40 kWh a day, 6 peak sun hours, 350 W panels
| Panels (350 W each) | 25panels |
|---|---|
| Array size8.66 kW needed, rounded up to whole panels | 8.75kW DC |
| Roof area41.7 m² of panel, before spacing and setbacks | 448ft² |
| Inverter1.2:1 DC-to-AC ratio | 7.29kW AC |
| Battery (1 day backup)1,042 Ah at 48 V, 80% depth of discharge | 50kWh |
| Expected yearly output | 14,755kWh/yr |
| Derate factor appliedPVWatts default — soiling, heat, wiring, inverter | 0.77 |
Notes
- 6 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.
Where the panel count comes from
40 kWh a day divided by 6 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 25 panels and about 448 ft² of roof.
Peak sun hours is not daylight hours
6 peak sun hours does not mean 6 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 50 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 7.29 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.
Nearby sizes
Read more
- Solar Cost per Watt — the Only Unit That Makes Two Quotes ComparableTwo quotes for "a solar system" are not comparable. Two quotes at dollars per watt are, and the conversion takes one division.
- Backup Generator vs Solar Battery — Cost per Hour of Outage CoveredA battery is cheaper to own and runs out. A generator is cheaper to run and never does. Which one wins is decided by how long your outages actually last.
- Solar Payback Period — the Four Inputs, and the One That Decides ItPayback is one division with four inputs. Three of them are knowable to within a few percent. The fourth — what your utility pays for exports — swings the answer by more than a decade.
- Home Solar Battery Cost — Priced per Usable Kilowatt-HourStorage is the worst-value component of a solar system on pure energy arithmetic, and often the right purchase anyway. Which of those is true depends on why you are buying it.