Temperature losses in UK solar panels: how heat quietly eats your kWh

When people think “solar”, they think “hot sun”.

If you want to sanity-check what this could look like where you live, start here: Find your postcode.

So it feels backwards when your monitoring app shows this pattern:

• A bright, cool day in spring hits a higher peak kW (for kW, kWh and kWp, see Glossary)
• A blazing August afternoon looks slightly muted, even with clear skies

Nothing is wrong. This is temperature loss, and it is built into the physics of silicon solar cells.

This guide explains:

• Why panels lose power as they heat up
• How big the losses are in normal UK conditions
• What makes some roofs run hotter than others
• How to spot the difference between normal temperature loss and a real problem

Along the way, we will use the same type of postcode context that underpins the SolarByPostcode calculator, without turning it into a lab report.

Quick Answer: What You Should Expect

If you want the practical version:

1. Compare a clear day in spring with a clear day in midsummer
2. Expect midsummer peak kW to be a bit lower if the panels were much hotter
3. Judge health by monthly and yearly kWh, not the single highest kW spike

If you are new to the “kW vs kWh vs kWp” confusion, start here first:
Why your solar panels will never hit their rated output (and why that’s completely normal)

Assumptions and variability

• We assume a modern silicon PV system with a typical temperature coefficient (the exact number varies by panel model).
• We treat the “loss” as a real-world effect driven mainly by cell temperature, not the air temperature you see in weather apps.
• Real roofs vary a lot: mounting type (in-roof vs stand-off), airflow behind panels, roof colour, and local wind exposure can change how hot panels run.
• Your monitoring app view can mislead: inverter clipping and export limits can mask temperature effects and change the shape of peaks.
• For the broader postcode-level assumptions behind SBP estimates, see: Data sources and methodology

The basic physics (in plain language)

Solar panels love light, not heat

A solar cell is a semiconductor. As it gets hotter, its voltage drops. That reduces the power you get out for the same sunlight.

Manufacturers capture this with a spec called the temperature coefficient. In simple terms:

• Above a reference temperature (25°C cell temperature), output falls in a roughly linear way
• The loss per degree is typically a few tenths of a percent per °C

You do not need to memorise the number to use this guide. Just remember the shape of the relationship: hotter panels, slightly less power.

Why your roof can be hotter than the weather app

Two different temperatures get mixed up:

• Air temperature: what your phone weather app shows
• Panel cell temperature: what the solar cells inside the panel reach

Cell temperature is what matters for temperature loss.

On a bright, still day, panels can run dramatically hotter than the air because they are absorbing sunlight and shedding heat slowly. Wind helps cool them. A ventilated mounting gap helps. Dark roofs tend to run hotter than light ones.

How big are temperature losses in the UK?

Most of the time, temperature loss is a “quiet haircut”, not a disaster.

Typical UK pattern:

• Spring: cool air, strong sun, good ventilation. Often the best peak kW.
• High summer: more daylight and more total kWh, but peak kW can be slightly lower on the hottest, stillest hours.
• Autumn and winter: temperature loss is minimal, but light is the limiting factor.

This is why people are often surprised when the “best-looking” day in the calendar does not produce the highest peak kW.

A simple worked example

Imagine a system rated at 4.0 kWp.

• On a cool, clear day your panels run relatively cool, so you might see peaks that get close to the inverter limit.
• On a hot roof day, you might see peaks that look 0.3–0.6 kW lower for the same clear sky window.

That is not your system failing. That is temperature loss doing its job quietly.

If you want to sanity-check your expectations against local conditions, start with your postcode estimate on the calculator homepage:
SolarByPostcode calculator

What makes some roofs run hotter than others?

Temperature loss is not the same everywhere, even in the same town. Roof details matter.

1. Mounting style and airflow

• Stand-off mounted panels (with a decent air gap) cool better.
• In-roof systems often run hotter because there is less airflow behind the panel.

Hotter panels means more temperature loss.

2. Wind exposure

Coastal and exposed sites often get more natural cooling. That does not always mean more sunshine, but it can lift peak kW on clear days.

If you want concrete postcode examples across the UK, compare the vibe of these areas:

• Bristol city area: BS1
• Brighton and Hove area: BN1
• Newcastle upon Tyne area: NE1
• Glasgow city area: G1

Do not over-interpret this. It is not “city X is always better”. It is simply to show how local exposure and typical weather can change what a “good day” looks like in your app.

3. Roof material and colour

Dark roofs absorb more heat. Flat roofs can trap heat differently than pitched roofs. These effects are real, but they are usually second-order compared with light level, roof orientation, and shading.

4. Inverter sizing and clipping

Sometimes the “summer is lower” story is made more confusing by inverter clipping.

