Energy Report, March 1-15, 2010

We moved into the house at the start of March. And we were also fortunate to have a lot more sunny weather in the first half of the month. We also got the wood stoves installed and inspected, so we could use them for supplemental heat on cloudy days and cold nights. So far we have only used them a few times though.


Average daily heating degree-days: 26.4
Average daily heating electricity used*: 3.0 kW h = 10,239 BTU
Average daily other electricity used: 11 kW h = 37,088 BTU
Average daily solar heat collected: 22.9 kW h = 78,326 BTU
Heating electricity per degree-day: 0.11 kW h = 388 BTU
Total heat (electric+solar) used per degree-day**: 0.08 kW h = 283 BTU

* This includes only the electricity consumed by the electric backup heater.
** This is a measure of how well the house is insulated and sealed, and how much passive solar gain it collects.


Wow! What a difference the sun makes! Early March was very sunny and about 10 degrees warmer per day than February. The milder weather combined with further improvements to the house have cut our heating electricity usage to nearly zero. In fact it has been exactly zero since March 7th when we shut off the electric backup heater. We attribute most of this improvement to the increased active and passive solar gains, plus some internal gains now that we're living in the house. We did use the wood stoves a couple of times on cooler days, but we didn't burn much wood (in the future we'll record exactly how much we burn).

Now that we're living here our general electricity usage has gone up to 11 kW h per day. That's not very much but we want to keep it well under 20 kW h, which is the estimated average daily production of our solar electric system.

The heat storage tank graph below shows an interesting pattern. For the first week we were getting significant gains each day as we had a week of sunny weather. But we also see a significant temperature loss each night, typically several degrees. After the middle of the graph, two things changed. Firstly the weather got cloudy, so we stopped getting heat gains during the day. And secondly we added insulation in the big opening between the heat storage tank and the mechanical room, so the nightly heat loss dropped to only about one degree. The excess heat was being transferred into the house so it wasn't entirely lost, but having a warm mechanical room doesn't do us much good. The remaining 1 degree of temperature loss is still significantly more than we predicted based on the insulation level of the tank, and we think it's being conducted through the copper pipes leading to the solar collectors. At some point we may splice in a length of PEX tubing as a thermal break, to see if it cuts the heat loss measurably.

Another pattern you can see is that the temperature at the bottom of the tank (the blue line) started dropping faster than the top in the last few days. We attribute this to increased use of domestic hot water, which cools the bottom of the tank (where the cold well water enters the heat exchanger coil) more than it cools the top.

This page was updated on Wednesday January 11, 2017