Cleaning the solar panels

The new solar system comes with a monitoring system that tells us how much power is being generated over time.  Now that we have had the system for 16 months, we have comparative generation for 4 months.

Notice that last year we generated more energy each month during the summer. 

Now we know that the weather varies, and it is possible this is simply because we had more sunshine last summer than this year, but that seems unlikely. 

We also know that solar panels decrease in efficiency over time, but a more than 10% drop in just a year seems extreme.

So we thought maybe the panels have gotten dirty and need cleaning.  Looking at the panels from down on the porch, that does seem a possibility.

We can't see the surface of the panels very well, but from this angle it certainly looks like they are covered with a yellowish dust -- maybe pollen from the oak and elm trees.

So we decided to wash them off.  We used an external faucet and hose, and Windex Outdoor Concentrated Cleaner, which we have previously used to wash the outside of our windows (and screens).  Searching on the web, it seems the main concern in cleaning solar panels is not to use an Ammonia based cleaner, and Windex Outdoor is Ammonia-Free.

We first sprayed the panels with just water, to wet them down and soften any dirt.  Then we sprayed them with the Windex Outdoor solution.  Checking with a clean rag, there was still a lot of grey dust on the panels, so we used a sheepskin applicator on the end of a long pole to gently wash the panels off.  Then we rinsed everything off with enough water that there was no more suds or dirt, just water rolling off the panels. This took about 200 gallons of water for the 14 panels.  And then just to be paranoid about the possible hard water deposits of tap water, we poured a gallon of distilled water on the top of the panels, letting it flow down and rinse everything off.

The result is panels that look much cleaner.

 We will have to wait until the end of October to see if we generate as much energy this year as we did in October last year.