Cleaning panels doesn't really matter - measurement and photo

[nwdiver]

That is not what he wrote. To say it another way, he got back 2/3 to 3/4 of the few percent of production he was down due to dirty panels. With a simply spray of a garden hose he recovered nearly all of the partial production that was lost to dirt on the glass. A more through scrubbing and rinse gets you back to the best output possible.

Ah... I get it now. So if production was down 10% he got back 6 - 8% not 10% and he's not losing 66 - 75% from dirt. ok. IMHO you're into 'margin of error' territory there.

That's why I just wait for rain. If I'm super bored I'll wash the panels but the production gain is lost in <2 weeks.

[J.P.M.]

Ah... I get it now. So if production was down 10% he got back 6 - 8% not 10% and he's not losing 66 - 75% from dirt. ok. IMHO you're into 'margin of error' territory there.

That's why I just wait for rain. If I'm super bored I'll wash the panels but the production gain is lost in <2 weeks.

As for margin/confidence in my numbers and methods, see the ret of my prior post if you interested.
As for the time/effort a simple hosing takes me under normal conditions (whatever that means/becomes in the future), that's why I will probably spend 5 minutes on my roof and ~ 15 to 20 gal. of H2O about 1X/month if it doesn't rain.

[nwdiver]

As for margin/confidence in my numbers and methods, see the ret of my prior post if you interested.
As for the time/effort a simple hosing takes me under normal conditions (whatever that means/becomes in the future), that's why I will probably spend 5 minutes on my roof and ~ 15 to 20 gal. of H2O about 1X/month if it doesn't rain.

Sure... but a 10mph breeze could increase PV output by almost 5% by cooling the panels. There's just A LOT of variables. I barely see a difference between clean and dirty let alone method of cleaning...

Regardless; assuming your numbers are accurate the other variables are frequency as you mentioned and oversizing. If you get a good rain right after you washed your panels then you didn't really get much return for your labor. And washing once a month means they're dirty at the end of the month so some production is already lost. You're not gaining a 10% increase in your annual production you're gaining ~10% the day after you clean them.

There's also oversized arrays. Instead of cleaning once a month I would just add 10% more panels. I've installed two arrays that are oversized by 50%. Between ~11 and ~3 most days it doesn't matter how dirty they are since the inverters are saturated and no amount of dirt that's able to stick to the panels seems able to change that

[azdave]

If I'm super bored I'll wash the panels but the production gain is lost in <2 weeks.

If I have to go up on the roof for other maintenance I'll wash them off while I am up there (soft water too) but the risk factor of a bad fall and a hospital bill could wipe out 10 or more years of savings in a heartbeat. I've washed mine 3 times now in 7 years.

[J.P.M.]

Sure... but a 10mph breeze could increase PV output by almost 5% by cooling the panels. There's just A LOT of variables. I barely see a difference between clean and dirty let alone method of cleaning...

Regardless; assuming your numbers are accurate the other variables are frequency as you mentioned and oversizing. If you get a good rain right after you washed your panels then you didn't really get much return for your labor. And washing once a month means they're dirty at the end of the month so some production is already lost. You're not gaining a 10% increase in your annual production you're gaining ~10% the day after you clean them.

There's also oversized arrays. Instead of cleaning once a month I would just add 10% more panels. I've installed two arrays that are oversized by 50%. Between ~11 and ~3 most days it doesn't matter how dirty they are since the inverters are saturated and no amount of dirt that's able to stick to the panels seems able to change that

To be clear, I don't care if arrays get cleaned or not. My purpose here is to describe what I think I've learned or confirmed from what studied. The two seem to have produced similar results in terms of an array's output as f(inputs).

By my calcs, which tend to agree well with a lot of empirical heat transfer correlations as well as text book stuff, lead me to believe that in common PV situations, a 10 MPH (~ 4.47 m/sec.) boost in wind velocity, will increase the convective heat transfer coeff. from the array from ~ 35.0 w/m^2*deg. C. to ~ 59.1 W/m^2 deg. C.

