Does Heat Cut Solar Panel Output in LA & the Valley?
Short answer: yes, a little — but less than most people fear. A panel's rating is measured in a lab at a cool 25°C cell temperature. On a hot Valley rooftop the cells can reach 60–70°C, and each degree above 25°C shaves output by roughly 0.29–0.34%. That’s a real midday dip — yet over a full year the effect stays modest, because California also gives you far more sun to work with.
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- Panel ratings are set at a 25°C cell temperature (STC). Real rooftop cells run hotter, so real midday output sits below the sticker number — this is normal and already baked into good production estimates.
- Modern panels lose about 0.29–0.34% of power per °C above 25°C (the temperature coefficient of Pmax). Exact value varies by model.
- Over a full year the heat effect is modest — California’s high sunshine more than makes up for hot-hour losses. A good temperature coefficient, roof ventilation, and smart module choice all help.
Why a hot panel makes less power
It feels backwards: the sunniest, hottest days should be your best, right? The sunlight part is true — but the heat part works against you. Solar cells are semiconductors, and like most electronics they get slightly less efficient as they warm up. The voltage each cell produces drops as its temperature climbs, so a scorching panel turns a given amount of sunlight into a little less electricity than a cool one would.
Every panel’s headline wattage — the “400 W” on the label — is measured under Standard Test Conditions (STC): 1,000 W/m² of light and a 25°C cell temperature. That’s a cool lab bench, not a July rooftop in Northridge. On a hot, sunny Valley afternoon the cells themselves — darker and sun-baked, sitting above a hot roof — can reach 60–70°C, well above the air temperature. The gap between that 25°C rating and the real cell temperature is where your midday “heat loss” comes from.
The temperature coefficient, in plain English
Every datasheet lists a temperature coefficient of Pmax — how much power the panel loses for each degree Celsius above 25°C. For modern panels it’s typically around −0.29% to −0.34% per °C, and the exact figure varies by model and cell technology.
The math is simple. Take the temperature above 25°C, multiply by the coefficient, and that’s your percentage loss at that instant. If a cell hits 65°C, that’s 40°C above the rating; at −0.32% per °C that’s about a 12.8% dip at that moment — a 400 W panel behaving like roughly a 349 W one. Below is an illustrative worked example. Treat the numbers as ballpark, not a promise for a specific panel.
| Cell temperature | Degrees above 25°C | Approx. output loss | Effective output |
|---|---|---|---|
| 25°C (STC rating) | 0°C | 0% | 400 W |
| 45°C (mild, NOCT-ish) | 20°C | ~6.4% | ~374 W |
| 55°C (warm rooftop) | 30°C | ~9.6% | ~362 W |
| 65°C (hot Valley afternoon) | 40°C | ~12.8% | ~349 W |
| 70°C (very hot, poor airflow) | 45°C | ~14.4% | ~342 W |
Illustrative only, at a single −0.32% per °C coefficient. Cell temperatures and coefficients vary by panel, mounting, and weather. Sources: SolarQuotes — Pmax & NOCT · Sandia PVPMC — NOCT.
Not all panels handle heat the same way
Panel technology matters. Older and value-tier PERC panels tend to have the steepest (worst) temperature coefficients; newer TOPCon cells — the mainstream choice in 2026 — do a bit better, and premium heterojunction (HJT) panels handle heat best of all. Here’s how those differences play out at a hot 65°C cell.
| Cell technology | Typical Pmax coeff. (per °C) | Loss at 65°C cell | Notes |
|---|---|---|---|
| PERC (older / value tier) | −0.34% to −0.37% | ~13.6–14.8% | Fine, but the steepest heat drop |
| TOPCon (mainstream 2026) | −0.29% to −0.32% | ~11.6–12.8% | Best balance of price and heat performance |
| HJT (premium) | −0.24% to −0.27% | ~9.6–10.8% | Best hot-weather performer, costs more |
Ranges vary by manufacturer and model — always check the datasheet. Sources: Solar-Stack — temperature effects · SolarReviews — reading specs. Compare real options in the best solar panels for California.
The “other” temperature number on the datasheet
Next to STC you’ll often see NOCT — Nominal Operating Cell Temperature. It’s the cell temperature a panel settles at under a defined outdoor scene: 800 W/m² of sun, 20°C air, and a light 1 m/s breeze. For most crystalline panels NOCT lands around 42–46°C, and the datasheet lists a lower “NOCT power” to match. It’s a more realistic yardstick than STC — but a still, hot Valley afternoon can push cells well past NOCT.
So why is the yearly hit still small?
Here’s the reassuring part. That 10–14% figure is a midday, hottest-hours number — not what you lose over a year. A few things keep the annual effect modest.
Heat losses are only the hot hours
Cells only run that hot for a slice of summer afternoons. Mornings, evenings, winter, and cooler days lose far less — so the yearly average loss is much smaller than the midday peak.
California gives you more sun
The Southern California basin gets very high solar irradiance. More sunlight per year means more total energy — which more than offsets the hot-hour dip that comes with it.
It’s already in the estimate
Good production models — like NREL’s PVWatts — fold temperature into an hour-by-hour simulation. A proper design already expects the heat, so your quoted annual kWh isn’t a surprise.
