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 1.6–2.0kWh; in midwinter it can be under a third of that.
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 roughly half on an overcast day. 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 | Runs |
|---|---|---|---|
| 100W | 0.4–0.5kWh | 0.15–0.2kWh | Phones, lights |
| 200W | 0.8–1.0kWh | 0.3–0.4kWh | Laptop, 12V fridge |
| 400W | 1.6–2.0kWh | 0.6–0.8kWh | Refills a mid-size battery |
| 800W | 3.2–4.0kWh | 1.2–1.6kWh | A van or small cabin |
Based on roughly 4–5 usable sun hours in summer and 2–3 in winter at mid-northern latitudes. Arizona and Maine are not the same place.
What that means for a power station
To refill a 1,000Wh station like the EcoFlow Delta 2 in a single good day you want around 400W of panel. For a 2,000Wh unit such as the Bluetti AC200LCheck price at BLUETTI, 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.