What each size costs, and what it covers
Systems are sold by capacity in kilowatts. At $3.75 a watt, that translates straight into a price, and at about 3000 watts a panel it translates into a panel count and a patch of roof.
Every size, priced nationally
| Size | Panels | Roof needed | Price | Makes each year | Saves, year one | Payback |
|---|---|---|---|---|---|---|
| 4 kW | 10 | 220 sq ft | $15,000 | 5,328 kWh | $930 | 14.1 years |
| 5 kW | 13 | 286 sq ft | $18,750 | 6,660 kWh | $1,162 | 14.1 years |
| 6 kW | 15 | 330 sq ft | $22,500 | 7,992 kWh | $1,395 | 14.1 years |
| 7 kW | 18 | 396 sq ft | $26,250 | 9,324 kWh | $1,627 | 14.1 years |
| 8 kWtypical | 20 | 440 sq ft | $30,000 | 10,656 kWh | $1,859 | 14.1 years |
| 10 kW | 25 | 550 sq ft | $37,500 | 13,320 kWh | $2,324 | 14.1 years |
| 12 kW | 30 | 660 sq ft | $45,000 | 15,984 kWh | $2,789 | 14.1 years |
| 15 kW | 38 | 836 sq ft | $56,250 | 19,980 kWh | $3,487 | 14.1 years |
| 20 kW | 50 | 1,100 sq ft | $75,000 | 26,640 kWh | $4,649 | 14.1 years |
National figures. Prices and payback vary by state more than by anything else on this page, so use the calculator or your state page for a number that applies to you.
Bigger is cheaper per watt, up to a point
Much of what you pay for is fixed regardless of size: the sales visit, the design, the permit, the inspection, the crew showing up, the inverter. Spreading those over more panels is why a 10 kW system rarely costs twice a 5 kW one.
The limit is what your utility will let you install and what your roof will hold. Most net metering rules cap a system at roughly your historical consumption, so building far beyond your own usage often means giving the surplus away. Sizing to cover most of your bill, rather than all of it, is usually where the arithmetic is best.