I would generalise this a little: The problem with buying a complex item is information inequality. The seller knows more about it than the potential buyer. When inspecting the item, the buyer therefore looks for signs of hidden defects or for evidence that the item is in good condition overall. In this way, cleanliness becomes a proxy for the general condition of the object. Similar mechanisms are at work elsewhere too. Take job applications, for example. A sloppy application is a red flag, even if the applicant’s references are excellent.

OT: a friend of mine recently bought a house. They were surprised to learn at closing that the seller had financed their solar panels and that wasn’t apart of the home purchase.

So they had to negotiate who was going to pay off the solar panel, and this all had to be completed before home sale.

Exactly the same reason you should probably clean the rest of the house when you're selling it.

Theoretically, the price of the house before cleaning should be equal to the value if the house after cleaning, minus whatever the cleaning costs. In reality, you'll get much less for your house without cleaning it, because human emotions.

Actually in some places panels devalue a house because some people consider them an eyesore

A patchwork of eg new paint is a good signal they're covering something up too

Also you don't need anything fancy when washing them, their surface is just glass.

I agree. Doesn't matter what you sell, you will get a higher price if it is cleaned and tidy when presented to the market - because the kinds of people who pay higher prices are looking for clean and tidy things.

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That's really impressive and well built solar system, for it to last thta long. It shows how durable quality panels are. I have a 6 month old ~3.7kW system (6 panels in total), planning on installing another 6 very soon. My house is surrounded by trees so it's been a bit of a struggle in the winter.

What the panels' technology / manufacturer?

> The drop off after cleaning could well be because some of the initial boost could come from the water cooling off the panels: solar panels become less efficient at higher temperatures so the drop of after they're all cleaned could be them warming up again after the water has evaporated.

This was almost certainly the case. AIUI, that cooling still makes a large difference.

Makes me wonder if it is worth it having a small esp32 or similar control a sprinkler system for the panels. Keep them cool during high ambient temps by spraying a litre of water on each panel every 30m.

An idea I had a while back (when I was considering panels) was running my mains water through the back of each panel (before the mains got to the hose) in a sort of radiator pattern.

People make the same claims about cleaning their air conditioner. Yeah it’s blowing cooler inside: you’ve just chilled the condenser and left some residual evaporative cooling.

Yeah, came here to say this. There are a lot of things that will affect efficiency and how clean your panels are is only one of them. How dirty your panels get is also highly sensitive to both location and installation. All of this gets horribly confounded by string inverters (especially if they're clipping the peak, which they often are).

I easily see 5% increases in efficiency after cleaning (with micro inverters), but also see ~3% increase in output on a hot day from just spraying the panels with water to cool them off, which goes away in 15-20min.

If you're going to clean once a year, do it after the "last" rain of the year before summer peak solar output. If you live somewhere rural with summer thunderstorms, with high angle panels, and without a lot of pollen (or dust storms), you may not need to clean them much at all.

If you're going to climb on a roof, do it safely! A 10' fall can kill or maim you.

Eh, compared to what though? Compared to speaking to licensed electrician, definitely. But compared to doing a raw web search and trying to put the information in the right order for the application you’re currently working on, I think it’s at worst a wash.

The system in question does not have a transformer so a grounding rod is not required as it is not a separately derived system.

The equipment grounds should be tied together to prevent any difference in potential though.

Funnily enough, there was a thread a while ago here where people were fixated on the idea that you had to wash solar panels, and thus proponents are underestimating maintenance costs. I had wondered where that idea had come from, as I thought it was fairly well understood that even quite dirty panels once much of an efficiency drop…

I asked a friend with a window cleaning business and they charge about $10 a panel. At that rate I'm tempted to try it but I'm not sure if it's even worth that.

Its an easy value add for a pressure-wash / soft-wash business

Are these older systems? The panels are cheap now but they used to be quite expensive, which would’ve changed the economics a bit.

A lot of these designs are an artifact of a time when the panels themselves would have been far and away the most expensive component in the system. The place I'm in originally had 10 125W panels that probably cost north of $8k when installed, and they were mounted on a rotating, tilt-adjustable post. Also the entire house was wired to run off DC as much as possible, with three different wiring schemes throughout and rare & expensive appliances. It reminded me strongly of RV power schemes, and maintenance was probably a huge headache.

I replaced it all with 9kW worth of panels (about $3k, shipped) on 4 simple 60° ground mounts, normal wiring and appliances. Angle is optimized for early spring / late fall - there's effectively no sun in winter, and the rest of the year I have excess power even with fixed, sub-optimal angles.

