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Solar Rooftop Payback Calculator
Estimate generation, bill savings, surplus export income and payback for a home rooftop solar system, using the public residential buyback rate as a default.
Year by year
| Year | Generation (kWh) | Net benefit | Cumulative after cost |
|---|---|---|---|
| 1 | 6,570 | 24,031.70 | -125,968.30 |
| 2 | 6,537 | 23,906.54 | -102,061.76 |
| 3 | 6,504 | 23,782.01 | -78,279.75 |
| 4 | 6,472 | 23,658.10 | -54,621.65 |
| 5 | 6,440 | 23,534.81 | -31,086.84 |
| 6 | 6,407 | 23,412.13 | -7,674.71 |
| 7 | 6,375 | 23,290.07 | 15,615.37 |
| 8 | 6,343 | 23,168.62 | 38,783.99 |
| 9 | 6,312 | 23,047.78 | 61,831.77 |
| 10 | 6,280 | 22,927.54 | 84,759.31 |
| 11 | 6,249 | 18,683.70 | 103,443.01 |
| 12 | 6,218 | 18,585.28 | 122,028.29 |
| 13 | 6,186 | 18,487.35 | 140,515.65 |
| 14 | 6,156 | 18,389.92 | 158,905.56 |
| 15 | 6,125 | 18,292.97 | 177,198.53 |
| 16 | 6,094 | 18,196.50 | 195,395.04 |
| 17 | 6,064 | 18,100.52 | 213,495.56 |
| 18 | 6,033 | 18,005.02 | 231,500.57 |
| 19 | 6,003 | 17,909.99 | 249,410.57 |
| 20 | 5,973 | 17,815.44 | 267,226.01 |
| 21 | 5,943 | 17,721.37 | 284,947.38 |
| 22 | 5,914 | 17,627.76 | 302,575.14 |
| 23 | 5,884 | 17,534.62 | 320,109.76 |
| 24 | 5,855 | 17,441.95 | 337,551.71 |
| 25 | 5,825 | 17,349.74 | 354,901.44 |
Ask about this page
The AI assistant answers only questions about the calculator on this page.
Estimate generation, bill savings, surplus export income and payback for a home rooftop solar system, using the public residential buyback rate as a default.
How it works
Generation in year k = size × daily sun hours × 365 × performance ratio × (1 − yearly loss)^(k − 1). Self-used energy is valued at your average bill rate; the rest earns the buyback rate only during the buyback period. Yearly upkeep is deducted. Payback is when cumulative benefit equals the installed cost, interpolated within that year. The default THB 2.20 per kWh for 10 years comes from the 2026 public residential rooftop programme.
Hand check: 5 kWp, 4 sun hours and 100% performance make 7,300 kWh a year. Using it all at THB 5 per kWh saves THB 36,500, so a THB 73,000 system with no upkeep pays back in 2.0 years. Exporting all of it at THB 2.20 earns THB 16,060 a year.
A simple energy estimate. Real output depends on orientation, tilt, shading, dust, temperature and equipment. Savings depend on future tariffs and Ft. Inverter replacement, loan interest and permit costs are excluded. Export rights depend on utility conditions and approval.
Frequently asked questions
Is self-use better than exporting?
Usually yes. Self-used power offsets the retail rate, which is higher than the export buyback rate, so running appliances in daytime shortens payback.
What sun hours should I use?
Use your area's peak sun hours from an installer or solar radiation data, then try a lower value to allow for cloud, dust and shade.
Can I install more than 5 kW?
Possibly, subject to permits, but the 2026 residential buyback programme is limited to 5 kW (AC) per applicant. Treat anything beyond that as self-use only.
Sources
Rates last verified 21 Sep 2026