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The short answer
A panel rated at 400W produces about 280–320W in genuinely good conditions, and far less the rest of the time. Over a day in summer that is roughly 1.6–2.2kWh; in midwinter it can be under a third of that.
Over a full year, the same 400W panel tilted toward the south makes roughly 450–720kWh across ten US cities in the PVWatts solar model, from Seattle at the low end to Phoenix at the top. That is about 1,100–1,800kWh a year for every kilowatt of rated panel. The city table has the figure for each.
The rating on the box is a laboratory number, measured under Standard Test Conditions that your roof will essentially never reproduce.
Plan on 70–80% of the rated figure in good conditions, and as little as 10–25% of it under heavy overcast (more like 30–60% through light haze or broken cloud). Size around that and you will not be disappointed.
Why the rating overstates it
- Temperature. Panels are rated at 25°C. They get much hotter in real sun, and output falls as they heat.
- Angle. Rated output assumes light hitting the panel square on. A flat or badly angled panel loses a lot.
- Cloud and haze. Even thin cloud cuts output sharply.
- Dirt. Dust and pollen measurably reduce it.
- Conversion losses. Charge controllers and inverters take their cut.
Realistic daily output
| Panel | Summer day | Winter day, northern states | Winter day, central and southern states | Runs |
|---|---|---|---|---|
| 100W | 0.4–0.5kWh | 0.1–0.2kWh | 0.3–0.4kWh | Phones, lights |
| 200W | 0.8–1.1kWh | 0.2–0.4kWh | 0.6–0.8kWh | Laptop, 12V fridge |
| 400W | 1.6–2.2kWh | 0.5–0.9kWh | 1.2–1.7kWh | Refills a mid-size battery |
| 800W | 3.1–4.3kWh | 1.0–1.8kWh | 2.4–3.4kWh | A van or small cabin |
Summer is an average June day and winter an average December day for a panel tilted 30° toward the south, scaled from the PVWatts runs in the city table below. The northern column covers Seattle, Chicago and Minneapolis, where a tilted panel gets about 1.5–2.5 peak sun hours a day in December. In the other seven cities in the table, from Kansas City and Denver down to Phoenix and Miami, it gets about 3.5–5.3. Arizona and Maine are not the same place: if you live away from these cities, run your own address with the method below.
Yearly output by city
These figures come from PVWatts, the free solar model run by the Department of Energy’s National Laboratory of the Rockies (formerly NREL). They were run on October 13, 2026 for one 400W panel tilted 30° and facing due south on an open rack, with the model’s default 14% system losses. Each is for a typical year of weather from the National Solar Radiation Database, so a real year will land somewhat above or below.
| City | 400W panel, per year | Per rated kW, per year | June day | December day |
|---|---|---|---|---|
| Phoenix | 719kWh | 1,800kWh | 2.2kWh | 1.7kWh |
| Los Angeles | 685kWh | 1,710kWh | 2.1kWh | 1.4kWh |
| Denver | 659kWh | 1,650kWh | 2.0kWh | 1.4kWh |
| Miami | 619kWh | 1,550kWh | 1.6kWh | 1.5kWh |
| Dallas | 610kWh | 1,530kWh | 1.9kWh | 1.4kWh |
| Kansas City | 587kWh | 1,470kWh | 1.9kWh | 1.2kWh |
| Atlanta | 577kWh | 1,440kWh | 1.8kWh | 1.2kWh |
| Minneapolis | 553kWh | 1,380kWh | 1.8kWh | 0.9kWh |
| Chicago | 535kWh | 1,340kWh | 1.9kWh | 0.8kWh |
| Seattle | 448kWh | 1,120kWh | 1.7kWh | 0.5kWh |
Location matters far more in winter than in summer. In June the ten cities sit within about 0.6kWh a day of each other; in December a panel in Phoenix makes more than three times what the same panel makes in Seattle. Miami is the odd one out: its best month in the model is March, and its June day is about the same as its December day.
PVWatts reports AC output from a grid-tied system, after an inverter rated at 96% efficiency; output before the inverter runs about 4–5% higher. A panel charging a power station feeds the station’s own charge controller instead, which takes its own cut, so treat these figures as a guide to what reaches the battery rather than an exact count.
Work it out for your own roof or balcony
The quick version: multiply the panel’s rating in kilowatts by the per-kW figure for the nearest city in the table. A 300W panel in Chicago comes to 0.3 × 1,340 = about 400kWh a year. For your exact spot, run PVWatts yourself:
- Location. Enter your address or ZIP code. PVWatts picks the nearest weather data.
- System size. Enter the panel’s rated watts as kilowatts: 0.4 for one 400W panel, 0.8 for two.
- Array type. Fixed (open rack) for a panel on a stand, frame or railing with air behind it. Fixed (roof mount) for panels close to a roof, which run hotter and make a little less.
- Tilt and azimuth. Tilt is the angle up from flat: 0° lying flat, 90° for a panel hung upright on a railing. Azimuth is the direction it faces, and 180° is due south. What facing west or hanging a panel vertically costs is worked through in our plug-in solar guide.
- System losses. Leave the 14% default for a spot with little or no shade. It already allows for dirt (2%), a little shading (3%), wiring and connections, mismatch between panels, the small drop new panels show in their first months, panels testing slightly under their label and 3% for downtime. Raise it if trees or buildings shade the panel for part of the day.
- Read the results by month. Divide a month’s output by its number of days for a daily figure. If the panel has to keep up year-round, size around the worst month, not the annual average.
Do not discount the result again for heat or the inverter. PVWatts models panel temperature from the local weather and treats inverter efficiency separately from the 14%, as its technical manual sets out. Heat, angle, cloud and dirt from the list at the top of this page are already inside the number.
What that means for a power station
To refill a 1,000Wh station like the EcoFlow DELTA 3 Classic Solar Generator in a single good day you want around 400W of panel. For a 2,000Wh unit such as the Bluetti AC200L, closer to 600–800W if you want it full by evening.
Full method in how to size a solar system.
Check the input limits before you buy panels
Every station states a maximum solar input in watts and a voltage window. Too many watts simply wastes the surplus. Too much voltage can damage the unit — and panel voltage peaks on cold bright mornings, exactly when people get caught out.
See solar panels and kits for matching panels to a station, or the picker if you are still choosing the battery.