Cleaning and Maintaining Solar Panels in New Zealand

For most New Zealand homes, you do not need to clean your solar panels at all. Our rain does the job. Independent research (including work cited by EECA) puts the output loss from a normally dirty panel at roughly 2 to 5 percent on a tilted roof that gets regular rainfall, and a good downpour washes most of that straight off. The exceptions worth caring about are salt-mist near the coast, bird mess, lichen and moss, and pollen or dust on near-flat panels. If none of those apply to you, the honest answer is: leave them alone, check the numbers in your monitoring app once a month, and save your weekend.

That probably sounds too easy, and it goes against what a few "annual solar cleaning" services would like you to believe. So let's go through exactly when cleaning genuinely helps in NZ conditions, when it is a waste of money (or worse, a warranty risk), and how to do it safely if you do need to.

Why rain does most of the work here

New Zealand is, by world standards, a wet and windy place. NIWA's long-term climate records show most populated parts of the country get rain spread fairly evenly through the year, rather than the long bone-dry spells you see in inland Australia or the American southwest where regular cleaning genuinely matters.

Panels are also mounted at a tilt. On a typical pitched Kiwi roof, that angle (often 15 to 30 degrees) lets rain sheet down the glass and carry loose dust and pollen with it. The combination of frequent rain and a tilted, smooth, self-shedding surface is why the real-world soiling loss on most NZ homes sits at the low end.

Put simply: if your panels are on a pitched roof, in a town that gets normal rainfall, and nothing is physically growing on or dropping onto them, cleaning will buy you almost nothing. You would spend more on the cleaning than you would ever recover in extra generation.

The exceptions where it actually matters

Here is where soiling stops being trivial and starts costing you real kilowatt hours:

• Near-flat panels (under about 10 degrees tilt). Common on flat-roofed modern builds and some commercial-style installs. Rain pools and dries rather than running off, leaving a dirt tide-line. These genuinely benefit from an occasional clean.
• Coastal salt-mist zones. Anywhere within a few kilometres of open coast, especially exposed west and south coasts, builds a fine salt film that rain alone does not always shift.
• Heavy bird traffic. A roof under a flight path, near pines, or next to power lines birds love to perch on. Droppings are opaque and sit in one spot, which is worse than an even layer of dust.
• Lichen and moss. The classic damp, shaded-corner problem, common on the West Coast, in Taranaki, and on south-facing roof edges nationwide.
• Pollen and agricultural dust. Rural Canterbury, the Waikato and Hawke's Bay during dry, dusty harvest spells, or heavy spring pollen near pine and wattle.

The salt-mist question, properly answered

This is the one coastal homeowners get wrong in both directions. Some panic and book monthly cleans they do not need; others ignore it until output quietly slips.

Salt-mist matters for two separate reasons, and only one of them is about cleaning.

The generation issue is mild. A thin salt haze on the glass scatters a little light. Regular rain usually keeps this in check, and the loss is small. A wipe-down once or twice a year in genuinely exposed coastal spots (think a beachfront at Raglan, Paekākāriki, or the open Otago coast) is plenty.

The corrosion issue is the one that actually costs money, and it has nothing to do with the glass. Salt attacks the aluminium frames, the mounting feet, the earthing connections and the cable glands. This is why your installer should have specified the right gear from day one. When you are getting quotes, coastal homeowners should ask point blank whether the racking and fixings are rated for a marine or severe marine environment, because that single line item matters far more than any cleaning schedule. If you are still at the quoting stage, it is worth lining up a few installers who actually understand coastal builds; we can sort that for you over here: https://nzsolaris.co.nz/get-solar-quotes/.

The unique bit nobody tells you: never use tank water plus a coastal roof as your excuse

Here is something the cleaning ads skip. If you wash salt off panels with hard or mineral-heavy water and let it dry in the sun, you can end up with a mineral film that is worse than the salt you removed. On a hot Northland or Hawke's Bay afternoon, droplets dry into spots before you have finished the array. If you clean coastal panels, do it early morning or on an overcast day, rinse with the cleanest water you have (deionised or rainwater beats hard bore water), and squeegee or let it sheet off rather than spot-drying. This single detail is the difference between a clean that helps and one that leaves a permanent haze.

Lichen, moss and the green-corner problem

Lichen is the one form of soiling worth taking seriously, because unlike dust it does not wash off and it does not stay still. It spreads. A spot of lichen on the lower edge of a panel in a damp, shaded West Coast valley or a south-facing Taranaki roof will slowly creep across the glass and, more importantly, can lodge under the frame edge and trap moisture against the seals.

The generation hit from lichen is real because it is opaque and it clusters, which can drag down a whole string of cells through the same shading effect a fallen leaf or a chimney shadow causes. If your output has slipped and you have green growth on the array, that is your culprit. We walk through how to diagnose a drop in output step by step over here: https://nzsolaris.co.nz/solar-output-dropped-troubleshooting-nz/.

