DC to AC Conversion Efficiency

November 20, 2014

One source of loss with a solar power system is the conversion of the DC power produced by the panels to the AC power used by the electric grid. This is done by a piece of equipment called an inverter. I was curious to see how my setup compared to the spec, so I pulled data for December 2013 through October 2014.

Solar Performance - October 2014

November 14, 2014

I managed to barely squeak through October with a net monthly surplus of power, just 5.68 kWh (for reference that's about a day's worth of usage). November will almost certainly be chipping away at my current annual surplus. Artisan Electric stopped by towards the end of the month to do my first annual inspection and cleaning and found that everything is right on track compared to their pre-install estimates.

Solar Nemesis: Rain

October 23, 2014

Over the course of the day my poor solar panels weren't even able to produce half a kilowatt-hour of energy; their total for the day was just 494 Wh. In comparison, on a sunny summer's day the same system was able to produce that much power in just 11 minutes during the peak period.

Solar Performance - September 2014

October 3, 2014

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).

Another Bill, Another Credit

September 20, 2014

Without any solar panels I would have used 427 kWh at a cost of about $21.60, so in terms of my bill the panels saved me about $29.40. The state incentives, as usual, made up the lion's share of the financial savings, with the 773 kWh generated being worth about $417.42 in incentives compared to $39.11 in sales.

Solar Shading, or, the Power of Pruning

September 11, 2014

Earlier this year I noticed that one of my panels, A2, was having a noticeable dip in power production compared to the other panels for a few hours every afternoon. A neighboring panel, A3, showed a similar problem but to a much lesser extent. The remaining panels nearby operated normally. Even though there's a tree nearby I'd originally dismissed shading as a problem since the none of the other panels were affected, but when the engineers at Tigo looked at my data they concluded that everything was operating normally and that shading must be to blame.

Panel A2 matches A1 for the first half of the day, then dives down for a few hours in the afternoon.

A few days ago I borrowed a pole pruner from a friend and was able to remove the offending branches:

Before pruning on the left, after pruning on the right.

Removing those branches seems to have fixed the problem! Just goes to show that even a few small tree limbs can significantly affect solar production. From January through August that little dip cost me a total of about 40 kWh of production. That's enough to drive a Nissan Leaf over 125 miles!

Solar Performance - August 2014

September 5, 2014

Production in August was down from July, and roughly the same as April. This makes sense given the shorter, cloudier days of late. Use was also down slightly as the cooler temperatures meant I didn't need to run the fans and air conditioning quite as much.

Solar production, grid purchase/sale, and use.

As expected, the system continued to generate a surplus of power for the month. It's on track to have a surplus on an annual basis as well! Unfortunately, panels A13 and A14 were out of commission for the first half of the month until the defective Tigo unit was replaced. Combined with the days they missed last month they are noticeably lower than the rest of the system. Panel A2 is definitely lagging due to the afternoon shading.

Percent of household energy use produced by solar.

Production is a bit under the installer's estimate, mainly due to the bad panels, but is still pretty much on target.

Breaking the data out by day there were only a handful of days that the system wasn't able to generate more power than I needed. Some of these were laundry days (the dryer is a killer!) while others were just very gloomy weather.

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

At the panel level, shading is still having a noticeable effect on panel A2, while the outages on panels A13 and A14 reduced production significantly.

Energy by panel, broken out by day. The lack of output on panel A14 (covering the dots for A13) is very apparent.

Noticeable loss of output on panels A2, A13, and A14.

This demonstrates why I wanted to make this chart: under-performing panels are easy to spot.

Hourly production shows a peak level similar to April but is shifted about an hour later in the day. I should try to find a solar elevation chart to see if this correlates.