TimesToCome

Geekiness and life on the third coast

How efficient is your home?




So where is your home on heating and electric usage? US Average kwh per year varies a lot depending on the source reporting. Most sources report about 9,500 kwh per home for electric usage in homes that do not use electric heat. A high efficiency home spends 36% of its utility expense on heat, some homes go as high as 58%. In the Houston area 7 kwh/sqft is average, a zero energy home here uses 4 kwh/sqft.

To find your heat use you must first deduct non heat use, then convert to BTU’s then divide by heating degree days and the square footage of your home. Start by getting your total gas or oil etc use and electric use for the year. Most utility companies are on line now and you can look up your account with them and get the last 18 to 24 months usage.

{this is one year’s use in Providence while we were still tightening up the victorian}
Gas (2003-2004): 1775 therms
Electric (2003-2004): 21,000 kwh

Gas for washer, water heater etc 30 therms a month or 3600 for 12 months ( this is also the average US )

Electric for lighting, appliances etc is 1100 /month ( wow! ) or 12,200 kwh for the year.

Gas for heat ( 1775 – 3600) = 1,415 { boiler with steam radiators }
Electric for heat ( 21,000 – 12,200 ) = 8,800 { This is 5 oil filled electric radiators and one pond heater. }

Now we want to convert the heat units to BTUS { British Thermal Units }
Gas therms * 100,000
Electric kwh * 3,413
Propane gallons * 91,500
Oil gallons * 138,000
Wood cord * 22 million
Coal ton * 26 million

So 1,415 * 100,000 + 8,800 * 3,413 = 171,534,400 BTUs for the 2003-2004 winter

Now we divide that by heating degree days. This takes the high for the day adds the low for the day divides by 2 ( average daily temperature ), and subtracts 65 from that. The data is already compiled by utility companies and can be easily found online for your zip code and the year you are investigating.

Providence, RI 2003-2004 ( 6,100 HDD )
171,534,400/6,100 = 28,120 BTUs/HDD

Now divide that by the square footage of the heated part of your home
28,120/2,500 = 11.25 BTU’s / HDD / square foot

So how’s your home stack up?
A super efficient home uses less than 3
12% of homes are less than 5
39% of homes are between 5 and 10
24% of homes are between 10 and 15
15% of homes are between 15 and 25
10% will be over 25

So we were about average in energy use at that point in time. Now it is 2007 and we’re down in Houston and this house is mostly energy efficient, just a few things left to do.

Total gas therms 385
Total electric kwh 12482

non heat/cooling use
gas 120 therms
kwh 8400

Total heat and cooling energy btus
gas 265* 100,000 = 26,500,000
kws 4082 * 3413 = 13,931,866

Total heating and cooling btus
(26,500,000 + 13,931,866 ) = 40,431,866 heating and cooling btus

heating and cooling btus divided by heating and cooling degree days
40,431,866 / 4368 = 9256

divide now by house square footage
9256/ 2876 = 3.2 btu/h-cdd/sqft

I expect once we replace the single pane clear glass windows with double pane low e, replace the 15 year old refrigerator and 15 year old heating and a/c systems we’ll come in well below the 3 btu/h-cdd/sqft that we are targeting.