Solar Panel Calculator

How many solar panels do I need? The full chain, with numbers

Every honest sizing estimate runs through the same four steps: how much electricity you use, how much sun your roof actually gets, how much of that survives the equipment, and how big each panel is. Everything else — quotes, financing, brand arguments — sits on top of that arithmetic. Here is the whole chain worked through with the default figures in the calculator above.

Start with a US home using 900 kWh a month, which is close to the national average of roughly 10,500 kWh a year. Divide by 30 and you need about 30 kWh per day. Take 4.5 peak sun hours (the rough US average) and 14% system losses, which leaves a derate factor of 0.86. Required array size is 30 / (4.5 x 0.86) = 7.75 kW. With 400 W panels that is 7,752 / 400 = 19.4 panels, and since nobody sells four tenths of a panel you round up to 20 panels.

Twenty 400 W panels is an installed 8.0 kW array. Run it forward: 8.0 x 4.5 x 365 x 0.86 = about 11,300 kWh a year against 10,800 kWh of usage, so roughly 105% coverage on paper. At about 21 square feet per panel including framing and row gaps, you need around 420 sq ft (39 m²) of unshaded roof — noticeably more than a single small roof face on many houses, which is why installers often split arrays across two planes.

What "peak sun hours" actually means

This is the single most misread input. Peak sun hours are not daylight hours. One peak sun hour equals 1,000 watts per square metre falling on the panel for one hour — the standard test condition panels are rated at. A location with 4.5 peak sun hours might have 14 hours of daylight in June; the metric compresses the whole day's varying irradiance into an equivalent number of full-strength hours. That is why Seattle sits near 3.5 and Phoenix near 6.0 despite similar day lengths, and why the NREL insolation maps are the right source rather than a weather app's "hours of sunshine".

UK readers should note the numbers run lower: much of England averages closer to 2.5 to 3.0 peak sun hours annually, so at 3.0 the same 900 kWh home would need about 11.6 kW — 30 panels rather than 20. That is a real constraint on British roofs and part of why UK sizing conversations lean on export tariffs and batteries more heavily than US ones.

System losses: why 14%

The 14% default is the PVWatts standard assumption and breaks down roughly as inverter conversion (2 to 4%), DC and AC wiring (2%), soiling (2%, more in dusty or pollen-heavy areas), heat derating (a panel at 65°C produces several percent less than at its 25°C rating), module mismatch, and light-induced degradation. Push it to 20% if you have dusty conditions, long cable runs, or a hot roof with no ventilation gap; drop it toward 10% only with microinverters, clean air and a cool climate. Note the calculator falls back to 14% rather than 0% if you clear the field, because a zero-loss assumption quietly inflates production by about a sixth.

Should you oversize the array?

This is genuinely policy-dependent, not a matter of taste. Under one-to-one net metering — where every exported kWh is credited at the retail rate — building to 100 to 110% of annual usage is usually the right call, since the grid acts as a free, lossless battery across seasons. Under net billing schemes like California's NEM 3.0, exports are credited at a fraction of retail, so oversizing sends surplus power out at a bad price. In those markets, sizing nearer 80% of usage and adding storage typically beats a bigger array. Check your utility's export rate before deciding — it changes the answer more than any hardware choice.

The shade reality check

Shade is where paper estimates die. A single chimney shadow crossing one panel can drag down a whole string on a traditional string inverter, which is why microinverters or DC optimisers are standard on complex roofs. Afternoon shade from a neighbour's oak costs far more than the same shade at 8am, because irradiance peaks midday. Before trusting any panel count, stand on your roof (or use a satellite tool) and check the arc from about 9am to 3pm year-round — winter shadows run three times longer than summer ones.

Where this estimate stops

Four things this calculator deliberately does not model: roof orientation and tilt penalties (east or west costs 15 to 20% of annual output), panel degradation (about 0.5% a year, so year 25 output is roughly 88% of year one), local permitting or interconnection caps that limit system size, and cost or payback. A professional site survey brings a shade analysis, a structural check on your rafters, and your utility's actual interconnection rules. Use the number above to know whether you are shopping for a 6 kW or a 12 kW system, then let a surveyor settle the last 20%.