100 Watt House
Our 1500 square-foot house was designed to minimize use of electricity and fossil fuels. It uses about as much electricity as a continuously running 100-watt light bulb – about 2-3 kilowatt-hours per day. Here’s how:
Passive solar heat with wood stove backup
The house uses a passive solar design for heating, and cooling. This means that it’s oriented with its long axis running east-west, and it has a lot more windows on the south side than anywhere else. When the low-angled sun shines through the south windows on a winter day, the rays strike the concrete floor and the heat is stored. It has been amazing to see how little supplemental heat we need. Our efficient and clean burning Jotul Castine wood stove may get fired up on a cold evening with a few logs. The house can usually coast through the night and day without a fire, with the temperature gradually increasing on a sunny winter day. We burn less than one cord per winter.
In the summer, the sun is much higher in the sky, and the 2’ overhangs prevent any direct sunlight from hitting the south windows beginning right at the spring equinox on March 21. The house has high ceilings and a loft, so the hottest air rises up there. The kid’s bedrooms have cathedral ceilings, each with a high dormer window that allows the hottest air to exit. During the summer we find that the temperature is manageable most of the time, but we sometimes have problems with mold growth during the hottest and most humid days in August. During those times, we use a small (5700 btu/h) Kenmore window air-conditioner installed in one of the loft windows. It works great – dehumidifying and cooling the whole house for little energy cost (approximately 1 kWh/day). We find that it works a lot better after modifying the intake by duct-taping a shoebox with a flexible duct onto it. We move the intake location 10’ away from the exhaust, so the unit doesn’t recirculate its own cooled air, and attach the intake 7’ off the ground, so the highest, hottest air is cooled.
In the summer, the sun is much higher in the sky, and the 2’ overhangs prevent any direct sunlight from hitting the south windows beginning right at the spring equinox on March 21. The house has high ceilings and a loft, so the hottest air rises up there. The kid’s bedrooms have cathedral ceilings, each with a high dormer window that allows the hottest air to exit. During the summer we find that the temperature is manageable most of the time, but we sometimes have problems with mold growth during the hottest and most humid days in August. During those times, we use a small (5700 btu/h) Kenmore window air-conditioner installed in one of the loft windows. It works great – dehumidifying and cooling the whole house for little energy cost (approximately 1 kWh/day). We find that it works a lot better after modifying the intake by duct-taping a shoebox with a flexible duct onto it. We move the intake location 10’ away from the exhaust, so the unit doesn’t recirculate its own cooled air, and attach the intake 7’ off the ground, so the highest, hottest air is cooled.
Instant hot water system
We love this system, which uses propane gas. We compared several other manufacturers, and chose the Rinnai model 2532 for its combination of features. The Rinnai uses 6W continuously, presumably to power the microprocessor. Fortunately, it can be cycled on and off; so, we installed a switch in the bathroom, just under the Rinnai control panel, and only turn it on when preparing to take a shower or bath (we wash our dishes and clothes with cold water). The control panel lets you set the temperature, so there is no need to overheat the water and then mix it with cold when showering. Winter showers are typically 106-110 degrees, while summer showers are 100-102 degrees or more often outside with cold water or solar-heated water from a coiled hose lying in the sun.
Our only complaint with the Rinnai is that it is somewhat loud, but since it's in the attic the sound is muffled. We have the inside-mounted version of the Rinnai, since the outside-wall mounted version uses an anti-freeze coil that consumes 75W whenever the temperature drops below 45 degrees F.
Our only complaint with the Rinnai is that it is somewhat loud, but since it's in the attic the sound is muffled. We have the inside-mounted version of the Rinnai, since the outside-wall mounted version uses an anti-freeze coil that consumes 75W whenever the temperature drops below 45 degrees F.
Gas range
We chose a special Gas range made by Peerless Premier, the only company we know of that offers models that don’t use a glow-bar in the oven. Glow bars use up to 300W whenever the oven is running – which would drastically increase power usage. After two and a half years, we had the propane tank filled and confirmed that we used about 5 gallons per month for cooking a solar hot water.
Daylighting
Our house has enough daylighting in most rooms that we don’t need lights before nightfall except on the cloudiest or rainiest days. The loft windows help tremendously by bringing light to the north side of the house, often a challenge in passive solar houses. The high transom windows in the bedrooms on the north side help distribute the loft’s light. We avoided skylights, which are great for light but gain too much heat in the summer and lose too much in the winter.
Compact fluorescent bulbs
All the fixtures in the house use compact fluorescent bulbs, either 13 or 14W for light equal to a 60W incandescent. These bulbs are so easy to use, and they save so much money and energy over their life, shouldn’t incandescents be outlawed?
Fixture selection
Turning on any light switch in the house uses at most 14W because we searched for single-bulb fixtures and separated fixtures onto individual switches, except for the attic switch, which turns on 2 bulbs and is used rarely. We added a manual timer to the outdoor front porch light, so we won’t accidentally leave it on all night (it’s not visible from inside the house). The outdoor lights are also compact fluorescent floods. The range hood is an inexpensive model, which works very well and uses a standard bulb, rather than the more common specialized, high-watt bulbs.
