TOPIC-05 · Engineering guide

Simple Solar Payback from Monthly Generation

With those zeros, simple payback is capital divided by the constant net cash flow. 6000 kWh produces 960 of annual benefit, 860 after maintenance, and 5000 / 860 = 5.814 years to three decimals.

Formula, variables and conditions

Annual energy is the sum of the twelve monthly AC values.
Self-use = annual × self-use fraction. Export = annual − self-use.
Year-one benefit = annual energy × (self-use fraction × buy tariff + export fraction × export tariff), before growth.
Net cash flow = benefit − maintenance.
When degradation, tariff growth and discount rate are all zero, simple payback = capital / net cash flow if that ratio falls inside the project years.

Variables and units

Monthly AC energy: twelve nonnegative values. The teaching case uses 500 kWh each month.
Capital and annual maintenance: currency units you enter. The fixture does not name a currency.
Self-use fraction: 0 to 1. It is not an hourly load match.
Buy and export tariffs: per kWh.
Degradation, tariff growth and discount rate: fractions. This example sets all three to 0.

Conditions

The general calculator still applies degradation and tariff growth year by year, and it finds payback by the cumulative-cash-flow crossing. The zeros make that crossing equal to capital / net cash flow.
PVWatts did not produce this generation. Online mode is a separate path and is not this case.
SRC-005 is stored with url: null and licenseStatus review-required. SRC-010 is the project model.

Worked example

English teaching case CASE-003. Ten project years.

Generation: 500 kWh × 12 = 6000 kWh
Self-use / export: 3600 kWh / 2400 kWh
Benefit: 3600 × 0.20 + 2400 × 0.10 = 960
Maintenance / net: 100 / 860
Capital: 5000
Simple payback: 5.814 years

5000 / 860 to three decimal places is 5.814 years. Cumulative net inflow over 10 years is 8600 before subtracting capital, or 3600 after it. The Chinese case on the same slug also reports two official 2022 emission factors. Those factors are not part of this English payback arithmetic.

Boundaries and common mistakes

Boundaries

Financing, taxes, incentives, replacements, demand charges and hourly matching are excluded.
A blank payback means the cumulative curve does not cross zero inside the years you set.
The result is a scenario, not a production or savings guarantee.

Common mistakes

Valuing exported kilowatt-hours at the buy tariff.
Treating a manual monthly profile as a weather-file result.
Mixing the Chinese case’s two carbon factors into one grid factor. This English page does not do that.