Panel cleaning

[BakerPV]

I washed my panels last evening before it got totally dark using Windex Outdoor Window Cleaner (the kind that screws onto the end of a hose) and my output at peak today was about 11% higher than yesterday (same weather conditions). It'll be interesting to see the total kWh I get at the end of today compared to yesterday. My system is fourteen 327 watt SunPower SPR-327NE panels and a SolarEdge SE5000, facing 100 degrees (East Southeast). They were installed at the end of April and have gotten some rain maybe twice. We have new homes going up near us and there is quite a bit of blowing dust from the empty lots. The panels had a very noticeable layer of dirt on them, and some spots of bird poop, and now they are actually blue/black again. I was thinking the lower output I was noticing over "just-installed" was due to the high temperatures we've been having lately (100+ degrees), but now that the panels are clean, I'm thinking dirt was the main culprit. Since I only have to stand on a ladder to get above a patio awning, and don't have to get on the roof, I believe I'll be hosing them off every other month or so now. I checked with SunPower before using the Windex and they said it was fine.
Kirk

[J.P.M.]

I vote for installing one or two more panels to begin with and then staying off the roof. Far easier than regular cleanings, smashed rain gutters and wear and tear on your roof surface. Bonus is no ladders, no dragging hoses and no risk to your health.

That's one way to do it. Opinions vary. I've suggested about 3-5 % oversizing for such opinions and live with the built in cost ineffectiveness. I'm still trying to get a better handle on how much and in what ways rain reduces the fouling and whether or not the rate of fouling becomes asymptotic and if so , in what ways and time frames.

[azdave]

I vote for installing one or two more panels to begin with and then staying off the roof. Far easier than regular cleanings, smashed rain gutters and wear and tear on your roof surface. Bonus is no ladders, no dragging hoses and no risk to your health.

[J.P.M.]

I read recently that dirty panels efficiency can be reduced from between 20 to 40%. Looking around online I've seen tools for cleaning the panels. One that I like is a brush that has super soft bristles, like the brush I have for washing my RV, and you can connect a hose to it so water runs through the brush and so you are gently washing your panels to remove all dirt. However, I think the catch is you need to used distilled water or de-ionized water so you don't leave any mineral deposits. Just a good rain won't wash away all the dirt and or bird poop so I think the brush idea is a good one. Do you have a solar panel cleaning service in your area that you can sign up with? maybe get your panels professionally cleaned about 3 to 4 times per year?? Obviously, if someone has invested in getting solar panels, you want them to be operating as efficiently as possible.

If the 20-40% claim were true, I suspect skylights would show a lot less transmittance than they do. Always possible, but my guess is more advertising hype than reality. I'd SWAG 10%. or so max.sThe folks in Death Valley, although they do not monitor input very well, do not see much of a decrease in output and they only clean their 1 MW array every 2 years. Thjey do not get a lot of rain.

There are panel cleaning services for ~ $1-$2/panel, but after 3-4 weeks the array will probably be 3% or so fouled. I hit the array with a hose and restore about 1/2 to 2/3 of the clean performance, save the money and don't break as many tiles as a cleaning service.

I cannot detect a difference from cleaning and then a distilled H2O rinse and a plain hose rinse. The hard water spots do not seem to impair performance in any way I've been able to measure. The eye is not the best discriminator of transmittance of solar radiation. There is no catch to D.I. or distilled H2O. IMO, it's not necessary or worth the hassle. I'd like to see hard numbers showing something else. More hype IMO.

[DanKegel]

Dew is as good as distilled water. The one time I tried, there seemed to be enough morning dew at dawn to clean a panel with a good sponge/squeegee without needing extra water. Has anyone else tried that?

[bcroe]

My advice, is don't wash them unless its dark; electricity and water are very dangerous. Bruce Roe

[Crowbar]

I read recently that dirty panels efficiency can be reduced from between 20 to 40%. Looking around online I've seen tools for cleaning the panels. One that I like is a brush that has super soft bristles, like the brush I have for washing my RV, and you can connect a hose to it so water runs through the brush and so you are gently washing your panels to remove all dirt. However, I think the catch is you need to used distilled water or de-ionized water so you don't leave any mineral deposits. Just a good rain won't wash away all the dirt and or bird poop so I think the brush idea is a good one. Do you have a solar panel cleaning service in your area that you can sign up with? maybe get your panels professionally cleaned about 3 to 4 times per year?? Obviously, if someone has invested in getting solar panels, you want them to be operating as efficiently as possible.

[Ian S]

We had heavy monsoon storms last week so I expect my panels were cleaned off to an extent. Still a lot of bird poop along the top edge of the highest row of panels. I still plan to install a row of bird spikes but it's just been too hot.

[J.P.M.]

Around my area - San Diego county, I look at PV arrays as I'm driving around the area and walking in my neighborhood. The last measureable rain at my house was 06/06/16, and that was only 0.03" - just enough to cake the dust on my panels. Many/most arrays I see are getting a nice and quite noticeable bathtub ring with the remainder of the panel getting pretty dusty looking.

From a bunch of measurements I've done and continue with, as best as I can give an educated guess, my array's performance degrades approx. somewhere between 0.75% and 1.0% per week as a result of dirt buildup on the panels if they are not washed. Without rain or washing, I believe that rate starts to get asymptotic after about 6-8 weeks or so in some way I can't yet estimate, quantify or model.

A decent rain or hosing the array down seems to restore about 1/2 to 2/3 or so of the clean array performance. So, if my array is fouled to the point of, say, a 6 % decline in performance due to dirt,
hosing it off will probably improve the array's performance by about 3 or 4 %.

A mostly south facing array around here will produce about 150 - 175 kWh/month per nameplate kW this time of year. 3% of that is ~ 5 kWh per nameplate (DC) kW. If in tier 4, or prime time T.O.U. that's ~ $0.35 or so per kWh --->>> approx. savings per installed kW ~ = $1.75/mo. So, for those with arrays that have seen no moisture in a few months, a few minutes with a hose and probably what amounts to less than 50 gal. of H2O, someone with a 5 kW array might well reduce their current electric bill by something like ($1.75) X (5) = ~ $8 or $9. per month or ~ 30 day billing period.

Caution: before you run out with a hose: ONLY HOSE AN ARRAY WHEN THE SUN IS NOT ON IT, PREFERABLY IN THE MORNING, AND NEVER AFTER ABOUT 8 A.M. AT THE LATEST.

REASON: THE PANEL GLAZING MAY CRACK FROM THERMAL SHOCK.

Just sayin'. No guarantees expressed or implied. Your mileage may vary.

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