Most of the lights are situated overhead so they light a large area – we have 7 ceiling fans, including one on the screened porch, and all have single-bulb fixtures. Larry’s favorite fixtures are the jelly-jar type, because they are so cheap (about $3). We have one in the hall, and in both bathrooms, and on the back porch. By changing the jar to a frosted globe, they look reasonable and shed a lot of light with a single bulb.
Minimized phantom loads
We’ve eliminated all constant loads except for the portable phone, answering machine, and smoke detectors. The smoke detectors are wired together and use about 2W total. We do have a microwave oven that has an LCD display, but the display’s power usage is negligible (tested using a Kill-a-Watt, which quickly displays the power used by almost any plug-in device). The portable phone and answering machine use about 5W together. We turn the stereo off when we’re not listening (it’s an oldie that has a REAL power switch that is really off when it’s off). And we disconnect our laptop computer using a power strip. The laptop is set to enter standby mode (using less than 2W) when not active for 5 minutes (this rarely happens as we usually turn on the computer, use it, then turn it off). When we need to print something, we plug in the printer’s wall adapter, print, then unplug it.
Tight, well-insulated construction
We paid close attention to air sealing during construction, using many many tubes of caulk. We sealed the bottom sill to the concrete slab. We sealed the joints in the OSB sheathing, and also carefully applied and taped house wrap. We sealed the drywall with caulk to the bottom plate – the top seal is made by the drywall joint between the wall and ceiling. We carefully sealed the voids between the windows and the adjacent studs to prevent air infiltration and leakage. The house was insulated with 12” of formaldehyde-free fiberglass batts in the cathedral ceilings (for R-38), and the walls were insulated with blown-in formaldehyde-free fiberglass. The latter is an incredibly tight product and we would probably have used it in the ceilings too if we had it to do again. Actually, now that expanding foam is available we'd strongly consider using that throughout. But we're not planning to build another house anytime soon!
Passive cooling with efficient ceiling fans
We don’t have any central AC (i.e. no duct work). Cooling is provided by several means. First, the slab-on-grade was left uninsulated in the interior of the house – we only insulated 4’ from the north, east and west sides and 8’ from the south side. This creates a reservoir of “cool” in the summer, since the ground temperature is 55 degrees F. Second, the open loft and transom windows allow hot air to move up and out of the house. Opening the loft and house windows at night allows this passive “stack effect” to pull cooler air into the house from outside as the high, hot air is exhausted. Third, we found very efficient energy-star rated ceiling fans, which use only 8 watts at their lowest setting, and have a high cfm/watt ratio of 193 at this speed. These were the contractor grade, inexpensive fans at one of the big boxes (no free advertising for these guys!) and we’ve been very happy with them. We used two smaller ceiling fans in the kids’ bedrooms, with shorter blades. Both of these are Craftmade brand and energy star certified, using 8 watts with 148 cfm/w on the lowest speed.
Finally, radiant barrier sheathing was used to minimize heat gain from the roof. This is OSB with a foil side. Surprisingly, the foil can face downward, as long as it faces an airspace. In fact, it works best when facing downward, since dust can substantially reduce its effectiveness. This stuff had to be special ordered, and cost about 30% more than plain OSB.
Finally, radiant barrier sheathing was used to minimize heat gain from the roof. This is OSB with a foil side. Surprisingly, the foil can face downward, as long as it faces an airspace. In fact, it works best when facing downward, since dust can substantially reduce its effectiveness. This stuff had to be special ordered, and cost about 30% more than plain OSB.
Efficient refrigerator and washer
Our Kenmore energy-star refrigerator uses about 1 kWh/day, the rated energy usage, confirmed by our own measurements with the Kill-A-Watt. We also have a front-loading Kenmore energy-star washer. In addition to using substantially less power, it uses less water and leaves the clothes drier than conventional top-loaders. We haven’t measured the power usage per load of laundry, so this is perhaps the only unknown electricity use in the house. Our dryer is the sun. We watch the weather and avoid doing laundry when it looks iffy – this does take some management, but it helps connect us with the Earth. Every once in a while we get surprised by a sudden shower and wet laundry on the line – in the winter we can dry it by the woodstove, or in summer, on the porch.
Other Appliances
We have a rice cooker, which we love for its simplicity and convenience. It takes about 0.5 kW-hr to cook 4 cups of dry rice. But now that we have the solar oven (see Solar Power), we are using the rice cooker only infrequently. We have a standard electric toaster oven and microwave, both of which use a frightening amount of electricity (about 1500 W and 1200W respectively), but fortunately never for long. During the summer months, we do most of our cooking on the screened porch, where we have a second (ancient) gas range. We move the toaster oven and rice cooker outside also, to prevent unnecessary heat and humidity from being created inside the house.
Slab for thermal mass
The concrete slab floor passively regulates the temperature of the house. Because of this large thermal mass, the house temperature simply can’t change very fast. This means that even on a very cold night without the woodstove, the house temperature doesn’t drop more than a few degrees. It warms up slowly on a sunny day, and has never overheated. In the summer, the slab helps cool the house, because the interior region is not insulated, but rather in contact with the ground.