Our gas bill for this month was

The reason I asked is: It is important that they be cleaned once a week, not just rinsed off. My Podiatrist has them installed on the ground specifically so he can clean them off good. Maybe you have easy access.

 

Once a week is totally overkill unless you really live in a super-dusty area. If my panels were mounted a bit steeper (have a near-flat roof), I would probably only need to go up once a year at most if I really had the urge. Because the panels are mounted near flat, dirt tends to accumulate over one row of cells due to the lip that holds the panel together. When this dirt gets pretty thick (it has to be pretty bad, in my experience) it can affect panel output by a decent amount. Light coatings of dirt/dust don't seem to have much effect - it takes a pretty good coating to drop output by even 5% where you might be able to notice the difference.

If the panels are mounted at least at a 10-15* angle, morning dew is enough to push most of the dirt off and the occasional rain plenty to wash them. I highly recommend against mounting panels at anything less than 10* because of this - if I were to do it again I'd make a bigger effort to tilt the panels - I didn't in my case because it would have likely raised the panels slightly above the roof-line which looked like it was going to cause permit issues. The combination of greater output thanks to tilting the panels further south and greater self-cleaning ability would have been worth it. Panels would have stayed a bit cooler, too being further away from the roof.

As it is now I've been going up there once every 3-4 months to wipe them down with a California duster. Takes about 15 minutes to wipe down 18 panels. Half the time when comparing the before/after numbers I can't tell a difference. I could just clean them in late May and then in mid August. To get that last 5% of power. Otherwise I could probably just clean them once a year in June. Certainly if you had to pay someone to clean them they would have to be VERY dirty to make it economical or you would have to have a fairly large system.

 

Are you using less or is your rate falling, or both? I suspect all of the fracking and horizontal drilling techniques are starting to bend the cost curve down.