How to compare

Pretty quickly I realized that just the daily production in kWh is not an easy metric, because how can you compare a 12-panel installation with a 20-panel installation? Modern panels have a capacity of up to 400Wp (watt-peak), whereas older ones can be less than 300Wp. I needed to find a better metric.

I came up with something new and simple: if you divide the production in Wh (Watt x time) by the capacity in Wp (Watt), you get something I call the eWPH: (equivalent) Watt Peak Hours. A eWPH of 1 hour for a certain day means: if all your panels would produce at top performance (defined as e.g. sun shining at 1000 W/m²) for 1 hours, they would have produced as much as they have today from sunrise to sunset.

In Belgium, where I'm based, we produce now (end of Winter) at around 1 eWPH. I found some stats on installation in India and there it's more than 4.

The big advantage is: this metric is independent of the size of your solar installation. If the average for Belgium is 1.2 eWPH for a certain day, and your installation only does 0.8, then it's not performing great, regardless of if you have 16 panels or 24.