Do Solar Panels Work Less Efficiently in Hot Weather? The Real Numbers

Yes — solar panels lose efficiency as they heat up, and a scorching July afternoon can produce less power per panel than a cool, clear day in April. This surprises most homeowners, because it runs against the obvious assumption: more sun equals more heat equals more power. In reality, sunlight and heat are two separate inputs, and only one of them helps your panels. The other works against them.

Here's the actual physics, the real production numbers, and what it means for sizing a system in a hot climate.

Last updated: 2026-07-14

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Why Heat Hurts Panel Output

Solar panels are semiconductor devices, and semiconductors generate more electrical resistance as they get hotter. That resistance eats into the voltage the panel can produce, even while the amount of sunlight hitting it stays the same or increases.

Every panel has a temperature coefficient, usually listed on the spec sheet as a percentage per degree Celsius above 25°C (77°F) — the standard testing temperature manufacturers use to rate output. A typical monocrystalline panel has a coefficient around -0.35% to -0.45% per °C. That means for every degree above 25°C the panel's surface reaches, you lose roughly a third to a half a percent of rated output.

The catch: panel surface temperature runs far hotter than the air temperature around it. On a 95°F (35°C) day with direct sun, a roof-mounted panel can reach 140–160°F (60–70°C) at the cell level, because dark panels absorb heat and rooftop mounting limits airflow underneath them.

Run the math on that: a panel sitting at 65°C is 40°C above its 25°C rating point. At a -0.4%/°C coefficient, that's a 16% efficiency loss — on what looks, from the ground, like a perfect solar day.

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Real Production Numbers: Summer Heat vs. Peak Efficiency

To make this concrete, compare a 10kW system's output on two different days with identical sun exposure but different temperatures, based on standard temperature-coefficient modeling:

Cool, sunny day — 60°F air / panel surface ~95°F (35°C):

• Temperature loss from rating point: ~4%
• Effective output: ~9.6 kW of rated capacity

Hot, sunny day — 100°F air / panel surface ~155°F (68°C):

• Temperature loss from rating point: ~17%
• Effective output: ~8.3 kW of rated capacity

That's a real-world swing of roughly 1.3 kW of lost capacity on the hottest days — precisely when air conditioning is driving your household's peak demand. This is why homeowners in Phoenix, Las Vegas, or Dallas sometimes see their system's best single-day output numbers show up in May or October, not July or August, even though those summer months still produce more total energy because the days are longer.

The distinction that matters: heat reduces efficiency, not total daily or monthly production. Long summer days with more peak sun hours generally still out-produce shorter, cooler months overall — the heat penalty is a discount on an already-larger number, not a wipeout. But it does mean your system's rated capacity and its real-world summer performance are two different figures, and any installer quote that doesn't account for this gap is overselling.

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Where This Matters Most: Hot-Climate States

The temperature penalty isn't evenly distributed. It matters most in states where summer heat is extreme and sustained:

• Arizona, Nevada, and inland Southern California — surface temperatures regularly exceed 65°C for weeks at a time
• Texas, especially inland areas without coastal breeze — see Texas solar guide for state-specific numbers
• Florida, where heat is paired with high humidity — see Florida solar guide
• Interior Southwest states generally

In these climates, the gap between a panel's rated capacity and its actual mid-summer output can run 15–20%, which is exactly why installers in hot regions should be sizing systems to account for it — not just quoting rated wattage and letting homeowners assume that's what they'll get every day.

If your installer's proposal doesn't mention temperature derating at all, that's worth asking about directly. EnergySage is a useful way to get multiple proposals side by side and see whether one installer is modeling this more conservatively — and more honestly — than another.

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For lighter backup needs — keeping devices charged and a fan or two running during a shorter outage — the Jackery Explorer 2000 Plus covers most scenarios at a lower price point, and can be recharged from solar if you already have panels installed.

Affiliate Disclosure: This article may contain affiliate links. If you make a purchase through these links, we may earn a small commission at no extra cost to you. We only recommend products we genuinely believe in. This helps support our work and allows us to continue providing free content.

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