15 kW solar system: cost, output and payback
A 15 kW solar system is 38 panels and costs about $42,750 installed, or $29,925 after the US federal tax credit. Across 673 measured cities it produces a median of 19,712 kWh a year and pays back in about 15 years.
Check whether 38 panels fit your actual roof → These are medians across real cities; your roof's size, pitch and shading change the answer.
Is a 15 kW system the right size?
This is more than most households need. A 15 kW array covers roughly 183% of an average home's 10,800 kWh a year, so unless you have electric heating, an EV, a pool or unusually high usage, you would be exporting a large share of it — usually at well below the retail rate you paid to install the panels.
What a 15 kW system produces where you live
Output depends almost entirely on local sunlight. The same 15 kW of hardware produces 26,966 kWh a year in Palmdale and 12,164 kWh in Fairbanks — a 2.2× spread across 673 cities we have measured from Google's rooftop imagery.
Where a 15 kW system pays back fastest
Payback is driven more by electricity price than by sunshine. Honolulu, HI tops the list at 5.2 years on 42.1¢/kWh, while Gatineau, QC takes 44.7 years on just 7.9¢/kWh. That is identical hardware producing 28% less electricity, but taking 8.6× as long to pay for itself — because the power it displaces is worth 5.3× less.
| City | kWh/yr | Rate | Payback |
|---|---|---|---|
| Honolulu, HI | 22,784 | 42.1¢ | 5.2 yrs |
| Lihue, HI | 22,657 | 42.1¢ | 5.2 yrs |
| Kahului, HI | 20,744 | 42.1¢ | 5.4 yrs |
| Hilo, HI | 19,253 | 42.1¢ | 5.6 yrs |
| Kailua-Kona, HI | 15,976 | 42.1¢ | 5.9 yrs |
| Quincy, MA | 19,393 | 30.1¢ | 7.7 yrs |
| Boston, MA | 18,437 | 30.1¢ | 7.9 yrs |
| Worcester, MA | 18,398 | 30.1¢ | 7.9 yrs |
| Fall River, MA | 18,220 | 30.1¢ | 7.9 yrs |
| Lowell, MA | 18,131 | 30.1¢ | 7.9 yrs |
| Somerville, MA | 18,080 | 30.1¢ | 7.9 yrs |
| Cambridge, MA | 17,901 | 30.1¢ | 8 yrs |
| Lynn, MA | 17,748 | 30.1¢ | 8 yrs |
| Newton, MA | 17,531 | 30.1¢ | 8 yrs |
| Greenfield, MA | 17,480 | 30.1¢ | 8 yrs |
Where it pays back slowest
| City | kWh/yr | Rate | Payback |
|---|---|---|---|
| Gatineau, QC | 16,460 | 7.9¢ | 44.7 yrs |
| Levis, QC | 16,907 | 7.9¢ | 44.3 yrs |
| Trois-Rivières, QC | 17,174 | 7.9¢ | 44 yrs |
| Longueuil, QC | 17,429 | 7.9¢ | 43.8 yrs |
| Drummondville, QC | 17,480 | 7.9¢ | 43.8 yrs |
| Terrebonne, QC | 17,595 | 7.9¢ | 43.7 yrs |
| Montreal, QC | 17,646 | 7.9¢ | 43.6 yrs |
| Laval, QC | 17,659 | 7.9¢ | 43.6 yrs |
| Quebec City, QC | 17,952 | 7.9¢ | 43.4 yrs |
| Winnipeg, MB | 17,710 | 10.1¢ | 34.1 yrs |
Other system sizes
15 kW solar questions
How many panels is a 15 kW solar system?
A 15 kW system is 38 panels at 400W each, the standard residential size in 2026. It needs roughly 76 m² (about 818 sq ft) of usable roof, and more in practice once you allow for edge setbacks, vents and access paths.
How much does a 15 kW solar system cost?
About $42,750 installed before incentives at $2.85 per watt, or roughly $29,925 after the 30% US federal tax credit. Quotes for the same 15 kW system routinely differ by thousands, because labour, margin and equipment vary far more than hardware cost does.
How much power does a 15 kW solar system produce?
Across the 673 cities we have measured, a 15 kW array produces a median of 19,712 kWh a year. The range runs from about 12,164 kWh in Fairbanks to 26,966 kWh in Palmdale — the same hardware, differing by 122% purely on local sunlight.
See the full dataset → every city, with sunlight, shading, roof size and rates.
Estimates generated 2026-08-22 from Google Solar API rooftop samples across 673 cities. Not a quote and not financial advice.