How Many Solar Panels Do I Need? A Simple Way to Size a System

“How many solar panels do I need?” has no single answer — but it has a simple formula. It comes down to two things about you: how much electricity you use, and how much sun your location gets. Get those two numbers and the panel count falls straight out.

Step 1: How much energy do you use?

Everything starts with your daily energy use in kilowatt-hours (kWh). For a grid-tied home, read it off your electricity bill — it usually shows your monthly kWh; divide by 30 for a daily figure. A typical home uses somewhere around 10–30 kWh a day.

For an off-grid setup (a cabin, van or tiny home) you build it up appliance by appliance: each device’s watts × the hours it runs per day, added together. That’s exactly what an off-grid load calculator is for — and the honest part is run-time: a fridge rated 150 W only runs its compressor part of the time, so use its average daily hours, not 24.

Step 2: How much sun do you get?

The second number is peak sun hours — and it’s not the same as daylight hours. One peak sun hour is an hour of sunshine at 1,000 W/m², the full “test” intensity. Your location’s peak sun hours bundle a whole day’s sunlight into an equivalent number of those full-strength hours: maybe 3–4 in a cloudy northern climate, 5–6 or more in a sunny one.

Two things matter here. First, look up your figure (a solar-irradiance map like NREL’s, or a peak-sun-hours tool). Second, for anything off-grid, use your worst month — a temperate site can get three to four times less sun in December than June, and a system sized for summer will fall short in winter.

Step 3: Divide

Now the arithmetic. One panel produces, per day:

panel watts × peak sun hours × system efficiency ÷ 1000  =  kWh per day

The system efficiency (~0.8) accounts for the energy lost to the inverter, wiring, heat, dust and mismatch — real panels deliver less than their lab rating. So a 400 W panel at 4.5 peak sun hours:

400 × 4.5 × 0.8 ÷ 1000 = 1.44 kWh per panel per day

Then divide your usage by that:

10 kWh/day ÷ 1.44 kWh = 6.9 → round up to 7 panels

Don’t forget the margin

That 7 is the bare minimum for average conditions. Real life has cloudy weeks, low winter sun, hot panels (which are less efficient) and gradual ageing (~0.5% a year). Adding roughly 25% — so 8 or 9 panels here — keeps the lights on when conditions are poor. Off-grid systems also need a battery bank sized to carry you through the sunless hours and cloudy days; that’s a separate calculation from the panel count.

Work out your own numbers

The solar panel output calculator does this in reverse — put in your panels, count, peak sun hours and efficiency and it shows the daily, monthly and yearly production, so you can dial in the array size against your usage. Pair it with the off-grid load calculator to nail down step 1, and the battery bank calculator to size storage. Like every LazyTools tool, they run entirely in your browser — no location access, nothing uploaded.

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This is a planning estimate using the standard solar production formula (daily kWh = panel watts × peak sun hours × system efficiency ÷ 1000). System efficiency of ~0.8 reflects typical real-world losses; NREL’s PVWatts uses about 14% DC losses plus inverter losses. Actual output varies with weather, shading, orientation and equipment — get a site-specific quote for a real install. Source: NREL PVWatts.