Washing array has little if any effect on output

[Cz10]

My system is 40 Suniva 265 w panels with Enphase M215 inverters. The system has been operational since July 2013.

I first cleaned the panels in Oct 2014 and could not detect and increase in production.

I just cleaned them again in October of this year with again no significant increase in production- maybe 2%, but certainly not more. They were REALLY dirty this time as we had multiple wildland fires in the area over this summer with lots of ash. I used a pressure washer and a soft brush that is meant for cleaning RV

The system is producing 110% of the estimated production (according to Enlighten) and has produced a peak output over 8.6 KW earlier this year before the panels were cleaned.

Is it because I am overdriving the inverters with the panel size, per a white paper on the Enphase site?

[sdold]

Hi Cz10, welcome to Solarpaneltalk. I think to get a fair comparison between before cleaning and after, you'd need to watch a pair of panels for a couple of days and then clean one and watch them again for a couple of days, to see the difference. I did that and got about a 5% difference when they were pretty brown from dust. My day to day production varies too much to be able to use the My Enlighten app or site without comparing individual panels.

[Cz10]

OK. I guess I expected a greater change

[J.P.M.]

OK. I guess I expected a greater change

Unless you have a way to compare clean vs. fouled performance that's reasonably accurate or at least consistent, either by before/after cleaning and comparing clear day instantaneous output measurements with adjustments for panel temps. and plane of array irradiance differences, or if a micro equipped array, using a method as Sdold describes, you won't get a decent estimate of performance loss due to fouling.

FWIW, I've found the eye is a poor predictor of array fouling. My array can look like it's completely derelict and still have a 6 % performance rolloff due to dirt/fouling.

Being a 4 + yr. old array, I suspect you probably have a sting inverter system, as micros were not quite as popular as they have become since.

If you get rain or overnite/morning dew on a regular basis, array fouling will probably level off at some point. It hasn't rained any appreciable amount at my house since early May. Morning dew/condensation has kept my performance penalty due to fouling to ~ 6 % or so, +/- some. Without rain or dew, my array's performance seems to drop ~ 1 % per week or so due to accumulation of dirt/fouling. This is far from an exact science. Your situation/location will be different, and the fouling rates will not be constant. 1%/week for my array is a ballpark number. Sometimes a bit more/less.

My array looks pretty dirty, but the fouling, which I measure whenever it's a clear sky at the time of minimum incidence angle on my array and I'm home, seems to stay at about that 6 % level, I'm suggesting that somewhat asymptotic limit of 6 % or so is from cleaning/runoff by the marine layer of moisture that happens quite regularly here.

I've also found/measured that simple hosing of my array at a rate of ~~ 3/4 gal. per panel if it doesn't rain seems to restore ~ 2/3 to 3/4 of the performance penalty from fouling. So a 6 % fouled array will improve to ~ 2-3 % fouled if I simply hose it off. If I wasn't measuring fouling rates and how rain/dew affects fouling, I'd hose my array every 4 weeks or so when it doesn't rain and live with what's probably something like an average 3 % performance penalty due to fouling.

I'd also not waste my time on commercial cleaning. That's a rip off.

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

[Cz10]

Being a 4 + yr. old array, I suspect you probably have a sting inverter system, as micros were not quite as popular as they have become.

I was an early adopter, I guess. Once I learned about Micro Inverters it seemed clear that it was the way to go.

Like I said, I thought it would be more along the lines of 10%+. At least now I don

[J.P.M.]

I was an early adopter, I guess. Once I learned about Micro Inverters it seemed clear that it was the way to go.

Like I said, I thought it would be more along the lines of 10%+. At least now I don

And like I wrote, how dirty a panel or array looks is not necessarily by itself a good indicator of how the performance has been impaired by dirt.

If you have micros and per panel monitoring, determining performance deterioration from fouling gets a lot easier. Further to Sdold's post:

Find 2 panels with as similar output as possible, hopefully adjacent to one another if possible, with one of them easily accessible as practical. Clean the accessible panel while trying to avoid cleaning the others. OR, better yet to keep output up, clean the entire array while covering one panel including considerations for making sure the covered panel glazing doesn't get wet during array cleaning. Remove the cover from the test panel after cleaning the rest of the array.

For either method, observe and record the output for the 2 comparison panels, either instantaneously or over time, or both. For same time periods, divide the smaller output by the larger output and subtract 1 from the result. That (negative) number will be a rough estimate of the performance penalty for the accumulated dirt. Keeping a record of the outputs can provide a lot of information about rates of fouling and how rain/dew/dry periods can affect system performance.