Workings

How the logging utility works.

The Fronius inverter provides a fair amount of instantaneous data about what it is doing, but it doesn't provide an accurate figure on how much energy it has generated today. The best it does is to provide a slightly optimistic figure in whole kWh - no decimal places.

To overcome this problem, the scripts query the inverter at regular intervals (2 minutes by default) and save the results of each query to a CSV file. A fresh CSV file is used for each day's output.

A second script then runs every 10 minutes and reads in all the records from the current day's file. It approximates the total energy production by assuming each power reading is representative of a 2 minute window, and then adding up all the chunks. It then reports the current readings to pvoutput.org which does all the hard work of producing graphs and what have you.

I find that over a whole day's production, the figure calculated for energy produced tends to be about 300 Wh too high (as compared with the production meter, which is theoretically more accurate). This seems to stay about constant regardless of the level of production. I've observed it on a day when I produced less than 2 kWh and the same on a day with more than 14 kWh. I think it may be connected to the length of the day - as the day gets longer it goes up a bit. I'm thinking about putting in a correction factor at some point.