That improvement will lower the array temp. However, in doing so, the (now lower) radiative heat transfer will be reduced by virtue of the panel temp. being closer to the surrounding temps. The net result will be a change in the array temp. - under common, summer, clear sky, midday conditions that will be less than the reduction in the convective heat transfer coeff. would lead one to believe. The actual drop in temp. will be something like ~ 5 to 6 deg. C. or so. So, using a bottom line ballpark for improvement in array output at ~ 0.5 % improvement per deg. C. drop in cell temp.: 0.005*5.5 = ~ 2.8 % improvement, not the 5 % you guessed at.
Doing a bit more involved stuff, my array's change in temp. per 1 m/sec. change in wind velocity is about 0.40 %/m/sec., or ~ 2% over 5.5 deg. C change in array temp. I believe my lower change/deg. C. may have something to do with the larger than normal array to roof clearance I have.

But unless I had a reliable source of wind blowing over the array, I'd not count on it too much. A clean array is something I have some control over - the wind, none and it's usually not blowing that hard in many locations.

As for frequency of cleaning, A decent 1/4" or so of rain seems to produce about the same effect on the cleanliness of my array as the simplified cleaning method I described of 5 minutes with a hose. If I lived in a place that got rain on a semi regular basis, I might never need to clean my array - depending on local sap/leaf/blossom conditions and the size of the local bird population and the bird's affinity for my array.

If my panels foul at 0.75 %/week and I use my method of simple hosing that removes ~ 2/3 of the fouling, I'll wind up with a more/less continuous fouling of 4.5 % after a few months. The math will change with the per week ave. fouling and cleaning schedule but the fouling penalty will level of dirt will level off. Your results will vary.

The choice between adding surface/capacity to an array vs. cleaning is an individual one.

IMO only and NOMB, I believe oversizing arrays is not the best way to implement sensible and cost effective alternate energy solutions any more than nuclear power is a sensible long term solution to meeting energy needs.

[PugPower]

So you guys inspired me and I decided to do my own un-scientific experiment. I cleaned my panels last night after 7PM and decided to compare the output from yesterday to today. By cleaning I mean I hosed them off from a stepladder. Took all of 10 minutes. I had not cleaned them since last year and they looked pretty dusty. I have a 6.7kW system consisting of 1 Solar edge 6000HD inverter and 20 LG 335w panels. Yesterday the high temp according to my personal weather station was 98.6 F, little to no wind, mostly clear with few clouds, and a light haze. According to PVOutput I generated 31.353kW. Today the high was 102.0 F, little to no wind, mostly clear with few clouds, and light haze (I honestly could't tell if it was worse or better than the day before, maybe a little better.). Today according to PVOutputs I generated 33.054kWh. A difference of 5.4% on a day which was 3.4 degrees hotter and maybe slightly clearer than the day before. Basically this un-scientific experiment confirms that cleaning your panels will not result in a big increase in production and like JPM says a hosing off is probably all that is needed and/or worthwhile.

[J.P.M.]

So you guys inspired me and I decided to do my own un-scientific experiment. I cleaned my panels last night after 7PM and decided to compare the output from yesterday to today. By cleaning I mean I hosed them off from a stepladder. Took all of 10 minutes. I had not cleaned them since last year and they looked pretty dusty. I have a 6.7kW system consisting of 1 Solar edge 6000HD inverter and 20 LG 335w panels. Yesterday the high temp according to my personal weather station was 98.6 F, little to no wind, mostly clear with few clouds, and a light haze. According to PVoutput I generated 31.353kW. Today the high was 102.0, little to no wind, mostly clear with few clouds, and light haze (I honestly could't tell if it was worse or better than the day before, maybe a little better.). Today according to PV watts I generated 33.054kWh. A difference of 5.4% on a day which was 3.4 degrees hotter and maybe slightly clearer than the day before. Basically this un-scientific experiment confirms that cleaning your panels will not result in a big increase in production and like JPM says a hosing off is probably all that is needed and/or worthwhile.