The diminished cost of panels alone over the past 20 years has transformed "off-grid" from a pastime for wealthy hippies into a reasonable solution for remote living.

It does no longer pay to be clever with solar panels; they're too cheap. Angle not quite right? Add a couple extra. Summer vs. Winter? Add a couple extra.

It may also be worth spraying them with water when they are too hot. As they get hotter the efficiency drops a little. It's only a few percent, and usually it happens at the time you need the efficiency the least because it's sunny, but if you can combine cleaning and cooling and get a few extra w out if it, why not.

> You can still wash the panels but I suspect it won't be necessary if there isn't too much dust accumulation? Isn't rain supposed to wash away the dust? What am I missing here?

In places where solar panels are used the most, like the Southwest US, there isn't much rain to speak of. Panels get a lot of dust accumulation from Feb-Nov typically, just when they're needed the most.

The author had ground level access. In that case, A microfiber covered sponge on a pole with a 5 gallon bucket of soapy water is probably the easiest thing. Especially if you do it annually, the dust wouldn’t be completely caked on requiring scraping and excessive elbow grease.

I do a similar procedure for my house windows. I use a squeegee on those but probably not necessary for panels.

Some places don't get much rain (and these are the places that tend to get even more dust - e.g. deserts).

Look at any outside surface in a rainy climate.

The dirt may not be piling up, but it's far from clean.

I am curious how much of a difference it makes.

I have 4kw of panels and 15kwh of storage. I only get down to about 80% most nights (I heat with wood and propane).

But my system will charge completely in about 2 hours of full sun. If it took 3 hours it wouldn't make a difference. So I generally don't clean them (well, the rain and snow do that work).

I do remove snow, because that has a very noticeable impact.

Is the difference that you are pushing back to a grid (I am not), so the power all gets used regardless of you being the one to use it?

> I clean my solar panels, 7kwh, every monmth or two.

This honestly sounds like a non-trivial strike against installing solar panels, especially if they’re installed on tall roofs.

I definitely wouldn’t want to have to clean them that often, or pay to have them cleaned that often.

If you’re spending only 10 minutes on them, then maybe you don’t have that many, and they’re not up high.

If export energy is 10% of your total output, and it jumped 35% then you got a ~3.5 - 4% gain in total output, not far from the author's experience.

Of course, you can math that any way you like to make the numbers bigger or smaller, depending on whether you're representing the buyer or the seller of the system.

> solar does not need artificial subsidies, and can pay for itself

That's true now, but wasn't true when those panels were installed. There were subsidies available to get the market kick-started.

They consist of an extra payment for every kwh you generate on top of the payment for delivering the power to the grid. The amount varied depending on when you signed up to the scheme, but at certain points it was as much as 50p/kwh (+ inflation, so more than that now).

Solar gets lots of subsidies in the UK. Homeowners who installed it 10 years ago get a ridiculous payment for generating excess, even if the grid doesn't use it.

> I am not sure what author is trying to hint. But solar does not need artificial subsidies, and can pay for itself!

At grid scale, yes.

For the house, not so much.

The feed in tariff was at one point 41 pence per kwhr which is probably £10 a day in summer.

but at 12p a kwhr it drops to £3.6

However, 10kwp of panels is ~£1800, but the micro inverters needed to drive them are >£3k (as its outside you can probably use two larger inverters.) Then mounting system is probably a good 800 quid too.

> I am not sure what author is trying to hint. But solar does not need artificial subsidies, and can pay for itself!

I think he is describing a subside that some governments gave years ago: you installed for example 5K, and get paid for whatever they generate at 300% market price, for a long time (25 years?). Today you find out that upgrading panels would made perfect financial sense, if you were selling at 100% and are going to sell electricity at 100% (you will be doubling the output for a tenth of the inversion 15 years ago), but as the subsides of 300% no longer exist, you will be actually losing money by upgrading.

when it first started it was basically 450GBP/MWh (In 2010 GBP). and that was paid for every kWh generated (whether exported or not). And the PPA lasts for 20-25 years.

As another anecdata-point, when we did this for a now already fairly old roof installation of about 15kWp, it was more than “just about worthwhile”, since the panel area is contiguous: cost seemed to scale up less quickly than the wattage/theoretical maximum output, and the cleaning guy (solo company, IIRC) even told us it wasn’t necessary to do this every year (we hadn’t had it done even once the first decade+ of the installation’s lifetime).

So, at least in our case, definitely worth it.

