A bloke knocked on my door last month offering to clean my solar panels for $250. Said they were “probably losing 30% output” from dirt buildup. I politely declined. Not because cleaning doesn’t help — sometimes it does — but because that 30% figure is wildly exaggerated for most Australian rooftops.

So what’s the real story? Let’s look at actual data instead of sales pitches.

What the research says

UNSW’s School of Photovoltaic and Renewable Energy Engineering studied this across multiple Australian test sites:

• Light dust and pollen: 1-3% output reduction
• Heavy dust (rural/agricultural areas): 5-8% output reduction
• Bird droppings (localised): up to 25% reduction on affected cells, but typically 2-5% system-wide
• Coastal salt spray: 3-6% reduction if not rinsed by rain

International research agrees — average soiling losses of 3-5% globally, with desert regions hitting 15-25%.

For a typical suburban Australian roof that gets regular rainfall? You’re looking at 2-5% losses. That’s real, but it’s not 30%.

When cleaning genuinely matters

There are situations where cleaning makes a meaningful difference:

Bird dropping hotspots. This is the big one. A single bird dropping on a cell can cause it to act as a resistor, reducing the output of the entire string. If you’ve got panels under a tree where birds roost, regular cleaning is worthwhile. I’ve seen monitoring data where one panel dropped 40% because pigeons were treating it as a toilet.

Rural and agricultural properties. Dust from unsealed roads, grain harvest, and livestock can coat panels heavily. Farmers near cropping land often see 8-12% losses during harvest season. A hose-down every few months makes sense.

Flat or low-tilt installations. Panels at less than 10 degrees don’t self-clean well in rain. Water pools, leaves accumulate, and dirt builds up. Most residential panels are at 15-30 degrees, which handles self-cleaning fine. But nearly flat panels (common on commercial roofs) need attention.

Extended dry periods or coastal salt spray. If it hasn’t rained in 6-8 weeks, dust accumulation gets noticeable. Coastal properties also get salt film that doesn’t wash off easily. A fresh water rinse helps in both cases.

When it probably doesn’t matter

Suburban homes in rainfall areas (most of coastal Australia). Rain does a decent job of keeping panels clean enough. If you’re getting 25+ rain days per year, your panels are getting a regular rinse.

Systems performing within expected range. Check your monitoring app. If your panels are producing within 5% of expected output for the current season, they’re fine. Don’t clean them just because they look dusty from the ground.

How to check if your panels need cleaning

Rather than guessing, use your monitoring data:

1. Compare day-to-day. Find two days with similar weather conditions (clear, similar temperature) a few weeks apart. Is production noticeably different? If one day is 10%+ lower with no obvious weather explanation, soiling might be a factor.

2. Compare year-on-year. Most monitoring apps let you compare this February to last February. If you’re down more than expected degradation (1-2% per year), investigate.

3. Visual inspection from the ground. Use binoculars. Look for bird droppings, leaf debris, or thick dust layers. A light even coating of dust is normal. Concentrated grime or debris is worth addressing.

How to clean them (if you decide to)

The free option: garden hose from the ground. Seriously, that’s it for most people. A spray with the hose on a cool morning (never spray cold water on hot panels — thermal shock can crack glass) will remove most surface dust and light soiling. Don’t use high pressure. Don’t use detergent. Just water.

The DIY option: soft brush on a pole. A telescopic window-cleaning brush with a hose attachment from Bunnings ($40-60) works well. Use it from the ground — do not climb on your roof. Rooftop falls are a leading cause of household injury in Australia, and wet panels on a tilted roof are extremely slippery.

The professional option: $150-350 depending on system size. Worth it for access issues (multi-storey, steep roof) or heavy soiling. Not worth it for a light clean on a single-storey roof.

The economics of cleaning

A typical 10kW system in Sydney produces about 14,000 kWh per year. If soiling is reducing output by 3%, that’s 420 kWh lost. At a blended value of 25c/kWh, that’s $105 per year in lost production.

If professional cleaning costs $250 and lasts 3-4 months, you’d need 3-4 cleans per year at $750-1,000. That’s wildly more than the $105 you’re losing. Even with 5% soiling losses ($175/year), professional cleaning doesn’t pay for itself unless you’re doing it once a year at most. A garden hose twice a year costs nothing and gets 80% of the benefit.

My recommendation

For most people: hose your panels from the ground twice a year (end of autumn to clear leaf debris, and mid-summer if it’s been dry). Check your monitoring data quarterly. Don’t pay for professional cleaning unless you have a specific problem — heavy bird droppings, post-construction dust, or an accessibility issue.

And if someone knocks on your door claiming your panels are losing 30% output from dirt, ask them for the monitoring data to prove it. They won’t have it.