Does Heat Cut Solar Panel Output in LA & the Valley?
Yes, but by less than most people expect. In NREL’s PVWatts model, a south-facing roof system in Northridge loses about 6% of its yearly output to heat with a standard panel (−0.37%/°C), and about 5% with the typical panel listed in California since 2024 (−0.30%/°C). In the hottest hours the loss approaches 20%, but those hours are few — and August is still the Valley’s most productive month.
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- Panel ratings assume a 25°C cell. On a Valley roof at midday in July and August, PVWatts puts the cells around 51°C; in the hottest hours, near 78°C.
- Over a year, heat costs a south-facing Northridge system about 6% of its output with a standard −0.37%/°C panel. Santa Monica loses almost as much, 5.9%, and still makes about 4% less energy, because it gets less sun.
- Modules added to California’s list since 2024 have a median coefficient of −0.30%/°C, against −0.36 for 2019–2021 listings — worth about 1% of yearly output in the Valley.
- Heat trims summer; it does not ruin it. August is the most productive month in both places.
- A sound production estimate already includes temperature. Ask whether yours does.
Why a hot panel makes less power
A panel’s nameplate watts are measured under Standard Test Conditions: 1,000 W/m² of sunlight, a 25°C cell and a standard solar spectrum (California Energy Commission). Cells on a sunny roof run far hotter, and silicon cells lose voltage as they warm. The datasheet’s temperature coefficient of Pmax says how much: at −0.30%/°C, a cell 20°C above 25°C produces 6% less than its rating at that moment.
California’s module list also publishes a more realistic PTC rating, taken at 20°C air rather than a 25°C cell. For modules listed since 2024 it is a median 94.5% of nameplate, so a 400 W panel is rated about 378 W there.
How hot the cells get depends on sunlight, air temperature, wind and mounting. The model PVWatts uses starts from NOCT — the cell temperature on an open rack at 800 W/m² of sun, 20°C air and a 1 m/s breeze — and adjusts it for how far the panel stands off the roof (Sandia PVPMC). The median NOCT of 350 W-plus modules listed since 2024 is 43.6°C. NREL’s PVWatts uses 49°C for roof-mounted arrays and 45°C for open racks, because a panel close to the roof runs hotter (SAM help: PVWatts).
How much heat costs in Los Angeles: the modeled numbers
We ran NREL’s PVWatts, version 8, hour by hour for two identical systems — 4 kW, facing south, 20° tilt, roof-mounted, default settings — one in Northridge in the San Fernando Valley and one in Santa Monica on the coast, on NREL’s typical-year weather. From the hourly output and cell temperatures, we calculated how much energy heat took.
| Measure | Northridge (Valley) | Santa Monica (coast) |
|---|---|---|
| Yearly output, kWh per kW | 1,700 | 1,630 |
| Yearly output lost to heat, standard panel (−0.37%/°C) | 6.1% | 5.9% |
| Same, typical panel listed since 2024 (−0.30%/°C) | 4.9% | 4.8% |
| Same, flattest residential panel on the state list (−0.215%/°C) | 3.5% | 3.4% |
| Typical cell temperature, July–August midday | 51°C | 48°C |
| Hottest hour of the year (standard panel) | 78°C cell, 35°C air: 19.6% lost to heat | 72°C cell, 36°C air: 17.3% lost to heat |
| Producing hours a year with cells above 60°C | 78 | 49 |
| Most productive month | August | August |
Our calculation from NREL PVWatts version 8 hourly output (API; 2020 typical-meteorological-year weather from the National Solar Radiation Database; defaults of 14% system losses, 96% inverter efficiency and a 1.2 DC-to-AC ratio). Heat loss is the energy the same array would make with its cells at 25°C, minus the modeled output. A different roof, orientation or panel changes the numbers.
Month by month: heat trims summer, but summer still wins
| Month | Northridge output, kWh per kW | Northridge heat loss | Santa Monica output, kWh per kW |
|---|---|---|---|
| January | 104 | 1.9% | 104 |
| February | 110 | 3.3% | 113 |
| March | 142 | 4.5% | 144 |
| April | 157 | 4.4% | 153 |
| May | 156 | 6.4% | 162 |
| June | 172 | 8.2% | 152 |
| July | 172 | 8.7% | 164 |
| August | 175 | 9.1% | 165 |
| September | 153 | 8.3% | 138 |
| October | 140 | 5.5% | 134 |
| November | 117 | 4.9% | 104 |
| December | 100 | 1.8% | 98 |
In the Valley, heat takes about 2% in December and January and 8–9% from June through September — yet those summer months still produce the most, because the extra sunlight outweighs the heat. On the coast, June comes in below May: the model’s sunlight on the panels drops from 6.92 to 6.77 kWh/m² a day, so it is less sun, not more heat. For sizing a system to your bill, see how many solar panels you need.
What the state’s module list says about temperature coefficients
The California Energy Commission lists every module eligible for the state’s programs, with its temperature coefficient and NOCT. Newer modules handle heat noticeably better.
| Added to list | Modules | Median coefficient | Middle half | −0.30%/°C or flatter | Median NOCT |
|---|---|---|---|---|---|
| 2019–2021 | 4,160 | −0.360%/°C | −0.377 to −0.342 | 2% | 45.2°C |
| 2024–2026 | 5,121 | −0.299%/°C | −0.332 to −0.293 | 54% | 43.6°C |
| 2025–2026 only | 3,122 | −0.297%/°C | −0.322 to −0.292 | 63% | 43.5°C |
Our tabulation of the CEC PV Module List (data as of September 21, 2026), modules with a nameplate of 350 W or more, grouped by listing date.
