Solar panels are the only part of a building that earns their keep every hour they sit in the sun, yet they’re also the only major asset we routinely leave coated in dust, pollen, and mineral film. The performance penalty is not theoretical. On real roofs, I’ve logged 5 to 12 percent production loss from soil and bird droppings, and I’ve seen systems under trees or near agriculture lose 15 percent or more after a windy season. Those numbers matter when power prices climb and ROI windows are already tight. Clean glass is efficient glass.
The question property owners ask next is predictable: can I just blast them with a power washing service and call it a day? The short answer is no, not with conventional high-pressure methods. The longer answer is this guide. I’ll explain how professional cleaning boosts yield, why low-pressure techniques protect your modules and warranties, what a proper workflow looks like, and when to fold panel cleaning into a broader maintenance plan that might include commercial pressure washing or Commercial window cleaning across the site.
What dirt does to photovoltaic performance
Photovoltaic cells need photons, and soil blocks them. A thin veil of dust reduces light transmission and increases reflectance, so fewer photons reach the cell. Bird droppings create opaque spots that act like tiny shades. In string-connected modules, shaded cells can drag a substring, and in uncontrolled conditions they can heat up relative to neighbors. Modern panels have bypass diodes that limit hot spots, but they don’t overcome severe soiling.
The effect varies. Desert installations see gradual loss as dust accumulates. Coastal arrays pick up salt crystals that attract moisture and leave mineral film as they dry. Urban roofs collect soot and rubberized particulate from traffic, which bonds to glass and resists a basic rinse. In agricultural zones, pollen spikes during flowering weeks and sticks to dew, forming a crust. I have watched a 500 kW system next to a grain facility gain 9 percent after a careful wash in spring, then another 4 percent after a second pass that removed silica film.
Pay attention to microclimate. If your roof slopes toward a tree line, sap mist and leaf litter will hit the leading edge. Panels tilted at low angles, typical on flat commercial roofs, shed water poorly, so contaminants accumulate in the bottom two inches where the frame meets the glass. On frameless bifacial modules, grime builds along clamps and cable routes.
Why pressure matters more than most think
The phrase “pressure washing service” can give installers heartburn because many people picture a contractor with a 3000 PSI wand peeling paint. Glass looks sturdy, but a module is a laminate of tempered glass, EVA, cells, and backsheet sealed with edge adhesives. That edge seal resists weather, not a pinpoint jet. High pressure risks driving water underneath the frame, past the backsheet, and into junction boxes. It also can lift labels or scuff the anti-reflective (AR) coating that improves light transmission.
I have tested panels after cleaning with a standard power washing service set near 2000 PSI at close range. The output looked fine the next day. Three months later, insulation resistance readings dropped on two modules, and corrosion showed near the connectors. Water intrusion rarely announces itself immediately. It creeps.
Manufacturers know this, which is why many warranties specify “low pressure, soft wash” techniques. You will see language like “do not use abrasive materials, high-pressure jets, or steam.” They are not trying to sell you proprietary tools. They are telling you the laminate and seals have limits. Respect those limits and your array will run for a quarter century. Ignore them and you may turn a simple cleaning into a slow leak that shows up as PID-like behavior or ground faults later.
Soft washing done right
A responsible pressure washing company brings more than a pump and a ladder. We bring water chemistry, low-pressure control, and tools designed for glass. When I scope a site, I start with water.
Municipal water can leave mineral spots that bake in the sun and etch the AR coating over time. The safest route is purified water, either deionized (DI) or reverse osmosis (RO) output. Portable DI tanks are common on service trucks, especially those that also handle Commercial window cleaning, because a spot-free rinse matters on glass. With DI water, you can often remove most grime with no soap at all, which eliminates residue that attracts dust.
For applied pressure, the target is gentle. Think garden-hose levels at the brush, often in the 60 to 150 PSI range at the tool, not at the pump. The water should sheet across the surface, not cut into it. Pair that with a soft-bristled brush or a rotating nylon head designed for solar glass, and you have the backbone of a safe process. Long carbon-fiber poles keep technicians off brittle tiles and away from array edges where a misstep can damage conduit.
Chemistry comes next. Avoid hydrofluoric acid, abrasive powders, and household detergents with brighteners. When we encounter baked-on droppings or oily soot, we use a neutral to mildly alkaline surfactant approved for PV glass, diluted per spec, usually in the 1 to 3 percent range. Dwell time matters more than elbow grease. Let the solution loosen the bond, agitate gently, rinse thoroughly with DI water, and inspect from several angles for rainbowing that signals residue.
