Solar Panel Size Calculator

The sizing formula, step by step

system kW = target yearly kWh ÷ (sun hours × 365 × 0.78)

The calculator runs the same four steps an installer's design software does. Take a home that uses 900 kWh a month, gets 4.5 sun hours a day, and wants to cover 100% of its usage:

1. Yearly usage: 900 kWh × 12 = 10,800 kWh.
2. Target production: 10,800 kWh × 100% offset = 10,800 kWh.
3. Yearly output per kW installed: 4.5 × 365 × 0.78 = 1,281 kWh per kW.
4. System size: 10,800 ÷ 1,281 = 8.4 kW.

From there the tool divides by panel wattage and rounds up (8.4 ÷ 0.4 = 21.1, so 22 panels at 400 W), allows 20 sq ft per panel for roof space (440 sq ft), and multiplies kW by your installed cost per watt: at $3.00 per watt the example is $25,290 before the 30% federal tax credit and about $17,700 after it. The how many solar panels do I need page runs the same math from the panel-count angle.

Where each input comes from

Usage, from your utility bill

The calculator converts your monthly bill into kWh by dividing by your rate. Better is to read the kWh directly: most bills print a 12- or 13-month usage bar chart. Add the 12 monthly figures rather than multiplying one month by 12, because a July bill in Texas or a January bill in Minnesota can be double the annual average. If you use the bill shortcut, take the rate from the same bill as total charge divided by total kWh, so delivery fees and taxes are included.

Sun hours, from NREL data

"Sun hours" means peak sun hours: the daily average of solar energy on the array, expressed as hours of full 1,000 W/m² sun. The National Renewable Energy Laboratory's solar resource maps put most of the Southwest at 5.5 to 6.5 hours, the Southeast and Plains at 4.5 to 5.5, and the Northeast, Pacific Northwest, and upper Midwest at 3.5 to 4.5. For a single address, NREL's free PVWatts tool gives the figure for your roof tilt and direction. When in doubt, use the lower end of the range.

Offset, your choice

Offset is the share of yearly usage you want the system to produce. 100% is the usual target under full net metering. Pick 70 to 90% if your utility credits exports below the retail rate or your roof is tight, and 110 to 120% only when a specific new load is coming.

What the 78% performance ratio includes

Panel ratings are measured at 25°C cell temperature under lab light. A roof in July runs far hotter, and every step between the panel and your breaker box loses a little. The 0.78 ratio bundles the typical losses:

• Heat: 5 to 10%, since output falls roughly 0.3 to 0.4% per °C above 25°C.
• Inverter conversion from DC to AC: 3 to 4%.
• Wiring, connectors, and panel mismatch: 2 to 4%.
• Soiling from dust and pollen, plus occasional snow: 2 to 5%.
• Availability and small shading losses: 1 to 3%.

Heavy shading or a poorly oriented roof are not included; handle those by lowering the sun-hours input. Full assumptions are on the methodology page.

System size by monthly usage and sun hours

System size in kW for a 100% offset at a 0.78 performance ratio.

kW = monthly kWh × 12 ÷ (sun hours × 365 × 0.78). A Phoenix home and a Seattle home with the same bill need systems that differ by more than half.

From kilowatts to panels

Panel count is system kW divided by panel kW, rounded up. Residential panels sold in 2026 mostly fall between 400 and 450 W; 350 W still appears in older quotes and budget lines.

Rounded up to whole panels; allow about 20 sq ft of roof each.

Common sizing mistakes

• Sizing from square footage. Floor area says nothing about whether you heat with gas or electricity, run a pool pump, or charge a car. Usage in kWh is the only reliable input.
• Sizing from one bill. A single summer or winter month can be 50% above or below the annual average. Use 12 months.
• Ignoring loads you are about to add. An EV driven 12,000 miles a year adds roughly 3,600 kWh, or about 2.8 kW of system at 4.5 sun hours; a heat pump replacing a gas furnace can add more. Fold these in now, since adding panels later means a second permit and possibly a new inverter.
• Oversizing where export credits are low. If your utility pays 4 cents for exported kWh and charges 17 cents for imports, every panel beyond your daytime self-consumption earns a quarter of what the first panels earn. There, a 70 to 90% offset or a battery usually beats a bigger array.
• Using daylight hours as sun hours. Fourteen hours of June daylight in Boston still averages only about 4.5 peak sun hours over the year.

Why an installer's number may differ

A site visit adds constraints this calculator cannot see. The usable roof shrinks once fire-code setbacks (typically 18 to 36 inches from ridges and edges), vents, and skylights are drawn in, so the installer may put fewer panels on the best face and a few on a less ideal one. They run hourly shade analysis rather than a flat derate, and they choose the inverter deliberately: a DC-to-AC ratio of 1.2 to 1.3 is normal, meaning a 9 kW array on a 7.6 kW inverter. That "clips" a little output on the brightest spring afternoons but raises production on every other day and keeps the inverter within interconnection limits. A modest gap from your estimate is normal; a proposal 30% larger with no new load to justify it is worth questioning.

Before settling on an offset, check how your state and utility credit exported power in the state-by-state cost and export-rules guide, since that decides whether the last few kilowatts pay for themselves. The cost calculator then turns your kW figure into a price with the federal credit applied.