How many solar panels to power a house?

The short answer (the three inputs)

The number of solar panels to power a house depends on three things: your annual electricity use, your local sun hours, and each panel's wattage. As a rule of thumb, divide your yearly kWh by (sun hours × 365 × a system-loss factor of ~0.84) to get the system size, then divide by panel wattage for the count. A typical home — using about 10,800 kWh a year (EIA RECS), at ~4.5 peak sun hours a day (NREL solar resource data), with 400 W panels — needs roughly 20 panels (an ~8 kW system). Your usage and roof drive the real figure.

Everything else is detail layered on those three inputs. Use more electricity and the count rises; live somewhere sunnier and it falls; fit higher-wattage panels and you need fewer of them for the same system size. The fastest way to a number for your home is to put your own annual kWh into the solar system size calculator — but the three steps below show exactly how that number is built so you can sanity-check any quote.

Step 1: find your annual kWh

Solar is sized to energy, not to roof area or house size, so the first input is how much electricity you actually use in a year. The single best source is your own bills: add up twelve months of kWh to capture summer cooling and winter heating in one figure.

If you do not have a year of bills handy, a representative US household uses about 10,800 kWh a year, with space heating, cooling and water heating dominating the total (EIA RECS). That national average is a starting point only — a small, gas-heated apartment might use half of it, while an all-electric home with a pool can use double. This is why floor area is a poor guide: two 2,000 sq ft homes can differ two-fold in kWh depending on what runs on electricity.

Step 2: apply your sun hours and system losses

A kilowatt of panels does not produce a kilowatt-hour every hour of the day. To convert your energy target into the size of array that can meet it, you need two location-and-hardware factors:

• Peak sun hours — the equivalent number of hours of full-strength (1 kW/m²) sunshine your site receives per day. A US-wide representative value is about 4.5 hours, but it ranges from roughly 3.5 in the cloudy Northeast to over 6 in the desert Southwest (NREL solar resource data).
• System losses (the derate factor) — real arrays lose energy to inverter conversion, wiring, soiling, heat and shading. PVWatts uses default system losses of about 14%, i.e. a derate factor near 0.84 (NREL PVWatts).

Put together, one kilowatt of panels produces about 4.5 × 365 × 0.84 ≈ 1,380 kWh per year. So the required system size is simply your annual kWh divided by 1,380.

Step 3: convert to panel count and roof area

Once you have the required system size in kilowatts, the panel count is arithmetic. Divide the system's watts by your panel wattage and round up to whole panels — you cannot install a fractional module.

Modern residential panels are typically 380–450 W; 400 W is a reasonable default. Each 400 W panel occupies about 21 square feet, so multiplying the panel count by that footprint gives the roof area before setbacks and walkways.

All three rows use 4.5 peak sun hours and a 0.84 derate; only the annual usage changes. Modeling the same method across three system sizes shows the relationship is close to linear — every extra ~1,400 kWh of annual use adds about one more panel. That linearity is useful for quoting: if you know roughly how your usage will change, you can predict the panel count without re-running the whole calculation.

One caveat the table hides: roof area scales with panels, but the usable roof rarely does. Setbacks, vents, chimneys and shaded sections all reduce what you can actually cover, so a 28-panel array can need noticeably more than its 588 sq ft footprint of physical roof. If roof space is tight, higher-wattage panels (450 W instead of 400 W) pack the same kW into fewer, denser modules.

Worked example for an average home

Here is the full calculation for a representative home, run through our solar system size calculator:

• Inputs: 10,800 kWh/year, covering 100% of use, 400 W panels, 4.5 peak sun hours, 84% derate.
• Production per kW: 4.5 × 365 × 0.84 ≈ 1,380 kWh per kW each year.
• Required size: 10,800 ÷ 1,380 ≈ 7.83 kW.
• Panels: 7,830 W ÷ 400 W = 19.6 → 20 panels (rounded up) → an 8.0 kW system.
• Roof area: 20 × 21 ≈ 420 sq ft.
• Estimated production: ≈ 11,040 kWh — about 102% of usage, because rounding up to whole panels slightly oversizes the array.

So the representative answer to "how many solar panels to power a house?" is about 20 panels — but that holds only for a 10,800 kWh home at 4.5 sun hours. Change any input and the count moves.

Ready to size your own roof? Use the solar system size calculator → Enter your annual kWh and local sun hours to get your exact panel count, roof area and system size in kW.

Common sizing mistakes (and sizing for a future EV or heat pump)

A few errors crop up again and again when people estimate panel counts:

• Sizing by house square footage. Floor area does not determine electricity use; appliances and climate do. Always size from kWh.
• Ignoring system losses. As above, dropping the ~0.84 derate undersizes the array by roughly 16%.
• Using nameplate sun hours. "Hours of daylight" is not the same as peak sun hours — use your location's figure from NREL, not the length of the day.
• Forgetting future loads. This is the big one. If you plan to add an EV or a heat pump, your future kWh will be much higher than your bills show today.

Sizing is only half the decision. To weigh whether the system pays for itself, read are solar panels worth it? and how much do solar panels cost?; if you are also weighing storage to use more of what you generate, see are home batteries worth it?.

The bottom line

To power a house you need enough panels to cover your annual kWh given your local sun hours and real-world losses — and for a typical 10,800 kWh US home at 4.5 peak sun hours with 400 W panels, that is about 20 panels in an 8 kW system, on roughly 420 square feet of roof. But the only number that matters is yours: pull a year of bills, add any EV or heat pump you plan to install, and run it through the solar system size calculator to get a panel count built on your own usage and location.