In winter, add a cold-weather note. Hot water can shock cold glass. Even on commercial pressure washing rigs with burners, keep solar cleaning water near ambient temperature. I’ve seen a panel crack from thermal stress when a well-meaning staffer used a hot tap on a frosty morning. If the glass squeals under the brush, it is too cold.
Safety and access on real roofs
Most arrays sit on roofs that were not designed for foot traffic. That fact governs everything from scheduling to equipment choice. On a standing seam metal roof, clamp to seams with approved anchors and use walk pads. On tile, keep weight on the lower thirds of tiles and use crawl boards. On ballasted commercial arrays, do not shift blocks to https://phxpwrgxc319.overblog.fr/2026/01/soft-washing-vs.power-washing-service-what-s-the-difference.html create shortcuts. Those blocks are there to resist uplift, and moving them changes the wind map of the array.
Fall protection sounds obvious until it isn’t. Harnesses, rated anchors, and a second set of eyes are part of basic practice. If a site refuses anchor installation, use weighted carts or parapet clamps where allowed by code. Indoors, tie off in warehouse skylight zones. If you are mixing services with a broader commercial pressure washing crew, be sure the team shifts from concrete to glass with a mindset reset. The same operator who safely scours a loading dock with a turbo nozzle must park that tool before stepping near modules.
Electrical awareness runs through the job. Panels produce power whenever they see light, so assume live DC. Do not open junction boxes. Do not spray directly at connectors, optimizers, microinverters, or conduit penetrations. Pre-plan your wash path so water does not pool over roof penetrations. If monitoring shows a rapid shutdown stage inactive, leave it alone. Cleaning is not commissioning.
The math that makes cleaning worthwhile
The business case for cleaning depends on local conditions and electricity value. In simple terms, cleaning pays when the gain in energy multiplied by the price per kilowatt-hour, over the interval until the next cleaning, exceeds the cost of the service. That is the clean, financial framing. The real world adds a few wrinkles.
A 100 kW commercial roof in a dusty corridor, with power worth 18 cents per kWh, might start the dry season with a 5 percent loss and reach 10 percent by midsummer. That is 5 to 10 kW of lost output during peak sun, roughly 25 to 50 kWh per day over five prime months, or 3,750 to 7,500 kWh. At 18 cents, that is 675 to 1,350 dollars. If a professional cleaning costs 700 to 900 dollars and restores most of the loss, the math works. On the same site, a second pass timed before the highest-billing months might be smart.
Residential cases vary more. In a coastal town with frequent rain and moderate pollen, the roof may self-clean enough that an annual visit is sufficient. In an arid place with construction dust or wildfire ash, semiannual cleaning pays. The case improves when you combine services. If we are already on site for Commercial window cleaning across a retail facade, adding the solar array may cost less than a standalone visit because travel and setup are sunk.
The hidden or avoided costs matter too. Clean panels run cooler. Cooler modules enjoy slightly higher voltage and lower operating stress. That effect is modest, but over years, lower operating temperature helps preserve output. Clean arrays also allow visual inspection of microcracks, delamination at edges, or sealant failures around mounts. I have found loose bonding jumpers and nibbled cable insulation while moving along rows with a brush in hand. Catching those early prevents bigger failures.
How a quality service call unfolds
People expect speed from a pressure washing service. Speed and care can coexist with the right flow. When we roll onto a site, the first ten minutes are quiet. We check interconnection points, look for damaged modules, and review anchor plans. Then we stage DI tanks, poles, hoses, and safety lines in a way that does not block drains or egress.
Rinsing comes first to remove loose dust. We work top to bottom so dirty water does not streak cleaned panels. On large roofs, we divide the array into lanes sized to match tank capacity, which avoids running out of DI water mid-lane and leaving half a string spotted. If surfactant is needed, we foam lightly, not soak, and we respect dwell times printed on the label rather than guessing. Agitation is gentle, one pass north-south, one pass east-west, then a thorough rinse. Final inspection happens with and against the sun to spot missed areas.
Documentation is part of the craft. Before and after photos from the same vantage point help owners see the difference. If you have monitoring, capture daily yield a week before and a week after under similar weather to measure the change. If you do not have monitoring, consider a simple meter reading at the inverter at the start and end of the day as context. You won’t isolate the cleaning effect perfectly, but you will build a history that informs future schedule choices.
