How Much Energy Does a Solar Panel Produce?
A single modern 400-watt panel in a sunny place like Los Angeles produces roughly 1.7–1.8 kilowatt-hours (kWh) a day — about 600–650 kWh a year. But that headline number hides everything worth understanding: what the wattage rating actually means, why real output is well below the “nameplate,” how location changes the answer, and how to estimate it for your own roof. This guide walks through the full picture, with a transparent formula you can check yourself.
Written by Cali Energy Research Team · Fact-checked by Cali Energy, CSLB #1032379 — B, C-10, C-39 · Last reviewed: August 3, 2026
- Output = panel kW × peak sun hours × a performance factor (for real-world losses).
- The watt rating is a lab (STC) figure; real output runs 15–25% lower after losses.
- A 400 W panel in LA makes roughly 1.7–1.8 kWh/day ≈ 600–650 kWh/year (estimate).
- Location is decisive — the same panel produces more in sunny LA than in a cloudier region.
The output formula
Every installer estimates panel output from the same simple relationship:
Daily kWh = (panel watts ÷ 1000) × peak sun hours × performance factor
For a 400 W panel in Los Angeles: 0.4 kW × ~5.5 peak sun hours × ~0.80 = ~1.76 kWh/day, or about 640 kWh/year. Change any input — a bigger panel, a sunnier or cloudier location, more shading — and the number moves. The rest of this guide unpacks each term.
What the watt rating really means (STC)
A panel labeled “400 W” is rated under Standard Test Conditions (STC): 1,000 watts per square meter of sunlight, a cell temperature of 25 °C, and a defined air mass. Those are laboratory conditions. On a real roof — hotter cells, imperfect angle, some haze — a panel rarely hits its nameplate. The rating is a comparison benchmark, not a promise of continuous output. See how panels are rated and compared.
Reference: DOE — Solar Performance and Efficiency.
Peak sun hours: the location term
“Peak sun hours” (PSH) is the number of hours per day that sunlight would need to shine at exactly the STC intensity (1,000 W/m²) to deliver the same total energy your location actually receives. It’s a convenient way to compress a whole day’s sunlight into one number. Crucially, PSH is a solar-resource input — it is not the same as your finished output; you still multiply by panel size and a performance factor.
Solar resource is mapped by NREL’s National Solar Radiation Database (NSRDB). As rough, approximate annual daily averages: much of the U.S. sits around ~4–5 PSH, California ~5, and sunny Los Angeles ~5.5 (values vary by exact location, dataset and how PSH is defined). Because these numbers move with your precise coordinates, treat them as ballpark and confirm with a modeled run.
Solar-resource data: NREL NSRDB.
The performance factor: where the losses go
A panel never delivers its full nameplate to your meter. Between the cells and the kilowatt-hours you actually use, real systems lose roughly 15–25% to a stack of effects. For a quick hand calculation, we bundle all of them into a single performance factor of about 0.75–0.85 — a Cali Energy simplified modeling assumption, not a universal constant (we use 0.80 as a middle estimate). The main contributors:
Temperature
Cells above 25 °C produce less than their STC rating — a real factor on hot LA roofs. See heat & solar output.
Inverter & wiring
Converting DC to AC and moving it through wires loses a few percent. See what an inverter does.
Soiling & shading
Dust, and shade from trees or chimneys, cut output. See shade & trees.
Orientation & mismatch
Less-than-ideal tilt or azimuth, and small panel-to-panel differences, trim the total.
You’ll sometimes see the single number “0.83” quoted as a PVWatts default — that’s a misreading. NREL’s PVWatts uses a default 14% system-loss input and separately models temperature, the inverter and the DC/AC ratio, so it does not reduce to one flat factor. Our simplified 0.75–0.85 performance factor is a hand-calc shortcut; PVWatts is the rigorous tool.
Estimate a panel’s output
Pick a panel wattage and your location’s peak sun hours to see a simplified daily, monthly and yearly estimate. This is a hand calculation — PVWatts models more detail.
Solar panel output estimator
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.
