How much energy do we use now? Pre net zero home.

By Matt

We currently are renting a 2,165 square foot house built in the late 90’s located in Northeast Rochester. The house has two levels, four bedrooms and two bathrooms. The lower level of the house is 1/2 to 3/4ths below ground with the master bedroom and kitchen on the upper level. A portion of the house, above the dining and living area, has a 15 foot vaulted ceiling and no attic space while the other half has lower ceilings with an insulated unused attic. Based on the thickness of the window wells the walls appear to be constructed of 2″ x 6″ lumber, and we assume the bays are filled with fiberglass batts. The windows are a mix of fixed, sliding, or turn out casements with double pane glass, presumably pretty good windows for the late 90’s.

The main heating for the home is forced air from a propane furnace supplemented in the lower level by a propane fireplace insert. The water is heated by a propane water heater. Cooling is central air with an outdoor condensing unit. The clothes washer and dryer are both electric. There are two refrigerators and an electric stove. We have two desktop and two laptop computers, a modern flat-screen TV, and the house has a mixture of incandescent, LED and compact fluorescent lights.

The house and water heating is the easiest to understand and calculate as we can just look at our propane bill.

We use the most of course in the dead of winter, approximately 180 therms, and the least in the middle of the summer approximately 23 therms. So from Nov 2016 to Oct 2017 a total of 801 therms of natural gas were used. The middle of the summer use would be entirely due to the hot water heater, so we can calculate our entire house heating load as 801 – (23 x 12). So 276 therms to heat water and 525 therms to heat the house. 1 Therm is approximately 100,000 BTUs, so 27.6 million BTUs to heat water and 52.5 million BTUs to heat the house. The amount of energy to heat the water (276 therms) was almost exactly what is printed on the energy saver guide on the side of the hot water heater, 268 therms typical yearly usage.

Our electricity use varies from ~1000 kWH to 600 kWH per month and we are uncertain about why we have such a large variability. It could be that after March there was typically no one home during the day to use electricity. Also our desktop computers finally started to enter hibernation mode when not in use much more reliably so they were not spinning during the daytime or at night. We use very little air conditioning during the summer, generally just a few hours a week during the worst heat, mostly to remove the humidity, and as such there is not a large jump in our use during the summer.

From December of 2016 to November of 2017 we used ~9000 kWH of electricity for an average of 750 kWH per month. This will have to be reduced when we get in our new Net-Zero house as we expect to generate 12,100 kWH of electricity from our solar panels and will be generating hot water and house heat by electricity. Things that may help reduce our usage will be all new high efficiency electric appliances, one less fridge, all LED lighting, large shade free windows, and the smaller footprint of the house. Since we will be monitoring our electric usage and trying to get to Net-Zero we should be more vigilant with turning things off when not in use.

How efficient is the heating/insulation of our rental house? Based on BTUs used per square foot per heating degree day (2017 HDD was 7343) the calculation is 52500000 BTU /2100 sq ft / 7343 = 3.43 which would put in the to 12% of homes in a 1997 survey. If you include hot water it would be 801000000 BTU / 2100 sq ft / 7343 = 5.19. I include the hot water so I can compare this statistic to the value calculated for our previous house in Ossining. A 1930s house with little to no insulation in the walls, single pane windows and an oil boiler. The water was heated with the same boiler so water heat cant be separated from the house heat in this calculation. We used approximately 950 gallons of #2 fuel oil per year (1 gallon = 138500 BTU). Ossining on average has 5500 HDD so the calculation is 950 gallons x 138500 BTU per gallon / 1450 square feet / 5500 HDD =  16.4. If you include the unfinished basement which has passively heated through waste heat and passage of heat through uninsulated floor then the square footage would increase 33% to 2175, and the BTU/Sqft/HDD would drop to 11.0. Not as bad as I thought, but more than 50% of households had lower numbers in that 1997 survey.

The current PHIUS passive house standards for home heating is 7300 BTU / SqFt / Year for zone 6 (Rochester). So that would be a BTU/SqFt/HDD for Rochester of 1. Can our net zero house approach an heating efficiency rating of 1? I think it may be difficult unless we hit some super tight air exchange rates (<0.5) and get lots of passive gains (solar gains, people and pets, appliances).


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