The short answer

For a typical home covering its whole bill, the count lands at roughly 16 to 22 panels — fewer (13–16) in very sunny places, more (22–25) in cloudy ones. With today's 400–500 W panels (around 440 W is typical), that's a system of about 6 to 9 kW. Small homes do fine with 8–12; an all-electric house with an EV and heat pump may want 25+. The best predictor isn't floor area — it's the kWh on your bill.

Exactly how we work it out

There are two layers. First we size the array, then we simulate a day to see how it actually performs.

1. Sizing the panels. The system size in kilowatts is your annual electricity use divided by your peak sun hours, times 365, times a performance ratio of about 0.85 (inverter, heat, wiring and dirt losses). Divide by panel wattage and round up:

On the default “Match yearly use” goal, that's the whole sizing story — which is why your battery doesn't change the panel count: panels generate kilowatt-hours, and a battery only stores them. Switch to “Maximise self-use” and we instead size enough panels to cover your daytime load and refill the battery each day, so a bigger battery does raise the count.

2. Simulating the day. This is where the battery, your daytime presence and the season come alive. We model each hour from midnight to midnight:

Solar is spread across the daylight hours as a bell curve peaking at noon, scaled by season — longer, stronger summer days, short weak winter ones. Your load is spread by which profile you pick: “out at work” puts a small morning blip and a big evening peak with a hollow midday; “home all day” fills the daytime in; “evenings” loads the night. Then, hour by hour: when solar beats load, the surplus charges the battery (up to its size) and any excess is exported; when load beats solar, the battery discharges to cover it, and only what's left is pulled from the grid — free if it falls in your midday window, otherwise paid.

From that we read off your self-sufficiency (the share of your load met by your own solar and battery rather than paid grid), how much solar you export, and your paid-grid share — and we run it twice, for summer and winter, so the Summer/Winter toggle and verdict tell you whether the system holds up year-round. It's a simplified model — seasonal swing really depends on your latitude, and a real design models your exact roof — but it's enough to see how the pieces trade off.

Why location matters most

"Peak sun hours" is the great equaliser. The same 440 W panel makes about twice as much energy in Phoenix as in Seattle, purely from available sunlight — roughly 2.5–4 hours a day in cloudy north climates up to 5.5–7.5 in the sunny southwest. Orientation and shade matter too: a sensibly tilted, unshaded roof facing the equator hits the headline figures, while one tree or chimney can quietly cost you a chunk.

Battery, day-at-home & the charts

This is what the simulation makes visible. Without a battery, a home that's out all day exports most of its midday solar and buys its evenings back from the grid — self-sufficiency often sits around 30–50%. Add a battery and you bank the midday surplus for the evening, pushing self-sufficiency toward 80–100% in summer. Being home in the daytime lifts it further, because you use the solar as it's made.

Winter is the honest test. Short, weak days mean even a big battery can't fully cover you — that's why the verdict flags winter as the weak point and the gap between the two seasons is the real measure of whether your system is "enough." Bigger batteries help less in winter than people expect, because the limit is how much the panels can make, not how much you can store.

Sizing for EVs, AC, off-grid & more

"How many panels" branches into a few common follow-ups. Each of these uses the same engine, tuned to a specific job:

It only looks complicated

Solar feels overwhelming because of the options — batteries, tariffs, orientations, free windows. But the spine is small: how much you use, how sunny it is, how big the panels are, and whether you store or shift the surplus. The simulation just shows those four playing out across a day so you can see the trade-offs instead of guessing.

Use it to land the right ballpark and check it survives winter — then take the printed assessment to a couple of accredited installers, who'll model your exact roof. Knowing the number is what stops you being over-sold.

Common questions

Game: solar facts

Five quick rounds on the numbers that drive a solar quote.

Method & sources

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