Start with the battery, not the panels
Panels exist to refill a battery. So the sequence is: work out your daily consumption, size the battery to cover it with buffer, then size panels to replace that consumption in the daylight you actually get.
Doing it the other way round — buying panels first — is how people end up with 800W of panel feeding a battery that cannot absorb it.
Panels never make their rated output
A 200W panel is rated under laboratory conditions: perfect angle, specific temperature, full sun. Reality includes cloud, dirt, heat, and a panel lying flat instead of aimed.
Plan on 70–80% of the rating in good conditions, and roughly half on an overcast day. A 400W array realistically returns 280–320W at midday in summer, and far less in December.
Working out panel wattage
Take your daily consumption in watt-hours and divide by the usable sun hours where you are — typically 4 to 5 in summer, 2 to 3 in winter at northern latitudes. Then add 30% for the losses above.
| Daily use | Summer panel need | Winter panel need |
|---|---|---|
| 500Wh | ~150W | ~300W |
| 1,000Wh | ~300W | ~600W |
| 1,500Wh | ~400W | ~800W |
| 2,000Wh | ~550W | ~1,100W |
Check the input limits
Every power station states a maximum solar input in watts and a voltage window. Exceeding the wattage simply wastes the surplus. Exceeding the voltage can damage the unit. Panels wired in series add voltage; in parallel they add current. Check both against the station before buying.
The most common wiring mistake. Wiring several panels in series to reach a higher voltage, then discovering the total exceeds the station’s ceiling on a cold bright morning — when panel voltage peaks. Leave margin.
Worked example: a van
Fridge 500Wh, Starlink 400Wh, lights and devices 300Wh gives about 1,200Wh a day. That points to a 2,000Wh battery for a day and a half of buffer, and 400W of panel to refill it in summer. In winter you will be topping up from the alternator or shore power, and no realistic van roof changes that.