Energy Report, January 2010

This is our first month of recorded data. We're not yet living in the house but we're keeping it heated in the low 60's using solar heat plus the electric backup heater. We are still missing insulation above some of the exterior window and door headers, and we still have some significant infiltration around some of the doors. Some of the temporary door thresholds also leak quite a bit of air. It's been cold and cloudy, so our solar heat collection hasn't been great. Despite all of that, and heating mostly with electricity, it's costing us less to heat than it would cost to heat a typical house this size. But we know we can do a lot better!


Average daily heating degree-days: 39.6
Average daily heating electricity used*: 54.7 kW h = 186,699 BTU
Average daily other electricity used: 3 kW h = 9823 BTU
Average daily solar heat collected: 12.6 kW h = 42,840 BTU
Heating electricity per degree-day: 1.38 kW h = 4709 BTU
Total heat (electric+solar) used per degree-day**: 1.70 kW h = 5790 BTU

* This is 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.


The solar heat collection system was working and we saw some significant temperature gains on some days, but with cloudy weather we quickly used up what heat we had collected and the system automatically reverted to the electric backup heater to keep the house warm. Even with six large solar collectors, the solar heat collection offset only about 20% of the electricity consumption. Clearly the answer is not to have more heat collectors, but to have less heat loss!

The graph below shows the temperature fluctuations in the heat storage tank. The red line is the temperature at the top of the tank and the blue line is the temperature at the bottom. See heat storage design for a discussion of why this temperature difference called stratification is desirable.

This page was updated on Wednesday January 11, 2017