Energy Report, November 2010


Solar electricity produced: 285 kW h (9.5 kW h per day)
Electricity consumed: 490 kW h (13.4 kW h per day)    Heating: 107 kW h, Non-heating: 383 kW h


Although winter doesn't officially arrive until December 21st, winter weather is upon us. Our solar electric array only collected enough electricity to meet 58% of our needs, so this is the first month where we have consumed more than we produced. Part of our higher consumption was due to a brief all-electric heating experiment described below, but even discounting that we didn't get enough sun to meet all our needs.  The graph below shows how we started the month with lots of heat "in the bank", with our 20,000 pounds of water in the heat storage tank at its maximum temperature of 140 degrees. We started using it rapidly in the first week of the month, got a bit of sunny weather and were able to warm it back up a bit, but then the second half of November brought mostly chilly temperatures and cloudy days that steadily depleted our store of heat. The system worked well and we extracted about 1 million BTU from the water as it cooled from 140 degrees to 90 degrees. But winter in Michigan means cloudy weather and we don't expect to really start warming up the tank again until March. If there's any good news, it's that our solar heat collectors are more efficient when the fluid is cold so at least we'll harvest as much of the scant sunshine as we can. Even at 90 degrees the storage tank still provides about 50% of the energy for our domestic hot water heating, with the on-demand electric backup heater providing the rest and consuming less than 1 kW h per day.

The graph below shows the temperatures outside (red) and inside the house (blue), recorded each morning and evening. We generally keep the thermostats set around 68 degrees, and it occasionally warms into the low 70s when we get a sunny day or when we have the wood stoves going.

We used the wood stoves on colder evenings, but during the coldest days near the end of the month we stopped burning wood and switched on the electric backup heater for a few days just to see how well it would perform without any heat from the wood stoves. It worked fine and kept the house warm but consumed about 40 kW h per day, which is four times our normal level of consumption. To us that seems like a lot, but we can heat a 3000 square foot house with electricity in typical Michigan winter conditions for about $4 per day which is not bad! We've now switched back to the wood stoves for heat and we have been carefully weighing and recording every armload of wood we bring into the house so we can measure our overall consumption.

With outside temperatures in the low 30s and cloudy days it takes about 30 pounds of wood per day, which is one large armload, to heat both the main house and the cottage. We expect to use a bit more as the weather gets colder, but not a lot more. At this rate it should take about one cord of wood for the whole heating season, which is about what we expected based on the heat loss calculations we did when designing the house. Incidentally, burning 30 pounds of wood produces heat energy equivalent to about 50 kW h of electricity and our stoves are about 80 percent efficient so they should deliver about 40 kW h of heat into the house - which is exactly what we measured during the few days when we heated with electricity. It's only a coincidence that the numbers came out exactly the same but it's safe to say this is a good estimate of our home's heating load under these conditions.

As a comparison, according to Michigan State University, heating an average 3000-square-foot home in Michigan uses about 130 million BTU per winter and we expect to use less than 20% of that.

This page was updated on Wednesday January 11, 2017