4 kW solar system: cost, output and payback
A 4 kW solar system is 10 panels and costs about $11,400 installed, or $7,980 after the US federal tax credit. Across 673 measured cities it produces a median of 5,256 kWh a year and pays back in about 9.8 years.
Check whether 10 panels fit your actual roof → These are medians across real cities; your roof's size, pitch and shading change the answer.
Is a 4 kW system the right size?
This is a partial-offset system. A 4 kW array covers roughly 49% of an average home's 10,800 kWh a year, so you would still buy a meaningful amount from the grid. That is not a flaw — smaller systems often have the best payback per dollar, because everything they generate displaces power you were going to buy anyway.
What a 4 kW system produces where you live
Output depends almost entirely on local sunlight. The same 4 kW of hardware produces 7,191 kWh a year in Palmdale and 3,244 kWh in Fairbanks — a 2.2× spread across 673 cities we have measured from Google's rooftop imagery.
Where a 4 kW system pays back fastest
Payback is driven more by electricity price than by sunshine. Honolulu, HI tops the list at 3.1 years on 42.1¢/kWh, while Gatineau, QC takes 32.9 years on just 7.9¢/kWh. That is identical hardware producing 28% less electricity, but taking 10.6× as long to pay for itself — because the power it displaces is worth 5.3× less.
| City | kWh/yr | Rate | Payback |
|---|---|---|---|
| Honolulu, HI | 6,076 | 42.1¢ | 3.1 yrs |
| Lihue, HI | 6,042 | 42.1¢ | 3.1 yrs |
| Kahului, HI | 5,532 | 42.1¢ | 3.4 yrs |
| Hilo, HI | 5,134 | 42.1¢ | 3.7 yrs |
| Kailua-Kona, HI | 4,260 | 42.1¢ | 4.4 yrs |
| Quincy, MA | 5,171 | 30.1¢ | 5.1 yrs |
| Boston, MA | 4,916 | 30.1¢ | 5.4 yrs |
| Worcester, MA | 4,906 | 30.1¢ | 5.4 yrs |
| Warwick, RI | 5,161 | 28.3¢ | 5.5 yrs |
| New Haven, CT | 5,005 | 29.2¢ | 5.5 yrs |
| Fall River, MA | 4,859 | 30.1¢ | 5.5 yrs |
| Lowell, MA | 4,835 | 30.1¢ | 5.5 yrs |
| Somerville, MA | 4,821 | 30.1¢ | 5.5 yrs |
| Pawtucket, RI | 4,998 | 28.3¢ | 5.6 yrs |
| Cambridge, MA | 4,774 | 30.1¢ | 5.6 yrs |
Where it pays back slowest
| City | kWh/yr | Rate | Payback |
|---|---|---|---|
| Gatineau, QC | 4,389 | 7.9¢ | 32.9 yrs |
| Levis, QC | 4,508 | 7.9¢ | 32 yrs |
| Trois-Rivières, QC | 4,580 | 7.9¢ | 31.5 yrs |
| Longueuil, QC | 4,648 | 7.9¢ | 31 yrs |
| Drummondville, QC | 4,661 | 7.9¢ | 31 yrs |
| Terrebonne, QC | 4,692 | 7.9¢ | 30.8 yrs |
| Montreal, QC | 4,706 | 7.9¢ | 30.7 yrs |
| Laval, QC | 4,709 | 7.9¢ | 30.6 yrs |
| Quebec City, QC | 4,787 | 7.9¢ | 30.1 yrs |
| Coquitlam, BC | 3,733 | 12.4¢ | 24.6 yrs |
Other system sizes
4 kW solar questions
How many panels is a 4 kW solar system?
A 4 kW system is 10 panels at 400W each, the standard residential size in 2026. It needs roughly 20 m² (about 215 sq ft) of usable roof, and more in practice once you allow for edge setbacks, vents and access paths.
How much does a 4 kW solar system cost?
About $11,400 installed before incentives at $2.85 per watt, or roughly $7,980 after the 30% US federal tax credit. Quotes for the same 4 kW system routinely differ by thousands, because labour, margin and equipment vary far more than hardware cost does.
How much power does a 4 kW solar system produce?
Across the 673 cities we have measured, a 4 kW array produces a median of 5,256 kWh a year. The range runs from about 3,244 kWh in Fairbanks to 7,191 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.