Should I size the array to cover 100% of my usage?

Usually somewhere between 80% and 100% is the sweet spot, and the deciding factor is your utility's export rate. Where net metering credits exports at retail rate, sizing to 100% is reasonable. Where export credits are low (California under NEM 3.0, for example), production you cannot consume yourself is worth much less, and a smaller array sized closer to your daytime load often returns more per dollar.

Do higher-wattage panels mean fewer panels and lower cost?

Fewer panels, yes — a 500W panel needs 20% fewer units than a 400W panel for the same array. Lower cost, not necessarily. What matters is dollars per watt, not dollars per panel, and premium high-wattage panels often cost more per watt. Higher-wattage panels genuinely help when roof space is tight; when it is not, they are a convenience rather than a saving.

Does adding a battery change how many panels I need?

Not directly — panel count is driven by how much energy you consume over a year, and a battery shifts when you use that energy rather than how much. Batteries do change the answer in low-export-rate states, where storing your own production is worth far more than exporting it. For outage-specific sizing, see our 72-hour outage guide.

How Many Solar Panels Do I Need to Run My House?

For an average U.S. home: 17 to 25 panels, or roughly 7 to 9 kW. Here is the formula behind that, and why your location moves the answer more than your house does.

The average U.S. home uses about 10,800 kWh a year and needs roughly 17 to 25 standard 400W panels — a 7 to 9 kW array — to offset all of it. That range is wide for a reason, and the reason is not house size. It is sunlight.

The formula

Two steps. First find the system size in kilowatts, then divide by your panel wattage:

System size (kW) = annual kWh ÷ (peak sun hours × 365 × 0.8)
Number of panels = system watts ÷ panel watts

Peak sun hours is not daylight hours. It is the number of hours per day during which sunlight hits your location at the equivalent of full-rated intensity — typically 3 to 6 in the U.S., versus 12+ hours of actual daylight. The 0.8 is the derate factor covering inverter losses, wiring losses, panel temperature, soiling and imperfect orientation. Skipping it is the single most common reason a hand-calculated array comes out undersized.

Worked example

A home using 10,800 kWh a year in a region averaging 4.5 peak sun hours:

10,800 ÷ (4.5 × 365 × 0.8) = 10,800 ÷ 1,314 = 8.2 kW
8,200W ÷ 400W per panel = 21 panels

Your location changes the answer more than anything else

Same house, same 10,800 kWh, same 400W panels — only the location changes:

Region	Peak sun hours	System size	400W panels
Southwest (AZ, NV, NM)	5.5 – 6.0	6.2 – 6.7 kW	16 – 17
South / Southeast	4.5 – 5.0	7.4 – 8.2 kW	19 – 21
Midwest	4.0 – 4.5	8.2 – 9.2 kW	21 – 23
Northeast	3.8 – 4.2	8.8 – 9.7 kW	22 – 25
Pacific Northwest	3.2 – 3.8	9.7 – 11.6 kW	25 – 29

A Seattle home needs nearly twice the array a Phoenix home does to produce the same electricity. This is why national-average panel counts are close to meaningless, and why any quote that does not start with your specific location and roof orientation is not a real estimate.

Will it fit on the roof?

A 400W residential panel is roughly 21 square feet. Twenty-one panels need about 440 square feet of usable mounting area — a modest fraction of a typical 1,500-2,000 sq ft roof. Raw space is rarely the constraint.

Shading and orientation are. A south-facing plane is the benchmark; east and west faces produce roughly 15-20% less, and north-facing planes are generally not worth mounting on in the northern U.S. Partial shade from a single chimney or tree branch can disproportionately hurt a string-inverter array, which is the main argument for microinverters or DC optimizers on a complicated roof.

Do not use the national average — use your own bill

Every number above depends on that 10,800 kWh figure, and real households range from under 5,000 to over 25,000 kWh a year. Pulling your actual consumption takes about ten minutes and changes the panel count substantially — we walk through it in how to calculate your home's daily kWh usage.

One thing that has changed since the federal tax credit ended: oversizing is now considerably more expensive to justify, because there is no 30% offset softening the cost of panels you may not need. Sizing to actual consumption rather than to a comfortable margin matters more in 2026 than it did in 2025 — see the payback math without the credit.