I can weigh in on this having installed solar for 18 years, (although I've been out of the game for nearly five years now).

As long as the panels are above 10 degrees pitch, are not under trees or a pigeon roost, and are of tier 1 manufacture with quality coatings, then most dust and pollution will wash off the next time it rains.

There is no 'need' to wash them. In India, I think it would be wise not to wait for the monsoons, but no such issue in Blighty!

Unfortunately, I no longer have the study we conducted with a customer in London who was right next to the North Circular in London. Not exactly Indian levels of pollution, but about as bad as it gets in the UK. He had 2 arrays both facing in the same direction and on separate inverters on a 3-storey building with no shading so ideal for testing purposes.

Specs for both were the same, the only difference being that the right-hand array was accessible from a balcony and could be cleaned regularly, which he did each weekend. He was meticulous with his records and we compared notes after 2 years.

When we visited the left-hand array was visibly different to the spotless R/H array, but the key thing was that the outputs were within a couple of kWh of each other.

These were Sanyo 190's - HIT cells so able to scavenge photons over a wider spectrum than conventional Poly or mono cells, but we saw similar results with all types.

Improvements in frame designs over the years made the bottoms of panels free-draining which also helped to reduce build-p.

Key dirt issues with panels are:

Build-ups of moss/lichen/dirt around the edges of the frames Normally this only occurs in less than a 10-degree pitch or when constantly pelted by leaves or pine needles.

Excessive bird poo. The odd dropping will be 'cooked' by the panels and become friable so will normally wash off with rain.

Pigeons living under the panels. Even if they don't soil the panels, (they will!) you would just not believe the mess they make underneath. Nor what the ammonia in their poo can do to cables, connectors, roofing felt.

It's the downside of living in a relatively clean air location I guess.
I hear all the anecdotal stories but I guess they are all dependent on the amount and type of soiling. Grime and dust is quite possible washed away by rain, but once lichen and algae gets hold my experience is no amount of water alone will shift it, the spores need killing off.
I plucked this off the net because it has a couple of pics. The first pic illustrates my "hard to reach" panels. The second pic. is extreme soiling and my second string is nowhere as bad it's but about half that. I'm unconvinced that this type of soiling won't affect production:-

https://solarsena.com/how-to-remove-algae-and-lichen-from-solar-panels/#:~:text=Cleaning%20Instruction%201%20Pour%20isopropyl%20alcohol%20in%20a,spray%20your%20solar%20panels%20off%20with%20cl

<blockquote><cite>Posted By: owlman</cite>It's the downside of living in a relatively clean air location I guess.
I hear all the anecdotal stories but I guess they are all dependent on the amount and type of soiling. Grime and dust is quite possible washed away by rain, but once lichen and algae gets hold my experience is no amount of water alone will shift it, the spores need killing off.
I plucked this off the net because it has a couple of pics. The first pic illustrates my "hard to reach" panels. The second pic. is extreme soiling and my second string is nowhere as bad it's but about half that. I'm unconvinced that this type of soiling won't affect production:-

<a href="https://solarsena.com/how-to-remove-algae-and-lichen-from-solar-panels/#:~:text=Cleaning%20Instruction%201%20Pour%20isopropyl%20alcohol%20in%20a,spray%20your%20solar%20panels%20off%20with%20cl" rel="nofollow">https://solarsena.com/how-to-remove-algae-and-lichen-from-solar-panels/#:~:text=Cleaning%20Instruction%201%20Pour%20isopropyl%20alcohol%20in%20a,spray%20your%20solar%20panels%20off%20with%20cl</a></blockquote>

Agreed, that type of growth on a panel is quite rare - our assumption over the years was that the lichen didn't like the heat of the panels during the day. We generally only saw that kind of growth where the panels were shaded for part of the day, or next to the sea, where I think the salt erodes the glass surface more and allows the spores to find a hold.

In terms of cleaning products, I'd say any panels made pre-2010 you can use whatever you like, as most manufacturers were using glass with very little surface treatment.

The old-style 'blingy' panels, (cobalt blue cells and silver frames like in the photos) were more prone to this, usually starting at the bottom where the water gathered in the frames and gradually creeping up the panel.

I had a personal theory that using washing up liquid etc on newer panels made them more likely to grow moss. I'd just use diluted vinegar for preference. - Use a soft cloth, not a brush or a scouring sponge except on the frame edges.

Sanyo/Panasonic changed their glass coating after one of their factories was damaged in the Tsunami and they were much better at resisting lichen as well as having a better draining frame design. However, the coating made the panels look 'patchy' like they were oil stained and it didn't look very nice.

