What a 400W panel actually produces: the loss ledger
Updated July 2026
A 400-watt panel under average US sun delivers about 1.39 kWh a day to your meter, not the 1.8 the sticker arithmetic promises. The missing 23% is not a scandal; it is an itemized list, and knowing the items is how you catch quotes that pretend the list doesn't exist.
Where the sticker number comes from
Panels are rated at Standard Test Conditions: 1,000 W/m² of light, cell temperature of 25°C (77°F), controlled spectrum. STC is honest and universal, which makes it perfect for comparing panels and useless for predicting your roof, because a summer roof is not 25°C. A cell in July sun runs 50 to 65°C, and silicon loses roughly 0.3 to 0.4% of output per degree above rating. Heat alone routinely shaves 8 to 12% on the days with the most sun to lose.
The ledger
| Loss | Typical bite | What it is |
|---|---|---|
| Temperature | 5–12% | Hot cells make fewer volts; worst on still July afternoons |
| Soiling | 2–5% | Dust, pollen, bird contributions; rain does most of the cleaning |
| Wiring & connections | ~2% | Resistance between panel and inverter |
| Module mismatch | ~2% | A string runs at its weakest panel's pace |
| Inverter conversion | 3–4% | DC to AC is efficient, not free |
| Availability & misc. | ~3% | Downtime, snow days, degraded clamps, the real world |
Multiply the survivors together and you land near 0.77, which is exactly the flat derate NREL uses for quick estimates and the one every number on this site uses: watts × peak sun hours × 0.77 = daily AC watt-hours. PVWatts models each line separately for a real address; our flat 0.77 is the honest shortcut, and the methodology page owns it as such.
What this means at quote time
Divide any quote's promised annual kWh by its DC kilowatts, then by 365. You get promised daily kWh per kW, which is just peak sun hours × derate. If the result implies 5.5 sun hours and your state's table says 4.4, someone is selling weather. It also works in reverse: a bid that assumes 4.0 hours in a 5.3-hour state is padding the panel count. The state table and this derate are the only two numbers you need to referee.
Also real but slower: panels age. Warranties typically guarantee 84 to 92% of rated output at year 25, about 0.4% degradation a year. Sizing for day-one output is normal; just don't size an off-grid system with zero margin and expect year-15 December to forgive you.
Questions people ask
How many kWh does a 400W panel produce per day?
About 1.39 kWh at the national-average 4.5 peak sun hours (400 × 4.5 × 0.77), 2.0 in Phoenix sun, 1.1 in Seattle. Per month, multiply by 30.4: roughly 42 kWh at the US average.
Do panels work on cloudy days?
At 10 to 40% of clear-sky output, depending on cloud thickness; the peak-sun-hours figure already averages those days in. What clouds change is the shape of production, not whether the annual math holds.