20 kWh a day, 4 peak sun hours, 350 W panels
| Panels (350 W each) | 19panels |
|---|---|
| Array size6.49 kW needed, rounded up to whole panels | 6.65kW DC |
| Roof area31.7 m² of panel, before spacing and setbacks | 341ft² |
| Inverter1.2:1 DC-to-AC ratio | 5.54kW AC |
| Battery (1 day backup)521 Ah at 48 V, 80% depth of discharge | 25kWh |
| Expected yearly output | 7,476kWh/yr |
| Derate factor appliedPVWatts default — soiling, heat, wiring, inverter | 0.77 |
- Daily use
- 20 kWh
- Peak sun hours
- 4 h
- Panel rating
- 350 W
Where the panel count comes from
20 kWh a day divided by 4 peak sun hours would suggest a smaller array than the one here. The gap is the derate factor of 0.77 — the PVWatts default that accounts for heat, soiling, wiring losses, imperfect orientation and inverter efficiency.
Skip it and you build an array that falls a quarter short of its nameplate, every day, forever.
That works out to 19 panels and about 341 ft² of roof.
Peak sun hours is not daylight hours
4 peak sun hours does not mean 4 hours of sunshine. It is the equivalent number of hours at full rated irradiance — a long hazy day and a short bright one can give the same figure. It is also an annual average: December in most of the US delivers 40–60% of it.
Battery and inverter
The 25 kWh battery covers one day with nothing coming in, at 80% depth of discharge. Off-grid with no generator, plan on three to five days instead.
The inverter at 5.54 kW is deliberately smaller than the array. A 1.2:1 DC-to-AC ratio clips a handful of peak hours a year and costs far less than the inverter that would catch them.
Notes
- 4 peak sun hours is a daily average across the year. Winter output in most of the US runs 40–60% of the annual average, so a system sized to the average will fall short in December unless you oversize or stay grid-tied.
- Roof area here is panel area only. Add 15–25% for row spacing, fire setbacks and obstructions — and check that the roof plane actually faces within about 45° of south.
- One day of battery backup covers an evening and a night. Off-grid systems with no generator normally carry three to five days.
Nearby sizes
- 15 kWh a day, 4 peak sun hours, 350 W panels
- 25 kWh a day, 4 peak sun hours, 350 W panels
- 10 kWh a day, 4 peak sun hours, 350 W panels
- 30 kWh a day, 4 peak sun hours, 350 W panels
- 20 kWh a day, 3.5 peak sun hours, 350 W panels
- 20 kWh a day, 4.5 peak sun hours, 350 W panels
- 20 kWh a day, 3 peak sun hours, 350 W panels
- 20 kWh a day, 5 peak sun hours, 350 W panels
- 20 kWh a day, 4 peak sun hours, 400 W panels
- 20 kWh a day, 4 peak sun hours, 450 W panels