I had a mate message me during last week’s Brisbane heatwave: “40 degrees, full sun, why isn’t my solar producing more?” It’s one of the most counterintuitive things about solar panels — they love light but hate heat. And understanding this helps you set realistic expectations and get the most from your system.

The temperature coefficient explained

Every solar panel has a temperature coefficient, typically listed in the spec sheet as something like “-0.34%/degrees C” for power output. This means that for every degree above 25 degrees C (the standard test condition), the panel’s output drops by about 0.34%.

On a 40-degree day, the panel surface temperature can reach 60-70 degrees C (panels run much hotter than ambient air temperature because they’re absorbing sunlight). At 65 degrees C, that’s 40 degrees above the standard test condition, meaning a 13.6% reduction in output (40 x 0.34%).

So on the hottest, sunniest days, your panels are producing 10-15% less than they would on a mild 25-degree sunny day. That’s a meaningful chunk of power.

Why summer days still produce more overall

Despite the heat penalty, summer days produce more total energy than winter days for two reasons:

Longer days. Summer in Brisbane gives you about 14 hours of daylight versus 10.5 in winter. More hours of production more than compensates for the heat reduction.

Higher sun angle. In summer, the sun is more directly overhead, reducing the atmospheric thickness that sunlight has to travel through. This increases irradiance.

The net result: a summer day in Brisbane might produce 60-70 kWh from a 13kW system, while a mid-winter day produces 35-45 kWh. Summer wins easily despite the temperature penalty.

But within summer, you’ll notice that the best production days are often clear days in the mid-20s (spring or autumn), not the 38-degree scorchers.

What you can do about heat losses

Mostly, nothing. You can’t control the weather. But there are some design considerations:

Roof ventilation. Panels mounted with adequate airflow underneath (at least 100mm gap) run cooler than panels flush-mounted on the roof surface. This is standard practice in Australia, but some installers skimp on standoff heights. If your panels are getting too hot, poor ventilation might be the cause.

Panel selection. Some panels have better temperature coefficients than others. N-type cells (like those in LONGi HiMO series and Trina Vertex N) typically have lower temperature coefficients (-0.29 to -0.32%/degrees C) compared to older P-type cells (-0.35 to -0.40%/degrees C). If you’re in a hot climate, this is worth considering when choosing panels.

Light-coloured roofing. A darker roof absorbs more heat, which radiates up to the panels. A lighter-coloured roof reflects more heat away. If you’re reroofing before a solar install, consider lighter colours. But don’t reroof just for this reason — the improvement is marginal.

Don’t stress about it. Seriously. The temperature effect is a few percentage points on hot days. It’s not worth losing sleep over. Your system will produce plenty of power through an Australian summer regardless.

Summer-specific tips

Since you’ll be producing the most power in summer, make the most of it:

Pre-cool your house. Run air conditioning during peak solar hours (10am-3pm) to cool the house down, then let it coast through the evening. You’re using free solar power to bank thermal energy instead of buying peak-rate grid power at 6pm.

Charge everything during the day. Phones, laptops, power tools, robot vacuums, cordless appliances — plug them all in during solar hours. These individual loads are small, but collectively they shift consumption to your free power window.

Run your pool pump during solar peak. If you’ve got a pool, summer is when the pump needs to run the most anyway. Align it with solar production and save $2-3/day in pumping costs.

Monitor your system more closely. Summer storms, hail, and extreme heat events can cause issues. Check your monitoring app after significant weather events and look for sudden performance changes.

The air conditioning paradox

Here’s the irony: the days when you need air conditioning most (extreme heat) are also the days when your solar panels produce slightly less than their potential. And air conditioning is the single biggest household energy load, easily pulling 2-5kW depending on the system and how hot it is outside.

For most solar households with appropriately sized systems, solar can handle air conditioning during the day without difficulty. But if you’re running a large ducted system at full blast on a 42-degree day, your solar might not keep up with that plus other household loads.

This is another argument for batteries. Charge the battery during peak production (late morning to early afternoon when solar output is highest), then let the battery supplement solar during the late afternoon when production is declining but heat (and air conditioning demand) is still high.

The Australian summer and solar are natural partners. Yes, the heat takes a small toll on panel output. But the long, sunny days more than make up for it. If you’re reading this during a heatwave and wondering why your panels aren’t breaking records, now you know why. And it’s totally normal.