Cleaning solar panels

The only cleaning done here was of tree sap dripping. Washing
would not remove it, took the 3 panels down and used a razor blade.
Bruce Roe

There's a guy on here JPM I believe he tracks his output meticulously, he could tell you the difference.

I'm guessing if you have to hire someone that's insured if they fall off your roof, the increase in production won't offset the cost

Paying a service to come out and wash my panels would cost me multiples times more than what I would recoup in energy due to dusty panels being cleaned. It's a low percent loss normally and with 26 panels and low relative energy costs here in Phoenix, the savings just don't add up. I go up on the roof usually once a year to check things out and that is the only time I might take a hose up with me to the spray them off. We've gone 124 days without rain and the panel are dusty but I still collect probably 95% of what I would if they were freshly cleaned. I also factor in my time and safety risk of extra trips to the roof and decided that staying on the ground with dusty panels is a better decision. I just wait for mother nature to pay a visit. We're entering our monsoon season so rain is likely not far from happening.

Like other have said, you will never recoup the cost of the cleaning if you have to pay someone to do it. You could clean them yourself, but, like azdave, I won't risk damage to my roof or myself by attempting this. I'll wait for the winter rains to clean my panels for free.

The answer to how much depends on the context of the question.

Panel or array fouling is quite variable depending on location, terrain, climate, weather, panel orientation and other factors.

If the panels have never been soaped up, washed with a soft brush and rinsed, such a good cleaning won't hurt and will probably improve array output by maybe 5 - 10 % or so it if you haven't had a decent rain in a month or so.

As Azdave seems to suggest and as I agree and believe, commercial cleaning is generally not economically viable and is probably a waste of money.
But that's not to say cleaning of some sort on a regular basis isn't a good idea. Read on to figure out why.

I live in a semi-rural area (zip 92026) that gets a lot of sun and about 25 cm of rain/yr. with most of that rain coming between Dec. and March.

Over the 10+ yrs. my array has been operational I've figured out that if it doesn't rain (mostly the other 8 months of the year) and I don't clean it, my array fouls at a rate such that the array performance deteriorates somewhere between 0.6% and 0.9% per week - I use a nominal 0.75 %/week or ~ 3% /month for a non-rain fouling rate. A decent rain of ~ 0.3" to maybe 0.5" will produce about the same result.

I've also come to the conclusion through observation, measurement and calculation that the rate of fouling of my array seems to pretty much stop at some point such that the total fouling penalty for my array seems to become asymptotic at somewhere between about 8% and maybe around 12% or so of the array's clean performance - although I'm not sure why.

As mentioned, array fouling mechanisms and rates are highly local and variable so take the above as no more than a guide. Your situation will probably be a different but I'd wager a small amount that you may find your situation to be similar to mine in many ways.

I designed my rooftop array with several features that most folks leave out. One such feature is that I can get at it rather easily for service and maintenance.
So, about 1X/month, I go up to the roof and clean my array.

A couple of things of many I've discovered about array fouling and cleaning:

First, once an array is clean, simply hosing it down with simple tap water (no brushing) at a rate of ~ 3-4 l/panel will remove most of the dirt and dust (but not guano) such that about 3/4 to maybe 90% of the performance that's been lost to fouling is restored. I also treat the big bird dumps and owl skrocks with window cleaner and a rag. Lots of soap and water perform no better for a lot of extra B.S. and work and the array fouling proceeds at the same rate.
I do that rinsing and cleaning - simple tap water and drip dry - ~ 1X/month and live with an array that's probably, on average, about 3% fouled.
That process takes me ~ 20 minutes or so. I do it in the early A.M. before the array heats up to avoid heat stressing and cracked glazing.

Second, the hard water spots left behind by tap water and drip drying may seem unsightly but do not seem to inhibit performance restoration in any way I've been able to measure. In other words, using D.I. or distilled H2O is a waste of money and resources.
It also turns out (perhaps serendipitously) that films of Group II chemicals that are responsible for most hard water and hard water spots (mostly Magnesium and Calcium) are pretty transparent to wavelengths of sunlight below ~ 1.15 or so micrometers - about the same cutoff wavelength for silicon hole pair formation - which in plain language means water spots don't matter for PV production if silicon bases cells are used.

