How Much Did LA Heat Waves Cut Rooftop Solar Output? Every Heat Wave Since 2022, Measured on 215 Roofs
Less than most people expect. On clear days of 95 °F or more, the median rooftop system in our fleet made 2.6% less per unit of sunlight than it did on clear days below 85 °F in the same weeks — about 2% for every 10 °F. Because heat-wave days in Los Angeles are also clear, the day’s total stayed close to normal: 3% below a mild clear day and about the same as a typical day of those weeks. Over a whole year, heat waves cost the median system about 0.2% of its output.
Solar cells lose a little voltage as they warm, so a panel on a 100 °F afternoon turns less of the sunlight that reaches it into power than it does on a cool spring day. The physics is laid out in how heat affects solar output in LA; below is what it adds up to on real roofs.
By daily high
Clear, smoke-free days, each system against its own mild clear days (below 85 °F) within 15 days.
| Daily high (ERA5) | Systems | System-days | Per unit of sunlight | Day total vs mild clear day | Day total vs typical day | Sunlight vs mild clear day |
|---|---|---|---|---|---|---|
| 85–89 °F | 216 | 13,421 | +0.2% | +0.6% | +3.4% | +0.1% |
| 90–94 °F | 215 | 8,153 | −0.9% | −1.1% | +2.1% | −0.6% |
| 95–99 °F | 214 | 3,050 | −2.2% | −2.4% | +1.5% | +0.0% |
| 100–104 °F | 205 | 759 | −4.1% | −5.6% | −1.2% | −2.7% |
| 105 °F or more | 143 | 347 | −3.1% | −3.3% | −1.0% | +2.2% |
| Each system’s average over its days in the band, then the median across systems. “Typical day”: the median of all the system’s days within 15 days, any weather. | ||||||
The loss builds steadily up to about 104 °F: +0.2% at 85–89 °F, −0.9% at 90–94 °F, −2.2% at 95–99 °F and −4.1% at 100–104 °F. The 143 systems with days of 105 °F or more lost 3%, no more than at 100 °F; those days came in only a few episodes, and a daily high says nothing about wind, which cools panels. Fitted system by system, the loss works out to 2.2% per 10 °F of daily high (middle half of systems 1.1–3.0%). That is an effective figure for the whole system, not a panel’s temperature coefficient, which is quoted per °C of cell temperature.
https://cali-energy.com/research/heat-wave-solar-output-los-angeles. Aggregated data are free to use under CC BY 4.0 with a link to this page.Every heat wave since August 2022
Runs of hot days at our San Fernando Valley weather point, with at least 40 measured system-days.
| Heat wave | Peak daily high, Valley (ERA5) | Systems | System-days | Per unit of sunlight | Day total vs mild clear day | Day total vs typical day |
|---|---|---|---|---|---|---|
| Aug 30 – Sep 9, 2022 | 107 °F | 50 | 67 | −6.5% | −6.8% | −2.3% |
| Sep 26–28, 2022 | 99 °F | 46 | 46 | −2.6% | −9.9% | +1.3% |
| Jul 14–17, 2023 | 96 °F | 55 | 220 | −1.2% | −5.5% | −0.6% |
| Aug 27–30, 2023 | 101 °F | 56 | 162 | −5.7% | +2.9% | +5.1% |
| Jul 5, 2024 | 101 °F | 64 | 64 | −5.2% | −5.5% | −1.9% |
| Aug 2–6, 2024 | 102 °F | 71 | 213 | −1.8% | −6.4% | +1.2% |
| Sep 3–10, 2024 | 112 °F | 150 | 665 | −2.9% | −2.3% | −0.4% |
| Oct 1–3, 2024 | 101 °F | 74 | 148 | −3.3% | −2.8% | +0.3% |
| Aug 19–28, 2026 | 101 °F | 172 | 730 | −3.7% | −0.9% | +0.3% |
| Oct 2–7, 2026 | 101 °F | 169 | 499 | −2.7% | −9.2% | −1.8% |
| Only clear, smoke-free days with mild clear days of the same system within 15 days. Heat waves with too few such days are left out. | ||||||
The deepest loss came in the long heat wave of Aug 30 – Sep 9, 2022: 6.5% per unit of sunlight on the median system, with a wide spread between roofs. It also had the fewest mild days nearby to compare against (50 systems, 67 system-days), so it is the least certain figure. The hottest by our weather data, Sep 3–10, 2024, reached 112 °F at the Valley point and cost 2.9% on 150 systems. On our daily fleet index that heat wave does not stand out: the fleet made 88–92% of a clear day during it and 90–92% in the days before.
