Energy Report, May 2010


Solar electricity produced: 658 kW h (21.2 kW h per day)
Electricity consumed: 292 kW h (9.4 kW h per day)


For the month of May we produced 2.25 times as much electricity as we consumed, and we exported a net surplus of 366 kW h. Our energy consumption per day was about the same as the month of April but solar energy production was 13% lower. We attribute this drop in solar production partly to more cloudy weather in May, and partly to the increasing sun angle that makes the system a little less efficient this time of year. Despite slightly lower production, we passed a milestone on May 15th - our first megawatt-hour delivered to the grid. This means that in the first 45 days of operation the system delivered one million watt-hours of energy to the electrical grid.

One way we are conserving energy is by not having an air conditioner. In May we had about a week of days with temperatures around 90 degrees F (32 C), but the house never got above 75 degrees (24 C) inside even without air conditioning. This is because the house is extremely well insulated, has a heat (cool) recovery ventilator, and has carefully designed overhangs over the south-facing windows to keep out direct sunlight this time of year. The following photo was taken around noon on May 29th, and shows how the overhangs shade the big south windows to prevent unwanted heat gain. The same windows are in full sun in wintertime when the sun angle is lower.

The solar heat collection system continues to operate well, and because the collector panels are oriented vertically (you can see part of them on the far left in the photo above) we collected much less heat per day than we did in the winter when the sun angle was lower. This is a good thing, because we're using it only for domestic hot water so we don't want a lot of heat collected. The following graph shows the temperatures at the top and bottom of the heat storage tank in degrees Fahrenheit. As designed, the system limits the maximum temperature to 140 degrees because that's the rated temperature of the polyethylene storage tank. Our well water is around 55 degrees F and after it passes through the heat exchanger coil in the tank it comes out about 10 degrees cooler than the water at the top of the tank. We can (barely) tolerate a 100-degree shower so we'd like to keep the tank above 110 degrees. With about 20,000 pounds of water in the heat storage tank at 140 degrees, we have about 600,000 BTU of usable heat energy for making hot water. Our low-flow showerhead uses only 1.3 gallons per minute and it takes about 30,000 BTU per hour to heat the water, so theoretically we could take a shower for 20 hours before running out of hot water! In practice after a 10-minute shower we don't even see a change in the tank temperature reading.

One interesting thing to note about the graph above is that the tank loses about 1 degree every night. That's actually quite significant, and represents more heat loss than we use for showers and laundry so we're actually losing more heat than we're using. It's no problem this time of year because we have far more hot water than we can use, but in the winter we would really like to cut that heat loss. Our best guess is that most of this heat loss is conducted through the copper pipe that leads up through the attic to the solar collectors, even though the pipe is insulated, so at some point we'll try inserting a length of PEX tubing to provide a thermal break in that pipe.


This page was updated on Wednesday January 11, 2017