Net Energy Balance

This graph shows the net accumulated difference in kWH between our electricity production and consumption since we first activated the PV system in April 2010. Each point represents the total amount of electricity that we generated to date minus the total amount consumed to date, and it varies seasonally because we get much less sunshine in the winter:

For the first two years we maintained a net surplus electricity production of about 400 kWh per year, which was about 7% more than we consumed on an annual basis. This excess energy was fed into the grid and sold back to our local electric utility Consumers Energy. Then in December of 2013 we both came down with a nasty case of the flu, and feeling too sick to keep the wood stoves tended we switched on the electric backup heater. That kept the house comfortable while we were sick but with the higher consumption and very low production in the cloudy winter months it put a really big dent in our energy balance, dipping down to about negative 400 kWh by March. It took us until June to get back to a net positive balance again. We've taken some steps to reduce our electricity consumption, and we're fixing the insulation problems that we uncovered using our thermal imager so we believe we can stay in net-positive mode from now on and we should once again produce about 6% to 7% annual surplus. That is, assuming that we don't get such a bad case of the flu again!

Heating Load

Although it's unfortunate that we used so much electricity for heating when we got sick this past winter, it did provide a useful opportunity to measure the total heating load of our home because for a time it was 100% electric heat without any help from the woodstoves. We have an electrical meter that measures just what the electric backup heater consumes so we can tell exactly how much energy was consumed for electric heat. Fortunately Liz, although she was sick with the flu, continued to dutifully record energy meter readings throughout December and January so we collected some useful data. For December and January our heating load averaged 45 kWh per day during the weeks when we heated only with electricity and didn't use the wood stoves. This is the coldest and least sunny part of the winter, so for November and February we'd estimate needing about 2/3 as much or 30 kWh per day, for a total of about 4500 kWh annually or roughly $500 in cost. Some people would consider that a very low heating bill for a house this size in our climate, but after fixing the insulation problems we estimate it would be more like $400 per year if we were to heat only with electricity. But assuming that we're able to tend the wood stoves (which is not too hard since we burn so little wood), we can keep the electric backup heater turned off and heat the house with about a cord of wood that we can harvest on-site, and still maintain net positive electricity production.


This page was updated on Wednesday January 11, 2017