Energy Report, December 2010


Solar electricity produced: 195 kW h (5.5 kW h per day)
Electricity consumed: 875 kW h (28.2 kW h per day)    Heating: 391 kW h, Non-heating: 484 kW h


The first half of the month was cloudy as is typical here in December, but we got some nice sunny days in mid-month when we were able to collect some useful heat in the storage tank. But overall there still wasn't much sun and we quickly used up what heat we were able to store.

The graph below shows the temperatures outside (red) and inside the house (blue), recorded each morning and evening. The inside temperature generally remained comfortable, with occasional spikes into the mid-70s when we cranked up the wood stoves. We had no problems keeping the house warm even with temperatures in the single digits, although we didn't keep the wood stoves going every day so we used the electric backup heat at times.

Our solar electricity production averaged a meager 5.5 kW h per day, and as expected this was the lowest monthly production we have recorded. Most of this is due to the generally cloudy weather, but even on sunny days our electricity production is hampered by the low sun angle because we get tree shadows on the PV modules. The following photo was taken at mid-day near the winter solstice when the sun is at its lowest angle. The tree shadows on the modules don't actually block a large percentage of the sun, but they greatly reduce the amount of power that the modules generate. We anticipated this when we chose the location of the house, but the drop in energy production isn't really all that significant because it's usually cloudy this time of year. And many of the trees that are creating these shadows are not particularly healthy trees, having been damaged during the logging that was done before we bought the property, so we plan to cut some of them down and replace them with fruit trees that won't grow high enough to block the December sun. On the plus side, you can see in this photo that our overhangs are working as designed, allowing full sun into the south-facing windows for passive solar heat gain. We estimate that we get about 20% of our heat from passive solar gains this time of year, and it should contribute much more in the coming months.

While our solar electricity generation reached an all-time low of 5.5 kW h per day this month, our consumption reached its highest level at 28.2 kW h per day. Nearly half of that was consumed by the electric back-up heater since we didn't burn the wood stoves every day and allowed the back-up heat to main temperatures as needed. The remaining usage of 15.6 kW h per day included about 1.3 kW h / day for domestic hot water heating. Even with our heat storage tank only around 85 degrees, it warms our well water from about 55 to 80 degrees before the electric on-demand heater warms it the rest of the way to 110, so the solar heat provides almost 1/2 of the energy needed to heat water for showers etc. We also used significant energy for the clothes dryer this month, as we didn't have many good drying days like the one shown above when we could line-dry clothes. Our total electric bill for all uses including heating the house was about $100 for December, which most people would consider quite cheap for this area, but we hope we can do better next year.

We had one additional significant energy consumer this month: our Heated Bird Bath. You can just see it in the photo above, sitting right above the ground in front of the windows on the left. When it gets very cold it consumes up to 2 kW h per day to keep the water ice-free and that seems pretty significant to us, but any consideration of whether a heated birdbath is "worthwhile" disappeared when the Eastern Bluebirds showed up! We have a group of about 8 bluebirds that have decided to spend the winter here (most of them fly south), and they were first attracted to the open water in the birdbath. Once they discovered the suet feeders they became regular visitors and we now see them daily.

Despite our meager production and fairly high consumption of electricity in December, we're still ahead in terms of overall electricity generation since we started the system in April. To date we have produced 32% more electricity than we consumed over the first 9 months, so we're still hoping to make it to this coming April with a net surplus for the year

This page was updated on Saturday January 14, 2017