Solar panels turn light, not heat, into electricity, and they work slightly less efficiently as they get hotter. Every panel has a temperature coefficient, a datasheet figure that states how much power it loses for each degree Celsius its cells rise above 25°C. For modern monocrystalline panels that figure is typically around -0.3% to -0.4% per degree. Because cells on a sunny roof run far hotter than the surrounding air, this small percentage adds up to a noticeable difference between a still summer afternoon and a crisp spring morning.

Why heat reduces output

A silicon cell produces electricity when light frees electrons and the cell's built-in electric field pushes them into a circuit (the cell structure is covered in what solar panels are made of). The strength of that push shows up as voltage. As the cell warms, more electrons are thermally excited even without light, and the voltage the cell can hold falls. Current rises very slightly with temperature, but nowhere near enough to make up for the voltage drop, so power, which is voltage multiplied by current, goes down.

This effect is reversible. A hot panel is not being damaged simply by being hot, and its output recovers as soon as it cools. Permanent, gradual loss of output is a separate process, explained in solar panel degradation explained.

Reading the three temperature coefficients on a datasheet

Panel datasheets usually list three coefficients, all measured against the 25°C cell temperature used in Standard Test Conditions. (These are the laboratory conditions that set a panel's rated wattage, not to be confused with Small-scale Technology Certificates, which share the same abbreviation.)

A smaller negative Pmax figure means better hot-weather performance. The gap between -0.30% and -0.38% per degree sounds trivial, but at a cell temperature of 65°C it separates a loss of about 12% from a loss of about 15%.

Cell temperature is not air temperature

The temperature that counts is inside the cell, and in full sun that is well above the air temperature. Dark cells under glass absorb a lot of solar energy, and only part of it leaves as electricity; the rest becomes heat. As a rule of thumb, cells in a typical rail-mounted roof array run roughly 20-35°C above the air temperature in strong sunshine, depending on wind, mounting gap and roof material.

Datasheets offer a clue in a figure called NMOT (nominal module operating temperature), or the older NOCT. It is the cell temperature reached under moderate test conditions: 800 watts per square metre of sunlight, 20°C air and a light breeze. Most panels land in the low-to-mid 40s. On a hot, still afternoon with stronger sunshine, cells climb well beyond that.

Worked examples: spring morning versus summer afternoon

Take a panel rated at 440 W with a Pmax coefficient of -0.35% per °C. The estimates below isolate the temperature effect only; sunlight intensity, dust and wiring losses also change real output.

This is why the highest instantaneous power readings on a monitoring app often appear on clear, cool days in late winter and spring rather than in January. Summer can still deliver more energy per day overall, because days are longer and the sun climbs higher. Heat trims summer output without cancelling that seasonal advantage.

The effect is easy to see in monitoring data. Compare two clear days with similar sunshine, one hot and one cool, and the hot day's production curve usually has a lower, flatter top. A difference of a few per cent at midday is normal temperature behaviour. A large or sudden drop that does not recover on cooler days is more likely to point to shading, soiling or a fault than to heat.

The cold side: rising voltage on frosty mornings

The voltage coefficient works in reverse as well. On a cold, clear morning, particularly in areas such as the Blue Mountains, the Southern Highlands and the Central Tablelands, cells can sit below freezing before the sun warms them, pushing string voltage above its rated value. System designers calculate the coldest likely cell temperature for the site and make sure the highest possible string voltage stays under the inverter's maximum DC input voltage. It is one reason the number of panels in each string is a calculation, not a rough estimate.

What influences how hot panels run

Panel temperature is not set by the weather alone. Several design and site factors make a difference:

• Air gap under the panels. Standard rail mounting leaves space for air to circulate. Panels sitting very close to a hot roof surface run warmer.
• Tilt and wind exposure. Tilted, breezy arrays shed heat more easily than flat, sheltered ones.
• Roof colour and material. A dark roof beneath the array absorbs more heat than a light-coloured one, warming the air that flows behind the panels.
• Cell technology. Newer n-type designs, such as TOPCon and heterojunction cells, generally have lower temperature coefficients than older p-type designs.
• Inverter location. Inverters also reduce output when they overheat. One mounted in full afternoon sun on a west-facing wall can throttle back on hot days, even while the panels perform well.

Any change to panel mounting or inverter position must be carried out by an accredited installer and a licensed electrician. Roof and electrical work is never a DIY job.

Next steps

If summer output looks lower than it should, the cause may be heat, shading, soiling or a fault, and monitoring data alone rarely tells them apart. The System Performance Audit (from $390 for a residential system) compares 12 months of generation against modelled output, and an Inverter Weather Cover (from $290, supplied and fitted) shades exposed inverters while keeping ventilation clearances. Both are on the energy market; prices are indicative and confirmed after a site assessment. For a system designed around your roof and local climate, request a free assessment from Blue Energy Solar.

Frequently asked questions

Do solar panels stop working in a heatwave?

No. Panels keep producing electricity in extreme heat, just at a reduced rate. On a very hot, still day, cell temperatures can climb high enough to trim output by somewhere around 15-20% compared with the rated figure, and the inverter may reduce output further if it overheats. Clear skies during a heatwave still mean plenty of sunlight, so daily production is usually solid.

Is it worth hosing panels down to cool them on hot days?

No. Any cooling lasts only minutes before the sun heats the glass again, and cold water on very hot glass puts it under thermal stress. Tap water can also leave mineral spots that reduce light reaching the cells. Climbing onto a roof or reaching panels with ladders and hoses is a serious fall risk, so cleaning should be left to trained professionals with proper safety equipment.

Should I choose panels mainly on their temperature coefficient?

Not on its own. The temperature coefficient is one factor alongside efficiency, degradation rate, warranty terms and the manufacturer's track record. Between quality panels, the difference in hot-weather output is usually a few per cent during the warmest months. Comparing estimated annual energy production for the whole system on your roof gives a better picture than any single datasheet figure.