Temperature is one of several losses
In a typical system, total losses (inverter, wiring, soiling, temperature) run ~15–25%. Temperature is a meaningful slice of that — not the whole story, and not a dealbreaker.
Bottom line: heat trims your peak midday output, but a well-designed LA or Valley system still produces strong annual energy. If summer performance is a priority, the levers that actually matter are below.
Heat vs. the other things that dent output
It’s worth keeping heat in perspective. On most LA and Valley roofs, a few other factors move your production as much or more than temperature — and they’re easier to act on.
Shade is usually the biggest one: a single tree branch or a neighbor’s chimney crossing the array in the afternoon can cost more than a hot afternoon does, which is why panel layout and, on shaded roofs, microinverters or optimizers matter. Soiling — dust, pollen, and the fine grit that builds up during a long dry Valley summer — can quietly shave a few percent until the next good rain or rinse. Orientation and tilt set your baseline: a south- or west-facing roof at a sensible pitch simply collects more sun across the year than a north-facing or shallow one. Next to those, the heat penalty is real but predictable — and, unlike shade or dust, there’s no maintenance you can do about it. That’s exactly why a good design accounts for all of them together rather than obsessing over any single number.
What actually helps in the heat
A better temperature coefficient
All else equal, a panel with a flatter coefficient (TOPCon or HJT) loses less on hot days. It’s worth comparing on the datasheet, not just on price per watt.
Roof ventilation & standoff
A rack that lifts panels off the roof lets air flow underneath and carry heat away, so cells run cooler than a flush, sealed mount. Standard racking already builds in this air gap.
Right-sizing for real output
Sizing to your actual usage — using heat-aware production numbers, not sticker wattage — means the system still covers your bill on the hottest days. See how many panels you need.
A clean, shaded-aware design
Good layout, tilt, and orientation matter far more to annual yield than chasing the last fraction of a temperature coefficient. Whether the whole system pencils out is covered in is solar worth it in California.
Northridge in July
On our San Fernando Valley installs, air temperatures routinely top 100°F in summer and rooftop cells run hotter still — so midday output sits below the panels’ STC rating on those days. That’s expected, and the systems are designed around it: the long, bright Valley sun season still delivers strong annual production. See our work across Northridge and Los Angeles.
Want a heat-aware production estimate?
We size every system with hour-by-hour, temperature-adjusted modeling for your exact roof — not sticker wattage — so the annual number you see is the number you get. Call +1-323-844-7777 or grab a free estimate.
Get a free estimateFrequently asked
Do solar panels really make less power in the heat?
Yes, a little. A panel’s rating is set at a 25°C cell temperature, and output drops by roughly 0.29–0.34% for each degree Celsius above that. On a hot Valley afternoon cells can hit 60–70°C, so midday output can sit around 10–14% below the sticker rating. Over a full year, though, the effect is modest — and it’s the sunlight, not the heat, that ultimately drives your production.
Is summer still my best season for solar in LA?
Usually yes. Even with the heat penalty, summer’s long, bright days mean many more sunlight hours, so total monthly production is typically at its highest. Heat trims the peak midday output on hot days, but it rarely undoes the seasonal gain from more sun.
What is a temperature coefficient, and what’s a good one?
It’s the percentage of power a panel loses per degree Celsius above 25°C, listed on every datasheet as the temperature coefficient of Pmax. Roughly speaking, closer to zero is better: modern panels range from about −0.24% (premium HJT) to −0.37% (older PERC) per °C. Anything around −0.30% or flatter is solid for a hot climate. Exact values vary by model.
Why is my panel rated 400W but never hits 400W on my app?
Because that 400 W is a lab number, measured at a 25°C cell temperature and full 1,000 W/m² light (Standard Test Conditions). Real rooftops are hotter and light varies through the day, so real output usually runs below the rating — especially at midday in summer. This is normal, and good production estimates already account for it.
Does mounting or roof type change how hot my panels get?
It can. Panels on standard racking with an air gap beneath them run cooler than a flush, sealed mount, because airflow carries heat away. A hot, dark roof under a tight mount pushes cell temperatures higher. It’s a smaller lever than panel choice or system size, but ventilation does help on the hottest days.
Should I pay more for a low-temperature-coefficient panel?
Sometimes. A flatter coefficient (TOPCon or HJT) genuinely loses less on hot days, which matters in the Valley. But layout, tilt, orientation, and correct sizing usually affect your annual kWh more than the last fraction of a coefficient. Compare on the full datasheet and total design, not one spec — we’re happy to walk you through both.
Related reading
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Get a free estimatePrepared by Cali Energy, July 2026. This article is for general educational purposes only and is not legal, tax, financial, engineering, or utility advice. Rates, incentives, codes, permit requirements, equipment specifications, prices, and program terms may change; figures and timelines are estimates, not guarantees. Confirm current requirements with the applicable utility, AHJ, program administrator, manufacturer, or a licensed professional. See our Content Disclaimer. Cali Energy · 19201 Parthenia St Unit E, Northridge, CA 91324 · CSLB #1032379 (B, C-10, C-39)