Seattle - Gas, Electric and Water Usage
Our gas bills are quite a bit lower than projected, although our electric use is not as low as expected. In July 2005 we added a 3KW PV system (the PV compatible meter came in l Aug). We subsequently continued to do minor improvement in efficiency. Starting in Jan 2005, we began contributing $6 per bill to the green power program (the costs listed included that $6). Here is our gas/electric usage to date:
|Month||gas (btus)||cost||elec (kwh)||cost|
Gas usage analysis:
Our predicted heat loss was 9854.7 BTU/deg/day, or about 44MBTU for a 4500 degree day year, of which our guesstimate was that 25% of that would come from solar, so we'd need about 33MBTU of supplemental heat. During our 1st two and a half years of occupation, we used around 27MBTU a year, which is very close to our estimated need if you add in the heat gain from the 4500kwh of electric used (of which only the part used in the heating season offsets needed heat). Although we have two gas dryers, one gas range, and our hot water is also gas, I've assumed that contribution is small, especially since our hot water usage is supplemented by a solar collector.
During the last two years our usage has gone up quite dramatically. While I have yet to identify the causes, I have a few theories. First, our base usage (which is due to dryers, cooking & hot water) has gone up by about 500KBTU per month (based on summer month numbers). This would account for about 6MBTU of the difference. Both units have also been more continuously occupied that the past, and so the heat is on more of the time. I'll guess that adds another 10%, since when we're gone we usually leave the heat at 60 (and even then it can take 2 days to get the house to 68), so maybe another 3MBTU there. Finally, we've been getting these cold snaps that make us put the HW tank up to 160, and then forgetting to turn it back down, thereby increasing our standby loss. I assume that accounts for the rest, but all these numbers are just estimates. It is also possible that the Polaris and/or solar collectors are not operating efficiently anymore, but I suspect this is not the problem.
Another way of looking at this is in energy used per square foot of heated space, or energy use per finished space. Our finished space is around 2500, and our heated space is about 2900SF. Our total energy use is from 36.18MBTU to 51.31MBTU (gas+electric), so for heated space that's a range of 12477BTU/SF/YR to 17693 BTU/SF/YR, and on finished space its from 14472BTU/SF/year to 20524BTU/SF/YR.
The best baseline comparison is from the dept of energy website http://buildingsdatabook.eren.doe.gov/docs/1.2.10.pdf , which lists single family residential houses as using about 44,800 BTU/SF/year. I also found alternative values for new houses down near 30,000BTU/SF/Year, which is probably a more valid comparison. Just based on straight utility averages for comparable homes, Seattle city light says that the average usage is 9200Kwh a year, and Puget Sound Energy says the average is 700 Therms(70MBtu), although I have no idea what the average house size those numbers represent. In my view these comparisons aren't really apples-apples because electric use is reasonably dependent on the number of occupants, plus our house includes a rental unit. Ignoring these issues, we use at most only half the energy of the average house, and possibly as little as 1/3.
Yet another measure I found is from www.2010imperative.org, a site dedicated to reducing energy use in buildings. They use essentially the same baseline usage (44,700BTU/SF/year), and say we need to cut that number in half immediately, and by 90% by 2025. In 2006, at 14KBTU/SF/year I'm already about 70% lower, but further lowering will not be easy (click here to read about the difficulty of building zero energy/zero carbon homes). Subsequently, my value of 20KBTU/SF/year is just over 50% better.
2010 update: we had a leak in the heating system in feb-mar, which when it was found was spraying water at the rate of a few gallons an hour. (We didn't measure the rate, its just a visual guess). The leak probably started slowly, but may have been spraying for a few weeks. No one noticed until there was mold. How much this affected gas use is unclear, but the resulting usage of fans sure drove up electric use. Our February use is also high, but there is no obvious reason.
