Solar Performance - September 2014

Another month, another set of charts.  As expected given the waning days and increasing number of cloudy days production was down compared to last month's. My usage was also down a bit thanks to the cooler weather (I have yet to turn on the heat). 

Solar production, grid purchase/sale, and use.

I'm still building up my surplus with Seattle City Light which should see me through the dark winter months. If I'm lucky then I might even generate a surplus in October. The cumulative production for each panel looks largely unchanged compared to last month, though all the bars are slightly higher now.

Percent of household energy use produced by solar.

Cumulative production by panel (DC).

This month the system did pretty well against the installer's estimates this month. DC production was slightly above the estimate (324 kWh versus 305 kWh), while the AC production was only 1 kWh below the estimate at 304 kWh. About 20 kWh was lost in the DC to AC conversion, roughly 6.2%.

Comparison with installer's estimate.

Looking at the daily numbers shows a clear trend. I have a daily baseline of around 4 - 6 kWh of use, with periodic weekly spikes in the 10 - 15 kWh range. These are laundry days, and I think there are more of them than usual this month since Pat is staying with me. This month had a few really standout days with great production, but also a couple of very poor days.

Daily solar production, grid purchase/sale, and use.

Percent of household energy use produced by solar.

The individual panels were much more consistent on a daily basis, with only a little variation between them. This is just what I want to see, everything working correctly with no shading effects.

Energy by panel, broken out by day.

The consistency is also clear in the monthly total for each panel, most of which are the same. Zooming in on the difference from the average reveals something interesting: the west-facing panels (A1 - A4, A7, and A8) are all out-performing the east-facing panels. I'm tempted to ascribe this to a typical Seattle weather pattern of cloudy mornings and sunny afternoons, though I haven't been collecting any weather data to corroborate that.

Fairly consistent production across all the panels.

West-facing panels were a bit more productive than their east-facing counterparts.

Taking a look at the daily numbers initially seems to dispute this idea, however. I'm consistently getting more energy from the panels facing east. After normalizing on the number of panels, though, the west-facing panels show better production on most days. I have 8 panels facing east and only 6 facing west. Of course the ideal direction is south but I don't have any roof sections facing that way.

More energy is coming from the east-facing panels as a group.

After dividing by the number of panels the west-facing group has slightly better production.

The curve for September is noticeably smaller than the curves for the previous 5 months, both in height and span. This is probably a reflection of the shorter days, cloudier weather, and maybe even a lower sun angle.

Production by hour for the past 6 months.

Financials are still looking pretty good. I'm excited for November to roll around to see how much of the cost I'll have recouped after the first year.

REC payments are dominating the net-metering savings.

Last but not least, the DC to AC conversion efficiency is showing a little bit of a drop-off at the lower power levels. I'll have to aggregate some more of this data from multiple months into a separate post.

DC to AC conversion efficiency was pretty uniform.