ArrayBudgetPanels, batteries and inverters sized to the load you actually have.

25 kWh a day, 4 peak sun hours, 450 W panels

Array size
8.55kW DC
19 × 450 W panels for 25 kWh a day
Panels (450 W each)19panels
Array size8.12 kW needed, rounded up to whole panels8.55kW DC
Roof area40.7 m² of panel, before spacing and setbacks438ft²
Inverter1.2:1 DC-to-AC ratio7.13kW AC
Battery (1 day backup)651 Ah at 48 V, 80% depth of discharge31.25kWh
Expected yearly output9,612kWh/yr
Derate factor appliedPVWatts default — soiling, heat, wiring, inverter0.77
Daily use
25 kWh
Peak sun hours
4 h
Panel rating
450 W

Where the panel count comes from

25 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 438 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 31.25 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 7.13 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.

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