If your array is oversized relative to the inverter, you might see:

• Cool spring days: a brief flat top at the inverter limit
• Hot summer days: less clipping because the panels cannot push as hard when hot

Both can be totally normal, and neither says anything negative about yearly kWh.

If you want that whole picture with diagrams, this section in the first guide covers it:
System design limits: inverter clipping peaks

What temperature loss does (and does not) change

It changes peak power more than yearly energy

Most people notice temperature loss because they watch their app at noon.

But your payback is driven by:

• Total kWh over months and years
• How much you self-consume versus export
• Your import and export rates

If you are thinking about sizing, batteries, or “how much is enough kWp”, this is the more useful next read:
Solar system sizing in the UK: choosing the right kWp without wasting money

It does not explain big, persistent underperformance

Temperature loss tends to be:

• Predictable
• Strongest on hot, still, very sunny hours
• Much smaller on cool days

So if your system looks weak all the time, temperature loss is not the answer.

If what you are seeing is a slow, steady decline year after year (rather than a summer-only dip), this guide is the relevant sibling:
Solar panel degradation in the UK: what you really lose over 25 years

How to tell normal temperature loss from a problem

Normal patterns (usually fine)

• Peak kW slightly lower in midsummer than in late spring, despite clear sky
• Output bouncing day to day with clouds and haze
• A windy clear day looking “punchier” than a still hot clear day

If the day-to-day “bounce” is your main confusion, this is the other key sibling guide in the same cluster:
Cloud cover vs solar output: what actually happens on overcast UK days

Patterns that deserve investigation

• A sudden step-change drop in kWh that persists for weeks
• A clear day that looks unusually flat compared with other clear days in the same month
• One string or panel group producing almost nothing
• Big gaps between your yearly kWh and a realistic postcode estimate (once you account for roof direction and shading)

If you have not already, sanity-check the biggest drivers first:

• Roof direction (see our Aspect Penalty Map)
• Standing charges and your tariff context (see Standing-Charge Drag Index)
• Shade and layout choices (chimneys, dormers, trees)

Temperature loss is real, but it is rarely the main reason a system is materially underperforming over a year.

Practical ways to reduce temperature losses (without doing anything silly)

You cannot change physics, but you can avoid making it worse.

1. Do not block airflow behind the panels

If you have a choice of mounting configuration, favour sensible ventilation behind the array. Avoid designs that trap heat unless there is a strong reason (and a clear explanation from the installer).

2. Do not obsess over cooling gadgets

If you see products promising to “cool your panels for massive gains”, be sceptical. In the UK, the economics are usually weak compared with the simple levers: good system sizing, sensible roof layout, and shifting self-consumption.

3. Optimise the decisions that really matter

If you want to improve real-world outcomes:

• Make sure your system size matches your usage and roof, rather than chasing a headline kWp
• Make sure your import and export rates are sensible
• Consider a battery only when the numbers work for your usage pattern

That is exactly what our calculator and guides are designed to support.

Frequently Asked Questions

Do solar panels work better in cold weather?

They often produce higher peak power in cool, bright conditions because the cells run cooler. But cold weather in the UK is usually accompanied by lower light levels and shorter days, so total winter kWh is still much lower.

Why does my solar output drop on the hottest days?

Because the panel cells heat up, voltage drops, and power falls slightly. It is normal for peak kW to be lower on very hot, still hours even with clear skies.

Is temperature loss a reason to buy a “better” panel?

Not usually. Most modern panels are in the same general range for temperature coefficient. The bigger determinants for real-world results are roof direction, shading, layout, inverter sizing, and your tariff economics.

Should I clean my panels to reduce temperature loss?

Cleaning affects soiling losses, not temperature loss. In the UK, cleaning is rarely about big performance jumps unless the panels are genuinely dirty. Temperature loss is about heat, airflow, and sunlight.

Bottom line

• Solar panels love sunlight, but they lose a bit of power as they heat up
• In the UK, temperature loss mostly shows up as slightly lower peak kW in midsummer
• Your highest peak kW often happens in April or May, not in the hottest week of August
• Big, persistent underperformance is more likely to be shading, roof direction, system design, or faults, not temperature loss

Next reads

• Why solar panels never hit their rated output (and why that is normal)
• Cloud cover vs solar output: what actually happens on overcast UK days
• Solar panel degradation in the UK: what to expect after 10, 20, and 30 years
• Solar monitoring: how to spot underperformance early (without obsessing)
• Solar system sizing in the UK: choosing the right kWp without wasting money

Run your postcode

Want a realistic benchmark for your roof? Start with your postcode: Check your area on SolarByPostcode

Run the calculator for your postcode