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, in my case an Eltek THEIA 3.8 HE-t UL. The manufacturer claims it is "up to 97.3% efficient," meaning that I'll lose at least 2.7% of the power produced by the panels. The specification for the inverter includes an efficiency curve showing how the efficiency varies depending on the amount of power being produced.
I was curious to see how my setup compared to the spec, so I pulled data for December 2013 through October 2014. I don't have the instantaneous numbers that are used in the spec (it would be quite a lot of work to collect and process all of that) and I don't have access to the direct data on the inverter, so I compared the DC power output from the Tigo power optimizers on the panels to the inverter output measure by my eGauge. I used daily totals for both numbers, then created a scatter plot of the efficiency versus the total power output for the day.
- Minimum: 58.45%
- Maximum: 95.08%
- Mean: 91.76%
- Median: 93.90%
The numbers are generally in line with with the spec though they tend to be a few percent lower. The low efficiency for low production days is one of the reasons that the winter is such a tough time for solar, since anywhere from 15 - 40% of the power coming from the panels can be lost due to conversion. Luckily that only works out to a handful of kilowatt-hours, and isn't a problem in the summer.