If you are standing outside at noon with a hose, ready to wash your dirty solar array, put it down immediately. You are about to make a very expensive mistake.
The golden rule of solar maintenance is simple: The best time to clean solar panels is strictly during the early morning (ideally between 6:00 AM and 8:00 AM) or late in the afternoon.
Never, under any circumstances, clean your solar panels during the middle of the day when the sun is blazing. Ignoring this rule doesn’t just lead to streaks; it can permanently destroy your solar investment through a phenomenon called thermal shock.
Here is why timing is the most critical factor in cleaning your panels safely.
The Science of Thermal Shock
To understand why midday cleaning is disastrous, we need to look at physics. Solar panels are designed to be incredibly efficient at absorbing heat and energy from the sun—they are essentially dark sheets of glass baking on your roof.
In a typical Australian summer, the ambient air temperature might be 35°C, but the surface temperature of your solar panels is significantly higher. It is not uncommon for panels to reach surface temperatures exceeding 60°C (140°F) during the middle of the day.
Now, consider the water coming out of your garden hose. Depending on your location and season, tap water is usually quite cold, often sitting around 15°C.
The Shattering Effect
When you spray 15°C water onto 60°C glass, you create an instantaneous temperature differential of 45 degrees. This triggers thermal shock.
- The Hot State: Under the midday sun, the glass substrate of your panels is expanded due to intense heat.
- The Cold Shock: The moment cold water hits the surface, the glass in that specific area contracts rapidly and violently.
- The Result: Because glass is brittle, it cannot handle the immense stress of one section shrinking fast while the surrounding area remains expanded. The result is immediate catastrophic failure—the glass shatters into thousands of tiny spiderweb cracks.
This damage is irreversible. It ruins the panel’s ability to generate power efficiently and, almost invariably, voids the manufacturer’s warranty instantly.
Cracked glass isn’t the only risk; using the wrong pressure settings is even worse. Read our guide on Pressure Washer Solar Panel Damage to protect your warranty and understand why high pressure is a no-go.
When is the Absolute ‘Best Time to Clean Solar Panels’?
To avoid thermal shock, the panels must be cool to the touch. There are two safe windows for this.
1. The Gold Standard: Early Morning (6 AM – 8 AM)
This is widely considered the best time to clean solar panels.
- Why: The panels have had the entire night to cool down and equalize with the ambient air temperature.
- The Strategy: Start as early as you have enough light to see clearly. You want to be finished before the sun rises high enough to start actively heating the glass surface.
2. The Alternative: Late Afternoon or Overcast Days
If mornings aren’t possible, late afternoon is your second-best option, but caution is required.
- Why: You must wait until the sun is low in the sky and no longer hitting the panels directly.
- The Warning: Do not assume that just because it’s 5:00 PM, the panels are safe. They can retain heat for hours after peak sun. You must ensure they have cooled significantly. Overcast, cool days are also generally safe windows, provided there are no sudden breaks in the clouds.
The Professional Difference: We Don’t Guess
The risk of thermal shock is why many homeowners choose to bypass DIY cleaning altogether. At Clean Solar Aus, we don’t guess about temperatures.
Before a drop of water touches a panel during the warmer months, professional cleaners should verify the surface temperature. We ensure the temperature differential between our purified water and the panel glass is within a safe, non-reactive range.
If the panels are too hot, we wait. Saving 20 minutes by rushing a job at noon isn’t worth replacing a $6,000 solar array.
Summary
Cleaning your solar panels is essential for maintaining peak efficiency, but when you clean them is a matter of safety. To ensure your system lasts for decades, stick strictly to early morning cleaning protocols. If you are unsure about the heat, don’t risk the shatter—call the professionals.