Electric usage analysis:
Our electric usage is mostly for the 2 refrigerators, lights, computers and other electronics and the various equipment motors. I use a kill-a-watt meter to measure most of the plug loads, and hence target which phantom loads to remove. I also installed a meter on the ADU in May 2007 to measure their consumption, which for the first year came to 2100kwh (out of about 4500 total for both units, including HVAC and other equipment). This isn't completely out of range, since they have virtually all the same load requirement, plus an electric stove, a less efficient dish washer, and a refrigerator that uses nearly twice as much electric. Upgrading their fridge would lower their use by about 300kwh/year.
For electric comparison, I ran the city's resource calculator: http://www.seattle.gov/conserve/homeprofile. That showed by data for Jun 21, 2004 thru Jun 21,2005 (pre PV install), so its a good comparison of our actual use. In that period, we used 17Kwh/day and the average comparable house used 25Kwh (single family with no electric Heat or H/W). Since we have an ADU, our comparable usage is pretty good, although a more fair comparison would probably be based on number of occupants. During the last year, we converted a significant portion of our lighting to CFLs, and upgraded one computer. Unfortunately the hibernate feature on the new computer works only when it feels like it, and the hibernate feature on the computer in the ADU doesn't work at all. In addition the refrigerator in the ADU uses 2.2Kwh/day, which could be reduced to 1.1Kwh/day by buying a new one. See PV section below for info on the PV aspect.
Update: Jan 2007: we've updated the computer in the ADU (hibernate works now), and installed a couple of plug strips on the TV/DVD to eliminate the phantom load. We now have a full year of solar data, but because we're away a lot, its not clear how to year by year comparisons.
On a more subjective basis, we find this is the most comfortable house we've ever lived in: there are no perceptible drafts, and when the thermometer says 70, it feels relatively warm even when its fairly cold out. The one exception is in sitting by windows, which alas still have relatively cold surface temperature. One other thing, is that the upper floors are no warmer than the lower floors. On cool sunny days, the main floor gets a bit warmer than any other floors because it has more window area per square foot of floor. In early 2005, we had unusually long sunny spell, and as a result we were often able to go days with little or no supplemental heat.
One positive surprise is how well the house performs on hot days. Even when the temperature was in the 90s out, the house never got warmer than 76, provided we kept the windows open all night and closed them by 10am or so.
Update: Jan 2009: while our electric use improved over the last couple of years (due to conservation--our PV amount didn't increase), our gas usage has gone up dramatically. There may be some equipment issues- the solar hot water and Polaris HW tank may not be as efficient, but I suspect its just that our usage went up. It also may be due to higher standby losses due to leaving the tank set at 160 most the time (see HVAC below for reason). On the other hand, I haven't noticed any changes in behavior either in our hot water usage or setting the thermostat, so it remains a mystery for now.
Update: Apr 2010: our electric use for the year was colored by having to run fans for a few weeks to dry out a leak we had, which added somewhere between 600 & 700kwh to our annual total.
In spite of not being able to use rainwater for toilets in the summer, our water use, remains very low. Here is a chart of our water usage:
|Usage Date||Usage (gallons/day)||Cost||Sewer+Water|
Our usage is nominally for four people, but we've been traveling about two months a year. Countering this is the fact that we work at home, so we're home more than most people. I'm guessing that the drop in usage for the Mar 2005 billing was due to the rainwater tank working, and the rise in Sept 2005 was due to the pump breaking. The low number for Nov 2005 is probably due to all of us being away a lot in Sept & Oct.
The low number in Feb-mar 2006, is again due to travel, and the higher number in Aug-Oct is because we ran out of water in the rainwater tank and had to hand water plants for a three weeks or so.
Update Jan 2009: Our water usage has crept up, possibly because we're home more, possibly just because we use more water. Still, with four occupants, our usage per person remains remarkably low. The higher water use may indicate more hot water use, which may be part of the reason for higher gas usage.
Update Apr 2010: our water usage has crept up again, with the dec-feb 2009 usage being unexplicably high. The Feb-Apr usage includes somewhere between 20 and 50 gals/day due to the leak, but then wern't home then, so the Apr-Jun period is similar. The summer was the hottest & driest I could remember, and as a result I was hand watering my not exactly drought tolerant landscaping.