.

Comfort! One of our main objectives when designing and building this house was to be comfortable in a house that uses no fossil fuels. We defined comfortable as keeping the Active Area’s temperature between 68 and 75 year-round. The Active Area is a large, open area on the south side of the house that includes the kitchen, dining & living room areas – the area where we spend 90% of our time during the winter, when not sleeping. Operationally, we will allow the bedrooms and basement temperatures to fluctuate, higher and lower, to give priority to comfort in the Active Area – more about this later.

As I mentioned above, house is not yet actually finished. For example we have no central heating system, which hopefully by next winter will be a combination of active solar thermal and a wood stove. But now, after bringing the house up to a stable 68 degrees, we are using a single electric space heater located in the Active Area to heat the entire house. This heater pumps out an impressive 4,915 BTUs/Hr (1440 watts x 3.413 BTU/watt).  

For 25 days (1/1 to 1/25) we heated the house using 452 kWh (sub-meter keeps track of this plug load) of pure electric resistance heat. During this period, I tried to keep the Active Area in the target range of 68 to 75. I had to do this manually because I do not have a reliable thermostat for the space heater. The Active Area was typically 67 degrees or warmer – with a range of 64 (two mornings) to 76 degrees during a few sunny afternoons. Note: whenever the temperature went above 69, the electric was off and the rise in temperature was due entirely to the sun. This was what we considered to be the minimally acceptable temperature for the Active Area. The upstair bedrooms were 64 or warmer (no door on the stairway yet), our bedroom was 60 or warmer, and the basement stayed right around 60 degrees.

Already we have seen that comfort is more than just air temperature. For example, it is often “chilly” in the Active Area at night when the temperature is 67, while at 60 the basement, where I have my computer, is very comfortable to sit and work. The chill in the Active Area is due to the large south facing windows, which during a sunny day are wonderful but at night cause a chill because people radiate heat to those black voids. We need to install insulated drapes to alleviate this problem. In the basement, on the other hand, everything (walls, ceiling, floor and all the stuff) is always between 59 and 63 degrees and quite comfortable because there is no heat loss through excessive radiation.

Another lesson learned was that our bedroom area (the north side of the house – floor plan to come) stayed at 60 to 63 with no direct heating whatsoever! This was so, even with our bedrooms interior walls being insulated, primarily for sound but also to control heat flow from the Active Area to the bedroom. I thought allowing the bedroom temperatures to fluctuate with the available heat, might bring our bedroom as low as 50 degrees. I had felt this was OK, because we are able to sleep comfortably in the Adirondacks at 40 degrees, so if we have enough blankets why can’t we sleep comfortably in our own beds at 50 degrees? Well, much to my wife’s delight, we will probably never need to test our Adirondack assumption!

Energy Requirements! This topic will be a major focus for years to come. My goal is to be comfortable with no fossil fuels, as little wood and electric for heat as possible, with as little technology as possible, and without being “grid dependent.” Why? The reason for no fossil fuels is obvious. We also want to limit airborne particulate and the physical effort of wood burning. We want to save a much electricity as we can from our rather large 6.2 kW PV array for a future electric car, to further reduce our dependence on fossil fuels. And finally, we don’t want to be dependent on the grid for basic household functions and comfort. In short, we want to “live well” at home with as little non-passive solar energy as possible.

While not having the active solar and wood stove in place seemed worrisome when we contemplated moving in, this situation gave us (me) the wonderful opportunity to measure exactly how much energy it takes to heat the house! We installed our PV system in October 2011. Without an adjustment in the “billing year” we would have had to “sell back” all the excess PV production on November 1, 2012. However, I adjusted the “billing year” to end on March 31, thereby allowing us to heat with electric, a normally wasteful approach, by using our “banked” first year’s production for this winter.

During the first 10 days, it seemed like we were going to blow through the 8,000 kWh we had saved ahead. In 10 days, we used nearly 2,000 kWh to bring the house up to operating temperature, 68 degrees in the Active Area. However in the next 25 days we used just 452 kWh to maintain the desired Active Area temperature.

From January 1st to the 25th there were 996 HDD for Albany, NY according to the National Weather Service.  Heating Degree Days can be used to estimate heating fuel requirements. In our region, the “normal” winter is about 6800 HDD. Extrapolating the energy requirements from 996 HDD to a normal winter indicates we will need about 10.5 million BTUs to heat our house to our minimal comfort requirements. [Math: 996 / 6800 = 6.8; 452 kWh X 3423 BTU/kWh X 6.8 =  10,500,000 BTUs/Heating Season] So the old adage, “Insulate a house well enough and you can heat it with a candle,” is not exactly true, but with our R-60 ceilings, R-50 walls above grade, R-40 basement walls, R-30 below the basement floor and air leakage of 0.6 ACH50 we would need less than 88 gallons of oil per year for heat (10,500,000 BTU / 139,000 BTU/Gal)/ .86 Annual Fuel Utilization Efficiency (good oil boiler) =  87.8 Gallons). This is not bad for a 2100 square foot home plus basement, occupied by two adults and two kids!

Furthermore, I doubt it will actually take 10 million BTUs for a heating season because a good portion of the 6800 HDD are days in September, October, April and May when we won’t need any heat.

Incidentally, I did construct energy models of our house before construction using TREAT (existing home performance software) and RemRate (new house Energy Star software). TREAT did better, due primarily to it ability to allow different temperature assumptions in different areas (active, bedrooms, basement), but it still estimated we would need over 25 million BTUs for the area temperatures we have maintained and over 15 million BTUs allowing the basement and bedrooms to go as low as 50, which we now know they won’t.

So, now that I have a pretty good estimate of how much energy is actually required to minimally heat our house for a year, we need to see if we can get most of that energy directly from the sun!  To be continued…  

Array of min-max*Note: All these temperatures mentioned here are Fahrenheit and based on thermometers that I have not yet fully calibrated. Residential thermometers are typically rated at +/-2 degrees. That is quite a swing when the human comfort range is just 60 to 75 degrees. For example, if you read 65 degrees, then the actual temperature might be any where from 63 degrees to 67 degrees – too much uncertainty for my purposes. I have ordered a digital thermometer that registers +/- 0.3 degrees. I will mark each thermometer +/- whatever is required, so I can adjust the displayed temperature closer to the actual temperature. It won’t be perfect, but it will better than what I was able to do with ice water, primarily because the calibration process will occur within the relevant range.

Dan Gibson is the Reporter and Chief Coordinator of Our Energy Independence Community ( Previously he performed home energy audits for five years in NYSERDA’s Home Performance program and new home ratings in the New York ENERGY STAR Home program. He is currently building a 100% Solar Home. He can be reached at

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