Pug: I don't know what your zip is or where you are in S,D. county but I'm in 92026. The data below sort of agrees with your observations with respect to atmospheric clearness.
FWIW, and FYI, PVWatts won't tell you what you generated today or any other day. It uses synthetic data for all weather and insolation. Was your reference to PVWatts a typo ?
PVWatts is a decent model, but using its modeled output for any day's actual output is useless.
PVoutput will probably get you a reasonable number for generation.
What's your array's orientation ? Got a lot of shade ? 31.353 kWh/day per 6.7 installed STC kW seems low compared to my output.

The last two days at my location:

Yesterday, 09/16/2020:
Temp. corrected daylong total measured GHI: 5.94 kWh/m^2
Average amb. air temp. at the array, 0800 to 1700 P.D.T.: 34.1 C. (93.4 F.)
Average amb. air temp. at ground level 0800 to 1700 P.D.T.:31.2 C. (88.2 F.)
Average wind velocity at the array: 1.8 m/sec (4.1 M.P.H.)
Total array output for the day: 27.85 kWh (5.32 kWh/installed STC kW)
Highest array measured cell temp: 63.3 C., 28.4 C. above amb. air temp. on roof. P.O.A. irradiance at that point was = 963 W/m^2
Modeled max. clear day output for my array using TMY temp. and wind data and clear sky irradiance model: 32.02 kWh/day (6.12 kWh/day per installed STC kW)

Today, 09/17/2020:
Temp. corrected daylong total measured GHI: 6.00 kWh/m^2
Average amb. air temp. at the array, 0800 to 1700 P.D.T.: 35.1 C (95.2 F)
Average amb. air temp. at ground level 0800 to 1700 P.D.T: 30.5 C (86.9 F)
Average wind velocity at the array: 1.5 m/sec (3.4 M.P.H.)
Total array output for the day: 28.21 kWh (5.39 kWh/installed STC kW)
Highest array measured cell temp.: 67.0 C, 30.9 C. above amb. air temp. on roof. P.O.A irradiance at that point was = 988 W/m^2
Modeled max. clear day output for my array using TMY temp. and wind data and clear sky irradiance model: 32.18 kWh/day (6.15 kWh/day per installed STC kW)

So today was a bit clearer (6.00 kWh/m^2 vs.5.94 kWh/m^2 GHI), and a deg. C or so warmer.

Measured by the array's recent output compared to clear sky output is improving. The worst day for output recently was 09/10 at 61 % of the max. possible value for production under a completely clear sky, with a more/less steady improvement to today's (09/17) value of 88% of the max. possible value for production for this date under a completely clear sky.

Depending on how far apart our locations are, my insolation data may/may not be useful to you.

[PugPower]

JPM, I meant PVOutput and have corrected the post. I'm a bit North of you and in high elevation. My array is almost due South, but I do have some shading in the early morning from trees and again in the late afternoon from my chimney. Earlier in September I was generating closer to 40kWh per day prior to this haze coming in. I think my best day was in May of this year when I generated 46kWh.

[J.P.M.]

JPM, I meant PVOutput and have corrected the post. I'm a bit North of you and in high elevation. My array is almost due South, but I do have some shading in the early morning from trees and again in the late afternoon from my chimney. Earlier in September I was generating closer to 40kWh per day prior to this haze coming in. I think my best day was in May of this year when I generated 46kWh.

Thank you.
Elevation here = 1,480 ft. above mean sea level.
Best production day here 06/12/2017, 35.47 kWh = 6.78 kWh/installed STC kW. The array was ~ 4 yrs. old at the time. Cool clear day. Ave. ground level amb. temp. was ~ 5-6 C. below average for the day. Max. cell temp that day was 50.8 C. That daily max for my array seems to jibe w/your best of 46 kWh/6.7installed STC k w = 6.87 kWh/day per installed STC kW.

