How many solar panels do I need?
The panel-count math in one line, worked examples from a fridge to a full house, and why batteries change the answer.
The math
Every panel-count question reduces to the same division chain — the energy you use per day, spread across the sun hours you actually get, adjusted for real-world losses, divided by what one panel produces:
Number of panels = daily Wh need ÷ sun hours ÷ 0.80 (system efficiency) ÷ panel watts
The 0.80 factor covers charge-controller losses, wiring, panel soiling, and temperature derating — real arrays deliver about 75–85% of nameplate in daily energy terms. If you're feeding a battery bank that then powers your loads, use 0.75–0.80 instead of 0.85–0.90, since the energy takes two lossy hops (charge, then discharge).
Worked examples at 4.5 sun hours, 400 W panels
| Load | Daily Wh | Array watts (÷4.5 ÷0.8) | 400 W panels |
|---|---|---|---|
| Fridge + lights + phone | 2,000 | ~560 W | 2 |
| Small cabin, weekend use | 4,000 | ~1,120 W | 3 |
| Full-time tiny home | 8,000 | ~2,230 W | 6 |
| Efficient whole home (no AC) | 15,000 | ~4,200 W | 11 |
| Average US home | 30,000 | ~8,300 W | 21 |
Sun hours vary hugely: 2–2.5 in northern winters, 6+ in southwest summers. Size for your worst month — the calculator's sun-hours presets pull typical low figures by region, and PVWatts gives exact local numbers.
Battery coupling changes the count
If your panels charge a battery and the battery runs your night loads, the bank adds losses and the array has to refill what the night drains. Two adjustments:
- Round-trip efficiency: lithium banks deliver ~90–95% of what you charge in; lead-acid ~80–85%. Budget the panel wattage 1.2× over the load-only figure.
- Autonomy days: a bank sized for 2 days of autonomy still needs to be refilled daily in normal operation — panels sized for one day's load refill a 1-day bank, but a 2-day bank needs roughly the daily load plus whatever was drawn overnight, every day.
Work the other direction with the panels-per-battery recharge math, or size the whole bank first with the battery bank sizer, then size panels to refill it.
Grid-tie vs off-grid changes the goal
Grid-tie panels only need to offset your annual bill — they can overproduce at noon and export. Off-grid panels must cover the worst day of the year, because there's no grid to fall back on. That's why two homes with identical loads can need very different arrays: the off-grid one is sized for December, the grid-tie one for the year.
Sanity checks before you buy
- Roof space: a 400 W panel is roughly 21 sq ft. Twenty-one panels ≈ 450 sq ft of usable roof.
- Inverter headroom: array watts should stay within your inverter's max PV input — see whole-system sizing for pairing.
- Reduce, then size: every 100 Wh/day you trim (LED swaps, an efficient fridge) removes roughly a quarter-panel of array. Efficiency is cheaper than panels.