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Panel output estimator
Updated August 6, 2026 · based on NREL PVWatts sun-hour data · by Cali Energy
How much energy does one solar panel produce? A 400 W panel in Los Angeles (~5.8 peak sun hours) makes about 1.9 kWh a day — roughly 56 kWh a month, or ~680 kWh a year — after a ~0.80 performance factor for real-world losses. Output scales with panel wattage, your local sun hours and how much you lose to heat, shading and wiring.
✓ Uses the standard output formula and NREL PVWatts sun hours. See every figure it uses ↓
kWh/day = watts ÷ 1000 × peak sun hours × performance factor.
Simplified estimate. Real output depends on tilt, azimuth, shading, temperature and soiling — run your exact address in NREL PVWatts.
The rates and figures this calculator usestap to verify
Output = wattage × peak sun hours × a performance factor. Here is every figure it uses — the sun-hour table links to its source.
| Los Angeles | ~5.8 |
| California | ~5.5 |
| U.S. (planning) | ~4.5 |
| Panel wattage | 350 / 400 / 440 W, or custom |
| Performance factor | 0.75–0.85 (0.80 default) |
| Daily output | watts ÷ 1000 × sun hours × factor |
| Monthly / yearly | × 30 / × 365 |
Sun hours are NREL PVWatts/NSRDB annual daily averages; your exact site moves with tilt, azimuth and shading — run your address in PVWatts for a precise figure. The 0.75–0.85 performance factor is a simplified Cali Energy planning factor — not an exact PVWatts derate — that rolls up inverter, wiring, soiling and temperature losses, with 0.80 a reasonable mid-point. This is one panel’s share; a full array is this figure × the number of panels.
Example results you can reproduce
Each row uses a 400 W panel and a 0.80 performance factor — pick the matching sun-hours option above to reproduce it.
| Location (sun hours) | Per day | Per year |
|---|---|---|
| Los Angeles (5.8) | 1.86 kWh | ~677 kWh |
| California (5.5) | 1.76 kWh | ~642 kWh |
| U.S. (4.5) | 1.44 kWh | ~526 kWh |
How this calculator works
It multiplies the panel’s wattage (in kW) by your local peak sun hours and a performance factor, then scales to a month and a year. Peak sun hours are a way of packing a whole day’s sunlight into a number of full-strength hours — Los Angeles averages about 5.8. Pick a preset location or type your own peak-sun-hours value if you know it. The performance factor (0.75–0.85) is what turns the lab nameplate into a real-world number, capturing inverter, wiring, soiling and heat losses. It’s a simplified estimate; PVWatts models your exact address.
How to read your result
You get one panel’s output per day, month and year. The nameplate wattage is a lab rating (STC); the daily kWh here is what you’d actually harvest on an average day. Multiply by your panel count for a whole-array figure — ten of these 400 W panels in LA make roughly 6,800 kWh a year.
What moves the result the most
Three levers: panel wattage (a 440 W panel makes about 10% more than a 400 W one), your local sun hours (LA’s 5.8 beats the U.S. planning figure of ~4.5 by nearly a third), and the performance factor — a shaded, hot or poorly-oriented install slides toward 0.75, a clean open roof toward 0.85.
Common questions
What is a peak sun hour?
One hour of full-strength sunlight (1,000 W/m²). A day’s scattered sunlight is expressed as an equivalent number of these full hours — about 5.8 a day in Los Angeles on an annual average.
Why is real output below the nameplate rating?
The wattage on the label is measured in lab conditions (STC). In the field, heat, wiring, inverter conversion, soiling and imperfect angles all shave output — which is exactly what the 0.80 performance factor represents.
What is the performance factor?
A single multiplier (here 0.75–0.85) that rolls up all the real-world losses between the panel’s rating and the energy you actually meter. 0.80 is a sensible default; a clean, cool, unshaded install can reach 0.85.
Does a 440 W panel make much more than a 400 W?
Yes, proportionally — about 10% more energy for the same sun and losses, since output scales with wattage. It also means slightly fewer panels for the same system size.
Does temperature really matter?
It does — panels lose a little output as they heat up, which is one reason a hot rooftop underperforms its lab rating. That loss is bundled into the performance factor here rather than modeled hour by hour.
Is this exact for my roof?
No — it’s a simplified estimate. For a precise, address-specific figure that accounts for your tilt, azimuth, shading and weather, run the free NREL PVWatts calculator.
Sources
- NREL — PVWatts Calculator
- NREL — PVWatts v8 documentation (system losses)
- NREL — National Solar Radiation Database (NSRDB)
- DOE — Solar Performance and Efficiency
Estimates only, not a quote. Assumptions, rates, incentives and program rules can change — confirm current details with your utility, the program administrator or a licensed professional.
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