This chart shows:
* Monthly kWh generated by the panels (blue bars).
* Monthly kWh used in the house (red bars).
* The difference, or net production (the green bars).
A positive difference (upward green bar) means we generated more electric energy than we used; a negative difference that we used more than we generated.
This is a two-year average from July 1, 2009 through June 30, 2011. There was relatively little difference between the two years. Bar charts of day-by-day PV generation for each of the 24 months are archieved below.
Note: The graph is based on PG&E "billing months," which are offset from actual months by 5 or 6 days.
Our hope was to be electrically neutral over the course of a year, having enough positive net production during summer to offset the heavier use and decreased generation of winter. Winter sees a big increase in useage, primarily from electric baseboard heaters used for supplemental heat upstairs.
We did not meet our goal. The averages for our "solar year" of July 1 – June 30 are
Electrical energy used: 6351 kWh
Electrical energy generated: 5526 kWh
We generated 87% of what we used. We would be very close to neutral with an 18-panel 3.33 kW system.
However, see further comments below about how this is a financial success.
With two year's data, we can now estimate the payback time for our PV system. Knowing exactly how much money we saved is difficult to compute exactly, due to the time of day pricing, but I estimate the PV system saves us between $750 and $800 in electric bills for the year. The system cost $22,600 installed. A California energy rebate gave a $6200 credit upfront, so our initial out-of-pocket expense was $16,400. Then there's a 30% federal tax credit, so we saved $4800 on 2009 taxes. Thus our net cost was $11,600. A straight-line payback calculation at $775/year gives a 15 year payback time. Rising energy costs will lower this estimate to, perhaps, around 12 years. Note that payback times, often quoted as 10–15 years, are quite sensitive to assumptions made and to the electricity cost in your area. In any case, the system should last ≈25 years with little or no maintenance, and it should have paid for itself in about half that time.
This graph displays the total kWh generated per day for July 2010. Summer is right at 20 kWh per day almost every single day. Always full sun here in the summer!
This graph displays the total kWh generated per day for December 2010. It was very rainy from about Dec 13 to Dec 25.
These curves (data recorded every 15 minutes) show the seasonal performance on a totally clear day. The late afternoon "wing" is presumably just from skylight on the panels since the sun has moved so far to the northwest by late afternoon that it is "behind" the panels. These are displayed as local time, not daylight savings time, so the the four can be compared
Solar noon at our longitude is almost exactly 12:00. The midsummer curve peaks right at 12:00. The peak shift of nearly an hour on the October 1 curve was at first surprising but can be explained partially from the panels facing about 15° east of south and partially from the eccentricity of the earth's orbit, which shifts the sun's position in the sky at noon.
The changing length of the day is clearly visible. Total daily generation on a sunny day in December is less than half of July, due more to the shorter day than to the lesser peak power at noon. The fact that April meets or even exceeds July's peak is due to the cooler temperatures boosting the panel's efficiency.