Output by panel wattage (Los Angeles estimate)
Using one transparent method — ~5.5 peak sun hours and a 0.80 performance factor (an LA illustration) — here is roughly what one panel produces by size:
| Panel | Per day | Per month | Per year |
|---|---|---|---|
| 350 W | ~1.54 kWh | ~46 kWh | ~562 kWh |
| 400 W | ~1.76 kWh | ~53 kWh | ~642 kWh |
| 440 W | ~1.94 kWh | ~58 kWh | ~707 kWh |
Computed as (W÷1000)×5.5×0.80×{365, 30}. An LA illustration; your address will differ — confirm with PVWatts.
From one panel to a whole system
Scale the same formula up. Per kilowatt of panels, LA produces on the order of 1,600–1,800 kWh per year in a PVWatts run with typical fixed-tilt residential assumptions (roughly 20° tilt, south-facing, standard module, 14% losses) — so a 6 kW system lands near ~10,000–11,000 kWh/year. Your real figure depends on roof pitch, direction and shading, which is exactly what a per-address PVWatts run captures. To go from output to number of panels for a target bill, see how many solar panels you need.
Output changes over the day, season and years
Across the day
Output follows the sun — near zero at dawn/dusk, peaking around solar noon. Peak sun hours already average this out.
Across the year
Long summer days produce more than short winter ones. Annual figures blend both. See solar in winter.
Across decades
Panels slowly degrade — an illustrative ~0.5%/year — so output drifts down gradually. See degradation.
Sources & methodology
Primary sources: NREL — PVWatts Calculator · NREL — PVWatts v8 documentation (system losses) · NREL — National Solar Radiation Database (NSRDB) · DOE — Solar Performance and Efficiency. The hand-calculation on this page uses a simplified performance factor (0.75–0.85) to bundle real-world losses; NREL PVWatts is more rigorous, applying a default 14% system-loss input plus separate temperature, inverter and DC/AC modeling. Solar-resource (peak-sun-hour) figures come from NREL’s NSRDB and are approximate; the only reliable number for a specific home is a PVWatts run for that address. All figures are estimates, not guarantees.
Frequently asked
How much electricity does a solar panel produce per day?
A modern 400 W panel in a sunny location like Los Angeles produces roughly 1.7–1.8 kWh per day. The estimate comes from the formula (watts ÷ 1000) × peak sun hours × a performance factor — e.g. 0.4 kW × ~5.5 PSH × ~0.80 ≈ 1.76 kWh. Cloudier locations, shading or a smaller panel lower it.
How much energy does a solar panel produce per year?
About 600–650 kWh per year for a 400 W panel in Los Angeles (roughly daily output × 365). Per kilowatt of panels, LA yields on the order of 1,600–1,800 kWh/year in a typical PVWatts run. Your exact figure depends on tilt, direction, shading and local weather — a per-address PVWatts run is the reliable number.
What are peak sun hours?
Peak sun hours (PSH) is the number of hours a day that sunlight would have to shine at the standard test intensity (1,000 W/m²) to equal the total solar energy your location actually receives. It’s an input to the output formula, not the output itself. Rough annual daily averages: U.S. ~4–5, California ~5, Los Angeles ~5.5 — values vary by exact location and dataset.
Why is a panel's real output less than its watt rating?
The watt rating is measured under lab Standard Test Conditions (1,000 W/m², 25 °C). Real roofs are hotter, angled imperfectly and occasionally shaded or dusty, and converting DC to AC loses a little more. Together these trim output by roughly 15–25%. For the simplified calculator on this page, Cali Energy uses a performance-factor range of about 0.75–0.85 — a modeling assumption, not a universal system-loss constant; PVWatts models several losses and operating conditions separately.
How do I calculate my own solar panel output?
Use (watts ÷ 1000) × peak sun hours × performance factor for a daily estimate, then × 365 for annual. Plug your panel wattage, your area’s peak sun hours and ~0.80 into the calculator on this page. For a precise, address-specific figure that accounts for your roof’s tilt, direction and shading, run the free NREL PVWatts tool.
Related reading
About this guide
An educational reference from Cali Energy's research team. The estimates here are simplified; for a production figure specific to your roof, run your address in NREL's free PVWatts calculator.
Prepared by Cali Energy, August 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)