Panels are susceptible to point shading, and the cells may be damaged by spots on the panels if they remain for a long period, so for this sort of growth or constant bird muck you should clean your panels - otherwise, I refer you to my earlier answer

Our panels (Sanyo) get moldy and even get lichen on them if left long. 36 degrees due south

Last April I gave them a clean on a perfect day. They were producing 500 watts more after cleaning. The sun was getting slightly higher but it was only about 20 mins between readings and late morning.

For me, that was conclusive that cleaning was worthwhile. I will try and upload a picture of the mold as they need to be done again

I'd heard too that vinegar was good and also, Industrial Denatured Alcohol (IDA) formerly IMS. Much of these treatments however seem to need getting up close and personal ( not always possible or safe),- with the panels as opposed to using the obvious telescopic water fed brush, hence my original query regarding a detergent I could spray on and leave for an hour or so before rinsing off.

I have a couple of pressure sprays, a knapsack and a 2 gallon bottle type both with lances. I can leave the bottle on the scaffold tower and just use the lance. I can walk alongside one end of the array up the lightweight roof ladder I mentioned. I'm hoping the spray would reach across 5 panels, otherwise it's straddling the ridge, it's a bungalow but still a high roof.

Posted By: sgt_wouldsOne other thing, if you feel 'tingly' when you wash them - STOP!

My friend got a shock he put it down to not being panel support not earthed. I suspect a lot of roof mounted are not earthed as being out of normal reach and not bolted to a roof that could become conductive (e.g. metal roof) are not counted as extraneous conductive part. A metal roof that is bonded would not require the array frame also being bonded if there is good contact between frame and roof, but supplementary bonding might be beneficial.
My panels on the ground are easy to clean and the frame and panels are grounded via an earth rod.

<blockquote><cite>Posted By: revor</cite><blockquote><cite>Posted By: sgt_woulds</cite>One other thing, if you feel 'tingly' when you wash them - STOP!
</blockquote>
My friend got a shock he put it down to not being panel support not earthed. I suspect a lot of roof mounted are not earthed as being out of normal reach and not bolted to a roof that could become conductive (e.g. metal roof) are not counted as extraneous conductive part. A metal roof that is bonded would not require the array frame also being bonded if there is good contact between frame and roof, but supplementary bonding might be beneficial.
My panels on the
ground are easy to clean and the frame and panels are grounded via an earth rod.</blockquote>

When I first started installing solar the gospel was not to bond the panels unless they could be touched from the ground or from inside the building via a skylight.

Part of this was to keep transient DC outside of the EPZ to avoid nuisance tripping. Also, there had been a study in Germany (?) in the early noughties which concluded that bonding the panels increased the likelihood of lightning strikes.

I don't know the truth of that - we only ever had 2 arrays that were damaged by lightning. One was a direct strike that killed the panels and the inverters, and the other was affected by an indirect strike nearby which tripped the board. The bonded one had the direct strike strike, make of that what you will.

With the introduction of micro-inverters which required AC up to the roof, the EPZ was extended outside and so the frames had to be bonded. This was around the same time as the MCS was introduced and for some reason, it became common for all arrays to be bonded. No one from MCS was interested in what the 3 original solar installation companies' experience was.

I still think it is a bad idea as the incidence of nuisance tripping increased with every bonded array installed. I'm hoping that has been solved in my absence.

Generally, a metal roof should not have an earth unless it has been assessed and approved by an LPS specialist. I don't know what that assessment involves, however.

All Micro inverter systems should have the frame earthed. I was never a huge fan of micro inverters unless they were directly accessible. The manufacturer's warranty only covers replacing the unit, not putting up the scaffold and the cost of labour to do it. Certainly, in the early days, the failure rates were too high to justify.

There is no reason not to fit the inverters inside the roof which means the frame doesn't need to be earthed, but this means bringing in lots of DC cables and isolators which increases costs and reduces the efficiency somewhat.

Also, some of the micros run bloody hot and on the occasions where we mounted internally we had to fit a wire cage and affix warning signs about surface temperatures.

The system we had with the lightning strike was microinverters, so bonded. In string inverter systems you have a physical disconnect in the inverter which would interrupt the path unless it was a direct strike, whereas with microinverters you've created an LPS direct to your consumer unit...

Yep, nothing wrong with the concept, but Enphase had a 10% failure rate and the customers were not happy about having to pay for scaffold and labour for replacement, albeit they got the actual replacement unit under warranty.

Just something to factor into lifetime system costs. For the old iron core string inverters we advised our customers to put aside the cost of an inverter and expect to replace within 15 years. When transformerless inverters came in we revised that down to 5-7 years.

For micros we had to advise the additional cost of scaffold and labour at least twice within the lifetime of the system.

Obviously, these were worst-case scenarios, but likelihood of replacing inverters is high - the likelihood of replacing panels except through damage or installation error is very low.

In all my time we only ever replaced 2 panels under performance warranty and that was only because they were slightly down and highlighted by having microinverers - so they can have their uses