Treat lichen gently and early. Do not scrape it with anything metal or abrasive; you will scratch the anti-reflective coating on the glass and that damage is permanent. A soft brush, lukewarm water and patience is the method. Stubborn growth may need a second pass a week later once the water has loosened it.

How to clean panels safely (if you actually need to)

The single most important point: nobody should be getting on a roof to clean solar panels unless they are set up to work at height safely. WorkSafe NZ data consistently shows falls from height are among the most common causes of serious harm in the building and trades sector, and a wet, sloped roof with glass on it is exactly the scenario that hurts people. A few percent of extra generation is not worth a trip to Christchurch Hospital.

Wherever you can, clean from the ground.

The ground-level method (preferred)

• Pick the right day. Early morning, cool, overcast, with the panels switched cool to the touch. Never on a hot panel; cold water on hot glass risks thermal stress.
• Use a soft brush on an extension pole. Many garden and window-cleaning poles reach a single-storey roof. A soft-bristle or microfibre head only.
• Plain water first. Most NZ soiling comes off with water alone. Rainwater or deionised water leaves no spots.
• For greasy film or bird mess, a few drops of mild dish soap in lukewarm water is fine. Rinse thoroughly.
• Never use: a water blaster, abrasive pads, metal scrapers, automotive solvents, or anything with grit. A water blaster can force water past the seals and void your panel warranty in one go.

If it truly needs roof access

Hire a professional with height-safety gear and public liability insurance, and confirm they will not walk on the panels themselves (standing on glass cracks cells you cannot see). Expect a typical residential clean to run somewhere in the region of $150 to $350 depending on roof access and array size, based on what NZ window and exterior cleaning operators commonly quote. Get the price before they start, and ask whether they use deionised water.

The warranty trap hiding in your cleaning

This is the part installers do not volunteer. Most panel manufacturers' warranties contain a clause excluding damage from improper cleaning. Use a water blaster, an abrasive, or harsh chemicals, scratch the coating or breach a seal, and you have potentially handed the manufacturer a clean reason to decline a future claim.

It gets sharper than that. Some installer workmanship warranties require that any work on the array, including cleaning, is done by an accredited person, or they reserve the right to inspect. Before you climb up or hire a random cleaner, it is genuinely worth reading your own paperwork. If a claim does come up later, knowing how the process works saves a lot of grief, and we have laid that out plainly over here: https://nzsolaris.co.nz/solar-warranty-claim-nz/.

What "maintenance" actually means for a NZ solar system

Cleaning is the part people fixate on, but it is the smallest part of keeping a system healthy. A grid-tied solar system has no moving parts; the real maintenance is mostly watching the numbers and a handful of physical checks.

The monthly habit that matters more than any clean

Open your monitoring app once a month and glance at production. You are not looking for a precise figure; you are looking for a sudden, unexplained drop, or one string consistently underperforming its twin. That tells you something real has changed: a tripped breaker, a failing optimiser, shading from a tree that has grown, or yes, lichen. If you are not sure how to read your app or what normal looks like through the seasons, we go through the main brands and what to watch for over here: https://nzsolaris.co.nz/monitor-solar-production-nz/.

The annual visual check (from the ground)

• Look up at the array with binoculars or your phone zoom: any cracked glass, lifted panels, green growth, or a build-up of leaves in the gaps?
• Check around the inverter (usually in the garage or on an exterior wall): any warning lights, error codes, or a fault buzzer you have been ignoring?
• Look for new shade. Trees grow. A fast-growing pittosporum or a neighbour's poplar that was fine at install can be clipping your winter sun three years on.
• Listen and feel. Inverters get warm and may have a quiet fan; a loud rattle or a hot smell is not normal.

The professional check every few years

Every three to five years, it is reasonable to have an accredited installer give the system a proper once-over: torque-checking electrical connections, inspecting the DC isolators (a known NZ pain point, as some older isolator models have been the subject of safety concerns flagged by the Electricity Authority and WorkSafe), checking the mounting integrity, and confirming the earthing is sound, especially in coastal and high-wind areas. The full picture of looking after a system over its life is something we cover in our ownership and aftercare guide: https://nzsolaris.co.nz/your-solar-system-ownership-guide/.

Does cleaning ever pay for itself? A worked example

Let's put real numbers on it, because this is where the "annual clean" sales pitch falls over for most homes.

Take a 5kW system on a pitched roof in Hamilton. EECA's guidance and typical NZ generation figures put a well-sited 5kW system at roughly 5,500 to 6,500 kWh per year. Say 6,000 kWh. Assume a blended value of 20 cents per kWh (a mix of self-consumed power offset against a retail rate and exported power at a buy-back rate; if you want to understand how those two numbers combine, that is worth getting your head around separately).

That system produces around $1,200 of value a year. A realistic soiling loss on a tilted, rain-washed Waikato roof is maybe 3 percent, so about $36 a year of lost value sitting on the glass. A professional clean at $200, even if it recovered every cent of that loss (it will not, because rain already recovers most of it), would take the better part of six years to pay back, and the dirt simply returns. The maths does not work.

