Monday, October 05, 2009

Solar Power - High and low usage and costs for the first month

The solar array continues to generate over 20KWh on a clear day, but our daily consumption varies a lot. On Sunday we were running the washing machine, dryer, pumping water from the well for a few hours, watching TV and baking/cooking/dish-washing for seven people, and we hit an all time high using almost 50KWh.

The next day, running pretty close to the minimum possible, we used about 13 KWh.

Our total generation for the month of September was about 700 KWh. The output is dropping as the day gets shorter and the sun gets lower, so our 4.2KW system peaked at about 3.4KW at the start of the month, after a rain shower cleaned the panels and the air we saw a peak of 3.7KW for a short period of one day, then it dropped back to 3.3KW and it ended the month at 3.2KW. The daily output on a clear day was 26KWh at the start of September and 24KWh at the end.

Our approximate total consumption for september was 1100 KWh, and since we are on the time-of-use E6 rate the net metering subtracted the 700 we generated and broke down the rest as -100 KWh at peak time (1-7pm), and 500 KWh mid and off-peak. We were billed $11.50 which is the base level for PG&E, and a total of $17 for 400 KWh was added to the annual accumulation.

In a year's time the net for the year is due to get settled. The very complex details are available from PG&E, but basically instead of costing us on a sliding scale of 11c-38c per KWh, the solar array keeps us at the bottom end of the scale and at the E6 rate it costs 8c off peak, 14c mid peak, and while we are running the meter backwards at peak PG&E pay us 30c per KWh. So our typical electric bill of $250 was reduced to $28.

I have been asked about the payback time for the installation. This is part of the calculation that the vendor sales-rep makes for your particular situation. In my case we made it much more complex by replacing the roof shingles, shutting down our use of propane, buying new appliances and increasing our use of electricity. However If we ignore all that and average $200/month savings, then after all the rebates the solar system installation of about $20K has a payback time of less than 8 years on a naive calculation. If we take into account a 5% interest rate on the installation cost it's more like 12 years, and if we assume that electricity costs would rise over that time it's somewhere in-between. I'm also assuming that the value of the house has gone up.

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