Heat, smoke and October 2024
Heat waves in Los Angeles often arrive with smoke, and smoke cuts output for a different reason. The figures above are for smoke-free days, so they show heat alone. What smoke does is measured in did wildfire smoke cut rooftop output.
Heat does not explain the weak October of 2024. The October 1–3 heat wave cost 3.3% per unit of sunlight, like the others, and the fleet index was no higher on the milder days that followed: 80–85% of a clear day on October 8–13, with Valley highs of 78–87 °F, against 83–86% on the three hot days. Whatever held October down, it was not the heat. How that month compares with other years is in why solar production drops from year to year.
Valley, basin and beyond
| Area (weather point) | Days of 95 °F+ a year, 2023–2025 (ERA5) | Systems | System-days | Per unit of sunlight | Day total vs mild clear day |
|---|---|---|---|---|---|
| San Fernando Valley | 18 | 113 | 2,416 | −2.5% | −2.9% |
| Santa Clarita Valley | 37 | 10 | 366 | −3.0% | −1.7% |
| Los Angeles basin | 8 | 85 | 1,334 | −2.5% | −4.0% |
| Ventura County | 2 | 7 | 40 | −3.4% | −7.4% |
| Ventura County and the Santa Clarita Valley have few systems in our fleet; treat their rows as a signal only. | |||||
Per hot day, roofs in the San Fernando Valley and the Los Angeles basin lost the same share of output per unit of sunlight, 2.5% at the median. The difference between areas is how often it happens: our Valley weather point saw about 18 days of 95 °F or more a year in 2023–2025, the Santa Clarita Valley about 37, the basin 8 and Ventura County 2. ERA5 highs are averages over a grid cell about 25 km across and run below the hottest station readings, so real roofs saw more heat than these counts suggest.
What it means for a homeowner
A heat wave is not a reason to call for service. Expect the day’s total to be within a few percent of a normal sunny day; if production falls by a third, something else is going on: smoke, cloud, or a fault worth checking. The year-long cost of hot weather in general is larger than the cost of heat waves alone: against the 77 °F (25 °C) cell temperature panels are rated at, warm cells cost a Valley system about 5–6% of a year in NREL’s model, as the heat explainer shows. What a heat wave changes more is the other side of the meter: on 95 °F days the metered homes we monitor used about 2.1 times the electricity of a 65–75 °F day, measured in how much electricity LA solar homes use.
Methodology
- Systems
- 215 Enphase-monitored rooftop systems in Los Angeles and Ventura counties with a verified DC rating, daily production checked as in the production benchmark.
- Weather
- ERA5 reanalysis (Open-Meteo) at four area points: daily high and ground-level sunlight (GHI). A clear day has at least 90% of the point’s clear-sky sunlight for that time of year. Aerosol: CAMS daily mean optical depth at 550 nm; days at 0.20 or more were treated as smoky or dusty and dropped.
- Comparison
- Each hot day (high of 95 °F or more) against the same system’s clear, smoke-free days below 85 °F within 15 days of the same year, at least three of them. “Per unit of sunlight” divides each day’s kWh per kW by that day’s GHI.
- Annual cost
- For each system-year 2023–2025 with at least 330 days: each day’s output times the median loss for its temperature band, as a share of the year’s output.
- Limits
- Daily totals only: the midday loss is larger than the daily figure, and hourly data are not available. ERA5 highs run below station readings. Period: August 4, 2022 (start of the aerosol data) to October 2026; the last hot day measured was October 4, 2026.
Frequently asked questions
Do solar panels produce less during a heat wave in Los Angeles?
A little. On clear days of 95 °F or more, rooftop systems in our fleet made a median 2.6% less per unit of sunlight than on mild clear days of the same weeks, and about 4% less at 100–104 °F.
How much output is lost per 10 °F?
About 2% per 10 °F of daily high for the median system, measured system by system; most systems fell between 1% and 3%.
Why didn't my production drop much on a 100 °F day?
Because hot days in LA are usually clear. The day’s total on a 95 °F+ day was +0.8% against a typical day of the same weeks.
How much does heat cost over a year?
Heat waves cost the median system about 0.2% of a year’s output against mild weather of the same weeks. Warm cells in general cost more, about 5–6% a year against the panel’s rating, in NREL’s model.
Do San Fernando Valley roofs lose more in the heat than roofs in the LA basin?
Per hot day, no: 2.5% per unit of sunlight in the Valley and 2.5% in the basin at the median. The Valley simply has more hot days.
More research
Next steps
Output looks low in the heat?
Prepared by Cali Energy, October 15, 2026. Figures describe systems in our monitoring fleet and are not a guarantee of what any specific roof will produce; orientation, tilt, shade, equipment and weather change results. Not engineering, financial or utility advice. See our Content Disclaimer.