We’ve now been in the house for about two months. But our electric bill runs (roughly) from the 21st to the 21st. So last week we received our first “full month” electric bill. Given that it’s the first full month, the information gleaned from the bill is, of course, very preliminary. But I thought I’d share it anyway. The important numbers were as follows:
Average Outdoor Temperature During the Period: 53 degrees
Total Electric Usage: 551 KWH
Total Cost for Electricity $90.94
Cost per KWH: 17 cents (all fees and charges included)
The last week of this billing cycle, it got fairly cold. Nightly lows were as low as 19 degrees. Daily highs were in the high 20s to mid 30s. But again, the average temp for the month was a relatively mild 53 degrees
The house is two floors plus a basement. Square footage is 1,000 per floor.
All lights are LED; 40 and 60 watt equivilants. Most are 60s. Only a few are 40s.
The house is completely electric. Oven is convection. Cook-top is induction. Water heater is heat-pump.
We have two Mitsubishi mini-split heat pumps. Both are 12,000 BTUs. Each has one cassette. One cassette is on the first floor. The other is on the second floor at the top of the stairs.
For all but the last two days of the month, we used only the first floor heat pump. Some days I didn’t use it at all. Other days I left it on. Mostly I was trying to get a feel for how it operated. I was surprised to find out that the fan on the indoor cassette unit always runs (albeit at a very low rate), even when it is putting out no heat.
The single heat pump, kept the house “comfortable” down to the 19 degree low. However, when the outside temp got below freezing, it appeared to put out heat all the time. This kept the first floor at a pretty consistent 67 or 68 degrees at night and 68 to 70 degrees during the day. The second floor temperature (which is where the bedrooms are) varied; at night it got down as low at 64 degrees, and during the day, with a little help from the sun, it rose to 68 or 69 degrees.
That’s why I put the word comfortable in quotes. I found this to be quite comfortable. After all, for us, almost all of the time spent on the second floor is spent under covers. But my wife was not quite as content. So for the last couple of days, I turned on the second floor heat pump. This quickly brought the second floor up to 68 and significantly reduced the working time for the first floor heat pump. Both heat pumps were set to 65 degrees. I’m not sure why a 65 degree set point on the heat pumps results in a 68 degree environment. But it does.
A couple of things seem clear to me. First, at 68 degrees the house is exceptionally comfortable. Not a hint of a draft, even when the wind is howling. And second, a clear, sunny day makes a measurable difference in the form of a reduced heat load.
Next week, I will install an eGuage energy monitor on the main feed and 12 circuits. So beginning then, I will have a much better read on the energy consumption for the heat pumps, water heater, and ERV. I will also install Hobo temperature/humidity data loggers on all three floors and outside. This will give me the ability to monitor and report on the performance of the house over the long term.
Based on this very preliminary information, I have a few initial thoughts. First, I was a bit disappointed in the first month’s electric bill. Both the KWH used and the overall cost were higher than I anticipated. But I’m thinking that this is due more to unrealistic expectations (and an unexpected 17 cent/KWH overall rate) than house performance.
One way to try to put this data in context is to compare this first full-month electric bill to my energy cost from one year ago, when we lived in our prior 2,800 square foot house, built in 1987, which used propane heat. The average temperature last year was 51 degrees; a couple of degrees cooler. At that house, I followed our energy usage pretty closely. Electric usage was easy (just read the bill). But I had to gauge propane usage by my readings at the end of each billing period. Each night, we set the thermostat back to 60 degrees and at least three days per week we left it at 60 degrees during the day because we were both at work. We never raised the thermostat past 69 degrees. But frankly even I thought that was uncomfortable. It pretty much always felt colder than either of us would like. At any rate, my total energy cost for that house during the same period last year was $428.43, and my electric usage was 881 KWH. But remember, we were heating an additional 800 square feet.
It will be interesting to see how things play out in the long run. Taking a SWAG based on this admittedly limited data, I’m thinking that when all is said and done, we have a good shot at being under 6,000 KWH total energy usage for the entire year. That would result in a total energy bill of about $1,000. And if it looks like that prediction is proving to be fairly accurate, I may start looking for a PV system next spring.
Energy updates will follow, once I get the monitoring systems installed.