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 ÷ 0.86 (system derate) ÷ panel watts

The 0.86 is 1 − 0.14: the default 14% total system loss in NREL's PVWatts Version 5 manual (Table 6). In plain terms, it is the energy a typical array loses to dirt on the glass (2%), distant shading (3%), panel-to-panel mismatch (2%), wiring (2%), connections (0.5%), first-sun light-induced degradation (1.5%), panels testing slightly under their label (1%), and downtime (3%). It does not include inverter losses, which PVWatts models separately, or battery charge and discharge losses — see the battery section below. The calculator's sizeSystem uses the same 0.86 and rounds array watts up to the next 100 W.

Worked examples at 4.5 sun hours, 400 W panels

The table uses 4.5 sun hours. Type 4.5 into the calculator's sun-hour field (it opens at 4) and the same daily watt-hours print the same array and 300 W panel count. The 400 W column is the calculator array ÷ 400, rounded up.

The EIA average-home bill, sized from the published annual kWh, is on what size solar system for an average home.

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 losses: the 0.86 derate does not include them. panelsForBattery divides by 0.85 on the recharge path. Do not stack another factor on top of that division.
• Autonomy days: a bank sized for more than one day still has to be refilled from the watt-hours you actually drew. Size that refill from the measured day, not from a second copy of the array formula.

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: check the module's own dimensions against the roof before you order the count.
• 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 about 26 W of array at 4.5 sun hours (100 ÷ 4.5 ÷ 0.86). Efficiency is cheaper than panels.

How a panel turns sunlight into the watts this count starts from is on how solar panels work. When several strings share a combiner, the series-fuse case is on how to fuse a solar panel array. Reading a nameplate before you order that count is on how to buy solar panels.

Compiled with AI assistance from the sources below. Spot an error? Email [email protected].

Sources

• NREL PVWatts Version 5 Manual, A. Dobos, September 4, 2014 — Table 6 default system losses, 14% total; 1 − 0.14 = 0.86. Accessed Oct 3, 2026.
• SolarSizer calculator, assets/app.js — sizeSystem: daily watt-hours ÷ sun hours ÷ 0.86, rounded up to the next 100 W, then ÷ 300 W panels, rounded up. Every table row is that function at 4.5 sun hours. Accessed Oct 3, 2026.

1,200 ÷ 4.5 ÷ 0.86 = 310.1 W, and sizeSystem rounds that up to 400 W. Each table row is the same function at 4.5 sun hours. Accessed Oct 6, 2026.

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