Anecdotally, for yesterday, I noticed the sky north of me seemed cloudier yesterday and I was almost on something of a demarcation line of clouds and grey as happens 1X/awhile when stuff drifts down out of L.A.

At 40 kWh/day/6.7 STC kW = 5.97 kWh/installed STC kWh, your early Sept est. of output is higher than mine for the period 09/01 to 09/06/2020. Mine for that period was 28.65 kWh/day/5.232 installed STC kW = 5.48 kWh/day average per installed STC kW for that period.


A bit off topic: The similar result between our arrays, while not a smoking gun, is similar to other data I've gathered since 2014 that tends to confirm a statement I've been making for years:
Arrays in the same location and similar orientation tend to produce about the same annual as well as short term output regardless of panel type.
The primary reason I purchased Sunpower equipment was to attempt to verify for myself whether or not the Sunpower claims of "highest efficiency" and higher output per installed kW had any substance or were and still are B.S.
IMO, the claims are B.S.
Beyond some basic quality level, panels are, and for the most part always have been, a commodity. Folks who spend more time/effort finding a quality installer will get more bang for their buck and a better installation than those who worrying/swallow the B.S. advert. hype. Caveat Emptor still applies.

[PugPower]

On June 12, 2020 I generated 45.192kWh. It was probably my 2nd day best since the array was installed in Oct. 2019. After looking at your data, I have to agree. There really is little real world difference when comparing similar wattage panels from the most common manufacturers: LG. Panasonic, Sunpower, etc. People paying more for advertised claims of more low-light power or specs showing fractions of a percentage of better efficiency are wasting their money. I have never known this as a definite, but have always assumed it.

[J.P.M.]

On June 12, 2020 I generated 45.192kWh. It was probably my 2nd day best since the array was installed in Oct. 2019. After looking at your data, I have to agree. There really is little real world difference when comparing similar wattage panels from the most common manufacturers: LG. Panasonic, Sunpower, etc. People paying more for advertised claims of more low-light power or specs showing fractions of a percentage of better efficiency are wasting their money. I have never known this as a definite, but have always assumed it.

Thank you for the information.

More Q.E.D. FWIW:

06/12/2020:
Your system: 45.192 kWh/6.7 STC kW = 6.75 kWh/day per installed STC kW.
Same day my system: 34.67 kWh/5.230 STC kW = 6.63 kWh/day per installed STC kW.
Given my stuff is ~ 5+ yrs. older, seems like output on that day per in stalled STC kW was about equal. Also, that data seems to add reinforcement to the idea that for mostly equator facing stuff, small variations in orientation don't matter a whole lot of difference in output.

From what I had learned over the years, the inference/claims of higher output through "highest efficiency" and such stuff - particularly Sunpower's hype - looked like B.S. to me.
So, while I had always planned to get PV, if for no other reason than something to provide more unprocessed data to fuel my curiosity, the S.P. choice, while still good equipment that has not failed once after going on 7 years, was made toward seeing if the S.P. claims of "highest efficiency" etc. had any validity.

Short conclusion: They to not. My conclusion is that S.P. stuff simply costs more but is no more fit for purpose in terms of annual output and reliability than other stuff that costs less.
Folks would be better off saving money on less overpriced equipment of about equal quality and maybe spending more time evaluating vendors. Their systems overall quality and construction - which is where problems usually originate in the first place - would benefit in terms of workmanship and professionalism.

To be clear: S.P. stuff is high quality and it does meet the output expectations their spec sheets will allow one to calculate.

But, "most efficient" ? No f-bombing way. Good quality ? - yup.

Overpriced with respect to quality, annual output and reliability compared to other stuff that costs less ? Fur sure.