Now I want solar panels merely so I have a reason to not have to clean my roof

In many countries it is actually illegal to use tall ladder etc. owners would have to erect a scaffolding to make access for work people to clean it safely.

20% seems high enough that if it was really _that_ effective, the companies making the panels would be tripping over themselves to buy this coating to apply it to the panels right in their factories. (And charge a premium for it.)

My solar panels are leaning in stacks in my yard, as our house is currently being removed with a new roof, etc. I'm planning on cleaning them while they're "loose", but I wonder if it also makes sense to apply a(n automotve-style) ceramic coating to them.

On Mars, the wind storms and dust devils are so fearsome that some of the robots there could rely on dust being blown clean off all the solar panels on a regular basis. Spirit and Opportunity benefited the most from these effects.

InSight was stationary and not so fortunate. But, the ground team got quite creative with its robotic arm to try and manipulate the sand that had accumulated on the solar panel, and gained a little bit more power.

https://en.wikipedia.org/wiki/InSight#Surface_operations

https://en.wikipedia.org/wiki/Martian_regolith

Yeah, the one tree branch that hits my array until 10a prevents it from charging at all even though it only obscures 1/4 of one panel in the 10-panel set.

But I like the branch and have plenty of full sun, so I haven't cut it.

Rain usually just cakes the dirt. Unless it's a downpour, it doesn't move with enough velocity or volume to remove the dirt. So the dirt just gets damp, maybe moves a few cm with gravity, then dries as a mud cake.

my experience is that unless you get some really torrential rain, it's not enough to clean the sand/dust/grime

Without trying to answer your "I believe the setup circumvents the fee" implied question, which you should find an actual answer to...

See reddit's r/solarDIY. There are a lot of setups like this, either for full house power or partial. I have a relatively big server in my basemeent that I power this way, for example, with a battery system that acts as both UPS and a "charge the battery from solar and run off inverter when possible". Something like an off-grid EG4 is popular among the reddit crew as a way to manage this; the device tries to run locally but accepts a shore power feed from the grid so if you need it you draw grid power. No grid tie involved.

> I believe that setup circumvents the fee since the only thing using energy is the battery, as though it were an off grid setup.

Entirely based on your jurisdiction, but that kind of zero-export requires the same permits and connection fee as a full net metering in mine. You can't end run the process by just being sneaky, electrical work needs permits and they'll either make you take it out (!!) or bring it into full compliance.

As an example, my utility is one of those ones that requires a separate manual disconnect, even though the vast majority of all-in-one inverters have a built in disconnect. It doesn't make any sense and it's at least two grand in permit + equipment, but that's the rule.

Having just put a battery into my existing system. For the power company I use the answer is no to your question. Many power companies charge that fee depending on the size of panels. Now if you do not hook to their system they do not care (off grid). But if you plug into it then they care and charge for the privilege.

Now you could pull particular items out of their grid. Just need the wires and panels to make sense. They would not care about that either. It is only if the panels are plugged into the grid they start to care. Downside is sometimes waiting for that battery to have enough juice to work. It goes faster than you think and some battery systems are decently expensive just by themselves, on top of the panels/inverters/controllers/install.

Ask a lawyer about the law.

That depends on what options are available to you for residential solar installation. Installing solar panels + micro-inverters is very cheap in cost of parts, but possibly expensive in labor. It will easily pay off your grid interconnect fee every month once you hit break-even from operating for a few years. Financing vs cash, cost of installation, tax credits, are the biggest factors in how quickly it becomes profitable. If you are extremely handy you can DIY most of it as a homeowner (depending on local/state laws) which cuts costs significantly.

You can do a whole home backup + solar panel input (aka ESS+PV) but it's much more expensive in parts and installation and is more complex. If you don't add grid interconnect it can be much cheaper as you can use "off-grid" parts (grid interconnect requires more expensive parts certified for certain standards).

But some jurisdictions may not allow cheaper "off-grid" parts/systems to be connected to the grid, even downstream. So an alternative would be to install a transfer switch, where you manually switch from the grid to off-grid. But then you literally have to choose whether you use the ESS+PV or the grid at any given time and manually throw a switch (you can't use grid to charge your system). And yes you can set this up for only specific circuits/appliances. Again it depends on your state and local laws.

AI can do most of the research for you. Keep a Git repo and add documentation and details to it over time, and you can build up an analysis of what works for you based on all available options, price comparisons, energy requirements, and local/state laws. It is unfortunately quite complicated because there are many options but not all will apply to your location. And before you do any of this, do a solar site survey to see if you can even reliably generate enough power from the sun!