When not to clean
There are days when the smart move is to delay. If wind gusts exceed safe limits for roof work, reschedule. If the roof is icy, wait for sun. If wildfire smoke is active and ash is still falling, hold off until the air clears or you will waste water and time. If the array has known electrical faults, call an electrician or the installer before bringing water onto the roof.
Do not clean modules with visible cracks, shattered glass, or missing backsheet. Tapping or brushing those risks injury and may spread damage. Flag them and escalate. Avoid cleaning during blazing hot hours. Cool glass cleans better, and you reduce thermal shock risk. Early morning or late afternoon often works best, and on eastern-facing arrays, afternoon avoids glare and gives better sightlines.
The right tools for glass, not for paint
Contractors love gear, and the right gear matters here. Carbon-fiber poles extend while keeping weight low, which keeps technicians off fragile areas and reduces fatigue. Brushes with flagged bristles hold more water and glide without scratching. A rotating brush head powered by water pressure can speed large arrays but must be set gentle enough to avoid pushing water into frames. For ground mounts, a low-pressure rinse bar helps sheet water evenly.
For chemistry, carry MSDS sheets and manufacturer approvals. If a product claims to “restore” glass or strip mineral film, read the fine print. Many glass restorers use acids that do not belong on AR coatings. If you must address hard water etching, test in a corner of a sacrificial panel or a sample pane first.
I keep a handheld TDS meter to verify DI water quality before we start. Anything under 10 ppm typically dries spot-free. Above that, we adjust resin or swap tanks. On a large commercial run, we log TDS every hour. It takes seconds and prevents disappointment when the sun dries panels faster than a technician can chase spots.
Integrating solar cleaning into site maintenance
Solar rarely exists in isolation. On retail centers, hotel properties, and campuses, a pressure washing company might already handle sidewalks, facades, and service yards. Folding solar into a twice-yearly maintenance window saves setup and keeps safety learning fresh. If your contractor offers Commercial window cleaning, they likely own the poles, pads, and DI rigs that transition easily to solar. The key is segmentation. The same truck can carry both toolkits, but the SOPs diverge: hot washers and harsh degreasers stay on the ground; soft wash and pure water go on the roof.
Consider seasonality. In pollen belts, schedule a post-bloom wash. In farm regions, clean after harvest dust settles. In coastal areas, plan a rinse after storm seasons that bring salt spray. If you’re operating a logistics roof with frequent drone inspections, clean before your annual thermography. Dirt changes emissivity and can obscure hot spots on a thermal camera. A clean array yields clearer diagnostic images, which save money during warranty claims.
Warranties and “approved methods”
Module warranties vary, but they share a theme: be gentle. Many manufacturers specify water pressure limits, brush types, and chemistry restrictions. Read your module data sheet and installation manual before you hire a contractor. If a pressure washing service is cagey about methods or insists that high pressure is “the only way to get it truly clean,” keep looking. Ask for references where they cleaned arrays of similar age and make. If they also maintain sensitive glass, like museum facades or solar carports with laminated glass canopies, that experience usually maps well.
Document that your contractor held pressure under stated limits, used DI water when applicable, and avoided junction boxes and the underside of panels. Those notes go into your O&M records. When a future performance question arises, you will want that trail.
Environmental stewardship and water use
Cleaning uses water, and on large roofs, the numbers add up. A 1 MW array can use 1,500 to 3,000 gallons in a careful wash, depending on soil load and technique. DI systems waste some water during regeneration or RO production. Mitigate by using efficient rinse bars, capturing first-pass rinse for use on heavily soiled edges, and avoiding clean-in-the-rain gimmicks that simply move dirt around. In drought regions, schedule crew start times to avoid peak municipal demand and coordinate with facility managers to minimize conflict with cooling tower loads.
Soaps matter downstream. Choose biodegradable surfactants and be mindful of drains that flow to landscaped areas. Many roofs route stormwater directly to detention systems. A neutral, low-foam cleaner used sparingly keeps you inside environmental guardrails.
What owners can do between professional visits
A good contractor does not just sell service. We teach owners how to preserve value between visits.
