Energy Report, January 2011


Solar electricity produced: 199 kW h (6.4 kW h per day)
Electricity consumed: 991 kW h (32.0 kW h per day)    Heating: 587 kW h, Non-heating: 404 kW h


This month we recorded our highest energy usage to date, mostly because we used the electric backup heater more than last month. On the brighter side (literally) our solar electricity production was 18% higher than in December. We used more non-heat electricity in December, which we attribute mainly to having company over the holidays. Even with the higher usage this month, we were able to meet 20% of our electricity needs with the solar array versus 19% last month.

The graph below shows our cumulative electricity production (red) and consumption (blue) since we started the system in April. It's interesting to note the abrupt jump in the blue line in early December that coincides with several days when, as an experiment, we used exclusively electric heat without using the woodstoves. To date we have still produced 8% more than we have consumed, and we're expecting the gap to start widening again in February due to typically sunnier weather and increasing sun angle, and because we don't expect to use the electric backup heat much more.

The graph below shows the temperatures outside (red) and inside the house (blue), recorded each morning and evening. Last month's graph showed the average of the main house and cottage temperatures, but this month we're only graphing the main house temps because we turned down the thermostat in the cottage when it wasn't occupied. Even on the very cold nights we had no problem maintaining a comfortable temperature with just a small fire in the wood stove.

Here's a graph of the heat storage tank temperatures, and you can see that we had a few days of bright sun when we saw some impressive daily heat gains. With about 20,000 pounds of water in the tank, each degree represents 20,000 BTU of energy which is equivalent to about 5.9 kW h of electricity. On the sunniest days we recorded about 10 degrees of temperature rise, equivalent to 200,000 BTU or 59 kW h. We typically collect about twice the usable energy from the heat collector panels than from the solar electric modules, and the heat collectors have only about half the surface area of the electric modules so they're roughly 4 times as efficient. Although we didn't collect nearly enough to heat entirely with solar energy, it was enough to meet a significant portion of our heating needs and the drops in tank temperature show where we transferred the heat into the house via the heated floor.


This page was updated on Wednesday January 11, 2017