Used to be S.P. systems cost ~ 20-25 % more for the same annual output and reliability compared to other decent but not high end stuff. That % premium/gouge seems to be coming down a bit for the last year or two but not enough. It also seems S.P. system reliability has taken a hit, particularly since their foray into micro inverters. I wonder if the Maxeon spinoff.
Also, their product support, which never was that good, now sucks worse.

I live in an HOA that has ~ 140 PV installations in the hills behind the Welk resort - you probably know where it is. Anyway, I do the project review/approval recommendations for the HOA for residential PV. Also, about a dozen or so PV owning neighbors allow me to monitor their systems output. All that has been an education in itself and pretty much provides further confirmation of my conclusions about high priced PV systems as well as a lot of other things.

That people are paying for advertising claims is, to my way of looking at it, a bit wide of the mark. A good part of the success of the S.P. yarn seems to be due to the solar ignorance of their marks (customers). I wrote some stuff for our community newspaper/blurb several years ago about some things to perhaps look out for in the way of scams and B.S. when looking for ways to reduce their electric bills - mostly to point out the value of conservation first and also how to spot B.S. from solar peddlers. I wasn't surprised at the vituperation and rancor from the usual suspects: old, entitled white guys whose conversations are often or usually more useful as ranting edicts than sources of useful information. However, I was a bit (but not too )surprised at how much of it came from owners of S.P. systems who drank the S.P. koolaid.

[PugPower]

There were some websites (can't find them now) one was from Australia and the other in the US, where solar enthusiasts/installers took dozens of different panels from different manufacturers and placed them in near identical real world installations and monitored the results over time. The result was that there was little difference between the major manufacturers. Of course some panels performed better than others, but it was not a significant amount. Similar technology, similar results.

I agree with you that focusing on the actual installation and choice of installer is time well spent vs. panel manufacturer. I feel I may have made a mistake not getting micro-inverters vs. string w/ optimizers. If I want to add more panels in the future I will have to upgrade my inverter. I didn't think about that at the time. I expect to have a excess of 2,500 - 3,000kWh at the end of my first year true-up. My wife wants to get a EV in the near future and I'm not sure if we will generate enough to 100% cover our current usage + the EV.

[secessus]

Are you writing that you believe cleaner panels have improved performance because they are running cooler ?

because they are temporarily force-cooled by the cleaning solution, yes.

[J.P.M.]

There were some websites (can't find them now) one was from Australia and the other in the US, where solar enthusiasts/installers took dozens of different panels from different manufacturers and placed them in near identical real world installations and monitored the results over time. The result was that there was little difference between the major manufacturers. Of course some panels performed better than others, but it was not a significant amount. Similar technology, similar results.

I agree with you that focusing on the actual installation and choice of installer is time well spent vs. panel manufacturer. I feel I may have made a mistake not getting micro-inverters vs. string w/ optimizers. If I want to add more panels in the future I will have to upgrade my inverter. I didn't think about that at the time. I expect to have a excess of 2,500 - 3,000kWh at the end of my first year true-up. My wife wants to get a EV in the near future and I'm not sure if we will generate enough to 100% cover our current usage + the EV.

Some closer sources, like poster Solarix, have done that or something quite similar. Not writing/speaking for him, but he's done some side by side stuff that seems to show string inverters perform quite well compared to micros or optimizers.

[J.P.M.]

because they are temporarily force-cooled by the cleaning solution, yes.

No. Probably not.

In spite of what you may think, such temperature reduction effects from sensible and evaporative cooling resulting from spraying water on an array will not have a measurable effect on array output.

For several reasons and as f(several factors), such cooling will last only for a matter of a few minutes (for example only, maybe about 3 to 10 minutes or so for my array, depending mostly on wind vector, dew point and air/water temp. diff.), and in addition, such spraying/washing better be done early or very late in the day at a time when irradiance on the panels will be quite low, making power output at such times quite low to begin with. So, very short term temp. reduction at a time of not much production in the first place is a double whammy for making it a very small effect in output.

Bottom line: Short term cooling as a benefit to washing an array is small enough to be pretty much non existent and can be ignored.