In the Valley model, each 0.01%/°C of coefficient is worth about 0.16% of yearly output. So a typical 2024–2026 panel at −0.30 beats a typical 2019–2021 panel at −0.36 by about 1% a year, and the flattest residential modules on the list — REC’s Pure-RXG series at −0.215%/°C — add roughly another 1.4%. That is real but modest next to price per watt, warranty and the quality of the install; compare all of them in the best solar panels for California.
What actually helps in the heat
A flatter coefficient
A lever you can compare on paper: about 0.16% of yearly output per 0.01%/°C in the Valley. Read it on the datasheet or the CEC list.
Air under the panels
Cells run hotter when the panel sits close to the roof. PVWatts assumes 49°C NOCT for roof mounts against 45°C for open racks, and the CEC notes high-temperature losses for arrays mounted close to or within a roofline. Standard racking leaves a gap; don’t give it up.
Keep heat in proportion
Heat is one loss among several. PVWatts’ default for everything else — soiling 2%, shading 3%, wiring, mismatch, availability and more — totals 14%, before the inverter (PVWatts manual). A shaded array can lose more than a hot one.
Heat-aware estimates
PVWatts and professional design tools model temperature hour by hour. A quote whose production is just nameplate watts times sun-hours will overstate summer output.
Hour-by-hour, for your roof
We model production hour by hour, with temperature, for your roof’s orientation, tilt and shade, so the yearly estimate reflects Valley or coastal heat. Actual output varies with the weather in a given year. Call +1-323-844-7777 or request a free estimate.
Get a free estimateFrequently asked
Do solar panels lose power in the heat?
Yes. A panel’s rating assumes a 25°C cell, and output falls by its temperature coefficient — about 0.30% per °C for a typical 2024–2026 panel — for every degree above that. In NREL’s PVWatts model, Valley roof cells run around 51°C at midday in July and August, or 8–10% below the rating at that moment, and near 78°C in the hottest hours.
How much yearly output does heat cost in Los Angeles?
In our PVWatts runs for a south-facing roof system, about 6% of a year’s output in Northridge with a standard −0.37%/°C panel, and about 4.9% with a typical panel listed since 2024 (−0.30%/°C). Santa Monica loses almost as much — 5.9% with the standard panel — but makes about 4% less energy overall because it gets less sun.
Is summer still the best season for solar in LA?
Yes. Even with 8–9% lost to heat from June to September in the Valley, August is the most productive month in both Northridge and Santa Monica in the model. On the coast, June dips below May because there is less sunlight on the panels, not because of heat.
What is a good temperature coefficient for a solar panel?
Closer to zero is better. The median for 350 W-plus modules added to California’s list since 2024 is −0.30%/°C; about 3% are −0.26%/°C or flatter, and the flattest residential series lists −0.215%/°C. In the Valley, each 0.01%/°C is worth about 0.16% of yearly output — worth comparing, but not more than price, warranty and install quality.
How hot do solar panels get in the San Fernando Valley?
In NREL’s PVWatts model for a roof system in Northridge, cells typically run around 51°C at midday in July and August, reach about 78°C in the hottest hour of the year, and spend about 78 producing hours a year above 60°C. The typical-year weather behind the model peaks at 38°C (100°F) air temperature.
Why doesn't my 400 W panel ever show 400 W?
Because 400 W is measured at a 25°C cell under 1,000 W/m² of light. On a roof the cells are hotter, the sun is rarely that strong at the panel’s angle, and wiring and inverter losses come off the top. California’s more realistic PTC rating for recent modules is about 94.5% of nameplate — roughly 378 W for a 400 W panel — before any of those losses.
Does mounting affect how hot solar panels get?
Yes. A panel close to the roof sheds less heat. NREL’s PVWatts assumes a 49°C NOCT for roof-mounted arrays against 45°C for open racks, and Sandia’s temperature model raises cell temperature as the gap under the panel shrinks. Standard racking leaves an air gap; flush or roof-integrated designs run hotter.
Related reading
Sources & methodology
Figures on this page come from the primary sources below and, where noted, from Cali Energy calculations using the stated assumptions. Rates, incentives, and program terms change; each was verified September 28, 2026.
- NLR (formerly NREL) — PVWatts Calculator
- NLR — PVWatts V8 API documentation (defaults: 14% losses, 96% inverter efficiency, 1.2 DC-to-AC ratio)
- NLR — SAM Help: PVWatts (module types and temperature coefficients; NOCT 49 °C roof mount, 45 °C open rack)
- NLR — PVWatts Version 5 Manual (default loss categories)
- California Energy Commission — Solar Equipment Lists: PV Module List (STC/PTC definitions, γPmax, NOCT)
- Sandia National Laboratories PVPMC — NOCT cell temperature model
Get a heat-aware estimate for your roof
We model production hour by hour, with temperature, for your LA or Valley roof, so the yearly estimate reflects local heat. Actual output varies with each year's weather.
Get a free estimatePrepared by Cali Energy, September 28, 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)