Solar Savings and Payback Calculator

Use bill and proposal data

Enter monthly demand from your bill and proposed generation from the installer. The example THB 4.5 and 400 kWh are not guaranteed rates or output.

Review several bills because daytime demand and seasons vary. Dividing a quoted annual generation by 12 gives only a trial monthly average, not a prediction for each month. Timing of household demand while panels produce power matters more to self-consumption than the monthly total alone.

Separate self-use and excess

The self-use share applies to generation but is capped by household demand. Value excess energy only when export eligibility and price are verified; otherwise use zero.

If generation is 400 kWh but demand only 100 kWh/month, even a 60% self-use entry is capped at 100 kWh, with 300 kWh excess. This prevents claiming retail savings above household use, but it still cannot simulate hourly timing. A more detailed interval study may lower realistic self-use.

Worked default example

At 400 kWh/month and 60% self-use, 240 kWh is used and 160 kWh remains. With zero export value and THB 4.5/kWh retail rate, gross value is THB 1,080/month or THB 12,960/year with zero maintenance. Blank upfront cost means no payback figure.

Compare 40% with 60% self-use while holding generation and rates constant. At 400 kWh and THB 4.5/kWh, self-use changes from 160 to 240 kWh, a THB 360/month difference if excess has no value. This sensitivity often matters more than rounding generation estimates.

Interpret simple payback

If upfront cost is entered, simple payback divides it by positive annual net benefit only. Otherwise there is no payback under these inputs. Degradation, taxes, financing and future rates are outside this model.

Before using payback to decide, obtain actual installed cost, warranty, maintenance and interconnection terms. Export compensation can differ from the retail rate or be unavailable. The estimate excludes panel degradation, equipment replacement and year-by-year cash flow.

Net benefit can be negative if entered annual maintenance exceeds electricity value. The calculator then avoids a misleading numeric payback. Even with positive benefit, dividing upfront cost by one annual figure assumes identical future years, so test sensitivity to generation and electricity prices.