Every year in April I look at how much energy my solar panels are generating. I pick a sunny day in early April and look at the maximum amount of energy generated per interval. In 2015, the panels are generating up to .75 kWh in the middle of the day. In an earlier year with freshly cleaned panels this number went up to .77 kWh. I use this check to see if I should have my panels professionally cleaned.
This winter California like most of the country had very strange weather. We had an exceptionally warm and dry weather with little or no snow in the Sierra Nevada. Technically the rainfall was reasonable in my immediate area, but California and the San Francisco peninsula depends considerably upon melting snow for its water. The few times it did rain, it rained a lot and these heavy rains seems to have washed off any debris from my solar panels.