A panel's wattage rating is a lab number, measured at exactly one sunlight intensity and one cell temperature. The sky almost never delivers that exact condition, so a panel almost never makes its nameplate number. That is not a defect and nobody is cheating you. Real output is the rating scaled by how much sun actually arrived and how hot the cells actually ran, and this guide shows you how to do the scaling yourself.
Sunlight has a strength, and we measure it
Sunlight hitting a surface carries power, measured in watts per square meter. The lab condition every panel is rated at, called STC (Standard Test Conditions), pins that at 1000 W/m² with the cells at 25°C. That 1000 figure is roughly what a clear sky delivers at midday with the sun high and the panel aimed straight at it. It is the definition of full sun for this whole industry.
Most of the day is not that. Morning and evening sun comes in at a low angle and through more atmosphere, so the panel might see 200 or 400 W/m². Clouds knock it down further. The datasheet for the Canadian Solar panels we stock actually plots this: its curves show output at 1000, 800, 600, 400, and 200 W/m², and output tracks the light almost proportionally. Half the sun, roughly half the watts.
Peak sun hours: the day compressed
You could add up the whole day's changing sunlight minute by minute, or you can do what the industry does and compress it. One peak sun hour means one hour's worth of full sun: 1 kWh of sunlight energy landing on each square meter. A day that delivers 5 kWh/m² of total sunshine, spread across 12 daylight hours of weak-to-strong light, counts as 5 peak sun hours.
That gives you the fastest useful estimate in solar: panel watts times sun hours equals watt-hours per day, as a ceiling. A 395W panel in 5 sun hours makes about 2kWh. Treat it as a ceiling on purpose, because clouds, heat, and roof angle all shave it.
For Gulf Breeze and the Florida panhandle, plan on roughly 4.5 to 5.5 peak sun hours a day depending on season. That range is approximate and stated here for scale only. For a real address, run PVWatts (NREL's free estimator) with the actual location, tilt, and azimuth, and use that number instead. Nobody should promise you production from the rough range, and we do not.
The part sun hours miss: heat
Here is the twist that surprises people. Panels lose output as they get hot, and Florida panels run hot. The same datasheet gives a second ratings table at NMOT (Nominal Module Operating Temperature), a condition built to look like real weather: 800 W/m², 20°C air, a light breeze, cells running at their natural operating temperature of about 42°C. Under those conditions the 395W panel produces 295W.
Worth pulling apart, because two effects hide in that one number. Dropping the light from 1000 to 800 W/m² would predict about 80 percent of nameplate, call it 316W. The remaining gap down to 295W is temperature: the cells sit around 42°C instead of 25°C, and this panel gives up 0.34 percent of its power for every degree above 25. Seventeen degrees times 0.34 percent is close to a 6 percent loss, and there are your missing 20 watts. Neither effect is a flaw. It is the same panel, honestly rated, meeting actual weather.
A worked example with real gear
Our stock off-grid design carries 24 of the Canadian Solar 395W panels, 9.48kW of DC. At an approximate 5 peak sun hours, the ceiling math says 9.48 times 5, about 47kWh per day.
A real day lands under that. Heat takes its cut as above. Wiring and the inverter's DC-to-AC conversion take small cuts, and shade or a string of cloudy afternoons take theirs. PVWatts models all of it for a specific address and tilt. The ceiling number is still worth computing first, because it catches nonsense fast: if someone claims that array will make 80kWh a day in December, no sun-hour figure on Earth supports it.
Season matters too. Summer days are long but hot, winter days are short but cool, and the design low temperature on our Pensacola-area jobs is 25°F. Sun hours swing across the year, which is why honest battery sizing works from the worst month, never the average.
Why the rating still matters
If the nameplate is a number the panel rarely produces, why size everything by it? Because it is the one condition every manufacturer measures identically. STC is the common yardstick. It lets a 395W panel from one factory be compared honestly against a 585W panel from another, and it is the number all the string math and code calculations key off. Treat the rating as the panel's position on a scale everyone shares, and treat daily output as a separate question, the one sun hours and PVWatts exist to answer.
Try this at home
Run PVWatts for your own address. It is free, and the short version needs just your location, roof tilt, and direction. Then do the napkin version for the same array: watts times sun hours. The gap between the two numbers is this whole lesson, and knowing both makes you very hard to oversell. When you are ready, bring either number to the system builder.
Sources
- Canadian Solar HiKu6 All-Black datasheet, models CS6R-380 to 405MS-HL, v1.1C25, Aug 2022: the STC definition, the NMOT condition and its 295W output for the 395W class, the 42°C nominal operating temperature, the 0.34 percent per degree power coefficient, and the output curves at 200 to 1000 W/m².
- The 316W and 20-watt decomposition is arithmetic from those datasheet values, labeled approximate.
- The 24-panel, 9.48kW reference array is a stock Ape Solar off-grid design.
- Pensacola-area design low of 25°F: ASHRAE design temperature data for Pensacola Regional Airport, as recorded on our area designs.
- PVWatts is NREL's free production estimator at pvwatts.nrel.gov.
- Gulf Breeze at 4.5 to 5.5 peak sun hours is an approximate seasonal range for scale only. Verify any real address with PVWatts.
- The peak-sun-hour definition and the near-proportional output-versus-irradiance relationship are standard physics and industry definitions, no document needed.