Take what you want of the above. Scrap the rest.

IMHO I think there is a concerted effort to build up a business opportunity to sell services to people who are clueless about their solar systems. Sort of like selling magic potions to dump down the toilet to fix septic systems or extended warranties. Sure, cleaning may be needed in very specific situations and areas but my guess is the ads I am seeing are just a way to make a buck.

Given how mentally and physically slothful Americans have become - and so seem to be leading the world in the headlong rush to mediocrity - I'm not at all surprised, but as a nation we've always been solar ignorant.

Maybe that's all being made worse by installers who never should have been installing PV in the first place and are now rooting around for some other (related) scams as the low hanging PV fruit is gone from the market and they're part of the crowd that's scrambling to make ends meet by cashing in on the collective ignorance.

Some things never change.

Thank you for the detailed analysis. We normally don't get any rain April-October however until someone comes up with a drone that does the cleaning, I'm going to pass climbing on my two-story house roof,

My last employer was heavily involved with ASME performance testing of power plants and I do remember some discussion at one point about coming up with a test procedure for field performance testing of large PV arrays and how to come up with a correction factor for panel cleanliness. Not sure if it ever was resolved as my guess was it was a prebid situation and we didnt get the job so didnt chase the details further.

You're welcome.

That was the reader's digest version.

My house is 2 stories as well.

Considerations for access for maintenance, including cleaning arrangements, are all part of the design process as I learned and practiced it so it's not a problem for me.

If nothing else, clean panels look better. From my experience though, i have to agree with J.P.M.'s findings.

I suspect what may have happened was the people looking at PV panel fouling discovered that the situation is not very amenable to closed form analysis or estimate, mostly because each application/site, or even areas within the same site (much like different sections and flow regimes within a heat exchanger) will have different fouling characteristics that are controlled by maybe a lot of variables that are not only not well understood but with some variables unknown.
Best shot in the case of flat plate PV generation may be to measure the in-situ fouling and come up with procedures that deal with it in cost effective ways at that site.

As a practical matter, and from the little anecdotal info I'm privy to, it turns out most large PV plants probably don't do much of anything about fouling and have usually found out like I have that the fouling often becomes asymptotic at some percent penalty of clean performance that's mostly site dependent and they accept that with maybe annual cleaning or less often in desert climates. The arrays in Borrego Springs and at Furnace Creeek in Death valley being two examples.

A bit off topic, but when I was employed, a couple of my employers strongly suggested that all the working engineers get more involved in one or two sub-specialties related to their proficiency areas(s). As a result, I served on two ASME pressure vessel subcommittees for a few years. One involved flow induced vibration in shell and tube heat exchangers. The other involved the broad topic of heat exchanger fouling. Both subcommittees mostly worked with the folks dealing with The PTC's 12.1 thru 12.5.

After I had retired, I had some interesting conversations with some folks at the ASME about what happened at the San Onofre Nuclear Generating Station.

Cleaning is based on your location/environment. Some do, some don't.
Solar panel owner, South Florida East Coast, since 2017, professionally cleaned by SolarShine, my neighbor, twice a year, June & December.
Bee Pollen, Pine Tree Pollen, and bird droppings are the three things Florida rain, at my home, doesn't wash off.
I have kept up with biannual cleaning since new.
My neighbor has shown me pictures of jobs he bids on that haven't been cleaned in years, sad, spending hard earned money and not keeping them clean.
My favorite was a 24 KW system, huge home, never cleaned, owner assumed the panels had age degradated, losing efficiency.
The forest that surrounds his home in Jupiter Farms resulted in heavy buildup that took 5 times longer than necessary to properly clean. Impressive before and after performance.
That home is now a twice a year appointment. Lesson learned.
Thank you for reading
Good luck to all

I have in-laws in AL and other relatives in coastal SC I visit on a regular basis. Pollen and tree sap is ubiquitous in their areas.
If not hosed down on a frequent/as needed basis, things can get pretty messy.
Taking care of such situations and other design considerations like snow removal is part of system design.
Nothing unusual there. It only means playing a little head's up ball at the preliminary design stages.