Electrical Power Production and Consumption
Maine is a "annualized net metering state." There are two meters on our house--one measures power "purchased" (in) from Central Maine Power Company and the other meter measures power "sold" (out) to CMP. If we export the same amount as we import, there is no charge, except for a $8/month hookup charge which all electricity customers must pay.
Here are the results for 1998. The monthly PV output reflects the amount of energy harvested by the PV array.
The 4200 watt array generated 4.246 megaWhr of electricity in 1998, even though June and November were particularly cloudy months. We "imported" 2.417 mWhr of power from Central Maine and "exported" 3.008 mWhr to the grid. Therefore in 1998, we generated 591 kWh more than we used.
While the bill has been figured on a monthly basis these past few years, in 2000 the bill became "annualized" - excess production from one month is added to the following month until a final 12-month net is determined. As you can see from these figures, we owed nothing for electricity - just the small monthly hookup charge of $8.00. As part of the new net billing rule, any excess will be given back to the utility at no cost and any deficit will be paid at the retail rate (currently 12 cents/kWh). Here is Maine's Annualized Net Metering Policy page.
Energy profile of March 10, 1996
Many people believe that in cold weather, solar houses suffer - it's just the opposite. The colder it is in the winter, the clearer the sky. Therefore, the panels are more efficient due to the cooler temperatures and the house absorbs heat more readily with the sun at a lower angle. If we don't open a window or lower some of the blinds when the full sun in shining, the winter temperatures inside the house will rise above 76 degrees.
The numbers on the left represent temperature, both outside air and water temperature inside the solar tanks. The day began with an outside temperature of 1 degree above zero, peaking at 28 by mid-afternoon. The water temperature continued to increase until about 2 p.m. (156F). The PV output is measured on the right, nearly reaching 4.0 kWh between 11 a.m. and 12 noon (peak output is calculated at 4.2 kWh). Quite an efficient day all around!