Now change the scenario. Near-flat panels on a coastal Mount Maunganui roof under a pōhutukawa, with regular bird traffic. Here the real loss might be 8 to 12 percent because dirt pools, salt films build, and droppings sit in fixed spots. On the same system that is $100 to $145 a year, and now a clean once or twice a year genuinely pencils out and protects your output. Same dollar figure for the clean; completely different answer. That is the whole point: it depends entirely on your roof, not on a generic schedule. If you want to model your own system's value before and after, our calculator lets you plug in your real numbers: https://nzsolaris.co.nz/solar-roi-calculator/.

Where this does not apply (the honest limits)

• If your output drop is sharp and sudden, cleaning is not your problem. Soiling builds slowly. A sudden 30 percent drop is electrical, not dirt. Stop reaching for the brush and start troubleshooting.
• If you have a steep two-storey roof, do not DIY it. There is no extra few percent worth a fall. Hire someone or wait for the rain.
• If your panels are old and the warranty matters to you, a wrong cleaning method can cost you the very protection you are trying to preserve.
• Snow, for the few Central Otago and high-country homes it affects, generally slides off tilted panels on its own as they warm. Knocking it off with anything hard risks scratching the glass; let it go.

Frequently asked questions

How often should I clean my solar panels in New Zealand?

For most pitched roofs in towns with normal rainfall, never on a fixed schedule; let the rain do it and watch your app for changes. If you have near-flat panels, heavy coastal salt-mist, regular bird mess, or lichen, once or twice a year is sensible. The right frequency is set by your roof and surroundings, not by a calendar.

Will dirty panels really reduce my power output?

On a tilted, rain-washed roof the typical loss is only around 2 to 5 percent, which rain largely keeps in check. Localised, opaque soiling like bird droppings or lichen hits harder because it shades whole sections of a panel string, which can drag down more than the dirty area alone.

Can I use a water blaster to clean my solar panels?

No. High-pressure water can force its way past the panel seals and the cable entries, causing internal moisture damage, and it is one of the fastest ways to void your manufacturer's warranty. Use a soft brush or microfibre head with plain water at normal pressure.

Is it safe to clean panels myself?

Only from the ground, with an extension pole, on a single-storey roof. WorkSafe NZ records show falls from height cause a large share of serious harm in trades, and a wet sloped roof with glass underfoot is a genuine hazard. If it needs roof access, hire someone with proper height-safety gear and insurance.

What should I use to clean salt off coastal panels?

Plain clean water and a soft brush, done early morning or on an overcast day so it does not spot-dry. Rainwater or deionised water is better than hard bore water, which can leave a mineral film. More importantly, make sure your frames and fixings were specified for a coastal environment in the first place, because corrosion of the hardware is the real long-term cost near the sea.

How do I get rid of lichen or moss on my panels?

Gently, with a soft brush, lukewarm water and patience, never a scraper or abrasive that would scratch the anti-reflective coating. Tackle it early before it spreads under the frame edges where it can trap moisture against the seals. Stubborn growth may need a second go a week later.

Does cleaning my panels pay for itself?

On a normal pitched, rain-washed roof, almost never; you would spend more on the clean than you recover in extra generation. It only pencils out where soiling is heavy and persistent, such as near-flat panels, exposed coastal sites, or roofs with constant bird traffic. Run your own numbers rather than trusting a generic annual-clean pitch.

Could cleaning void my warranty?

Yes, if you do it wrong. Most panel warranties exclude damage from improper cleaning, and some installer workmanship warranties limit who can work on the array. Read your own paperwork before you climb up or hire a cleaner, and avoid water blasters, abrasives and harsh chemicals.

My output has dropped suddenly. Is dirt the cause?

Almost certainly not. Soiling builds gradually over weeks and months, so a sudden drop points to something electrical: a tripped breaker, an inverter fault, a failing optimiser, or new shading. Check your monitoring and the inverter before you reach for a brush.

The bottom line

For the average New Zealand home, solar panel maintenance is mostly about paying attention, not scrubbing. Our rain handles the cleaning on most tilted roofs, and the few percent you might claw back rarely justifies the cost or the risk of getting up there. Save your energy for the things that actually move the needle: a monthly glance at your production figures, an annual look up at the array, and a proper professional check every few years.

Where cleaning does matter (flat panels, heavy salt-mist, lichen, bird traffic), do it gently, from the ground where you can, with clean water and a soft brush, and never anything that could scratch the glass or breach a seal. If you are coastal, the bigger win was made at install time with the right corrosion-rated hardware.

Where to go from here

If you take one thing away, let it be this: watch the numbers, not the dirt. A monthly look at your monitoring app will catch a real problem long before any amount of scrubbing would, and it costs you nothing but a minute on the couch.

If your numbers have slipped and you are not sure why, start with the troubleshooting walk-through over here: https://nzsolaris.co.nz/solar-output-dropped-troubleshooting-nz/. And if you want the full picture of looking after a system across its whole life, our ownership and aftercare guide pulls it all together: https://nzsolaris.co.nz/your-solar-system-ownership-guide/.