- Monitor production weekly, not just monthly. If output drops relative to expected weather and temperature, consider soiling as a cause and call for a check. Trim overhanging branches that drip sap or drop leaves across the array. A modest tree trim can reduce cleaning frequency by a season. Keep rooftop gutters and scuppers clear so rainfall sheets across panels instead of backing up and leaving sediment at the lower edges. Prohibit untrained staff from “helping” with pressure washers or abrasive pads. One well-meaning maintenance day can void a warranty. Photograph problem spots, like a bird roost over a specific row, so your contractor can pre-treat that area and save time.
Myths worth retiring
I still hear three claims that do not hold up.
First, “rain cleans panels.” Light rain moves dust, but it leaves mineral film and does not remove hardened grime. Think of your car windshield. After a storm, you still need wipers and washer fluid to see clearly.
Second, “high pressure is faster and cheaper.” On small, lightly soiled arrays, maybe it looks that way, right up to the point a seal fails or an AR coating dulls. The cost of one damaged module outweighs the minutes saved.
Third, “panels are slippery and dangerous, so best not to touch them.” They are slippery, and they demand respect. With harnesses, anchors, and poles, we clean them without stepping on glass. The risk is manageable with training.
Case notes from the field
A supermarket we service sits beside a four-lane road. Their 300 kW array had slipped from a daily average of 1,400 kWh in April to about 1,250 by mid June on comparable weather. Our inspection found black streaks at the windward edges and baked-on soot behind HVAC exhausts. We washed with DI water, used a 2 percent neutral surfactant on the worst zones, and spent extra time on the bottom six inches of each module where sediment had formed a lip. The next week, daily yield averaged 1,365 kWh under similar irradiance. That 9 percent gain paid for the visit in roughly seven weeks.
A private school in a leafy neighborhood saw bird pressure spike when a flock nested in nearby cedars. Droppings landed heavily on a courtyard canopy. We installed discrete bird deterrent spikes near favored perches and added a monthly pure-water rinse to keep panels from developing opaque crusts. Output stabilized, and the canopy regained its visual appeal, which mattered to donors and parents as much as the meter.
On an industrial warehouse, a general contractor did a site-wide power wash before turnover, including the roof. The crew used a rotary nozzle at close range to remove roofing adhesive overspray. The panels looked spotless. Three months later, the owner filed a warranty claim on delamination along the lower edges of several modules. The manufacturer asked for maintenance records and photos. The contractor’s method voided coverage for those panels. The lesson still stings.
Choosing the right partner
If you are evaluating a pressure washing company for solar, ask about:
- Water treatment equipment and TDS monitoring to ensure spot-free rinses without harsh chemicals. Written pressure limits and tool types, including soft brushes and carbon-fiber poles, not just wands and nozzles. Safety practices for roof work, from anchors to access plans, plus experience around live electrical equipment. Documentation of methods, before-and-after evidence, and familiarity with module manufacturer guidelines. Bundled services that sensibly pair solar cleaning with Commercial window cleaning or ground-level commercial pressure washing, without mixing harsh ground techniques onto the roof.
A contractor who can talk through these points clearly is far less likely to improvise under pressure.
A practical schedule that respects reality
Set a baseline. If your array is new, run a season without cleaning and watch production closely. Note loss during dry spells and recovery after rains. If loss stays under 3 percent and rain restores it, annual cleaning may suffice. If you see 6 to 10 percent seasonal drift that rain does not correct, plan two visits aimed at your dirtiest months. Tie visits to your energy billing cycles. It is worth cleaning ahead of peak-cost periods in time-of-use markets, because the kilowatts you regain then are more valuable.
For most commercial roofs in dusty or pollen-heavy zones, twice per year works. For coastal and rainy regions with low dust, once per year can be fine, with a light rinse added after major storms that leave salt. For ground-mounts near agriculture, expect quarterly touch-ups during heavy field activity. Whatever cadence you choose, keep it flexible. A wildfire week or a nearby construction project can add an unscheduled clean to protect your yield.
The bottom line
Solar panels are not fragile ornaments. They are robust, engineered assets that reward attentive care. A gentle, professional approach to cleaning with the right water, low pressure, and purpose-built tools recovers real kilowatt-hours without risking the integrity of the modules. The practice borrows from both a power washing service and high-end glass care, and it belongs in the toolkit of any pressure washing company that services modern buildings. Fold solar into your maintenance plan alongside Commercial window cleaning and commercial pressure washing where it makes sense, and you will see cleaner glass, steadier production, and fewer surprises when you need your array to perform at its best.