How We Calculate: Solar Panel Payback

What This Calculator Does

The solar panel payback calculator answers “How long until my solar panels pay for themselves?” by simulating year-by-year energy generation, savings, and cumulative returns over a chosen analysis period (default 25 years).

Unlike simple payback calculators that divide cost by annual savings, this model accounts for panel degradation, rising energy prices, and the difference between self-consumed and exported electricity — all of which significantly affect the real payback period.

How the Year-by-Year Simulation Works

For each year from 1 to the analysis period:

1. Calculate annual generation: System Size × kWh/kWp × (1 − degradation rate)^(year−1)
2. Calculate effective self-consumption (if battery is present):
   • Daily generation = Annual generation ÷ 365
   • Daily surplus = Daily generation × (1 − Base Self-Consumption Rate)
   • Battery capture = min(Daily surplus, Battery Capacity × Round-Trip Efficiency)
   • Effective Self-Consumption = Base Self-Consumption Rate + (Battery capture ÷ Daily generation), capped at 95%
   • Without a battery, effective self-consumption equals the base rate
3. Split into self-consumed and exported: Generation × effective self-consumption rate = self-consumed kWh; the remainder is exported
4. Apply tariff rates: Self-consumed kWh × current electricity tariff (saves buying from grid); Exported kWh × current export tariff (earns income)
5. Escalate tariffs: Both tariffs increase by the energy price inflation rate each year
6. Subtract maintenance: Annual savings = self-consumption savings + export income − maintenance cost
7. Accumulate: Add annual savings to the running cumulative total
8. Check payback: When cumulative savings first exceed the net system cost, payback is reached

The payback month is interpolated: if the crossover happens partway through a year, the calculator estimates which month within that year the break-even occurred.

How Each Variable Affects the Result

Electricity tariff is the most impactful variable. Every 1p/kWh increase in tariff improves annual savings by roughly £10–£40 depending on system size and self-consumption. UK rates (24.5p/kWh) make solar significantly more attractive than low-cost US states.

Self-consumption rate determines the value of each kWh. At 24.5p import vs 4.5p export, a self-consumed kWh is worth 5.4× more than an exported one. A 5kWh battery with a 30% base self-consumption rate raises effective self-consumption to ~76%, nearly doubling annual savings compared to no battery.

System cost is the denominator of the payback equation. Lower installation costs shorten payback proportionally. Battery costs add to the numerator but improve the self-consumption rate.

Energy price inflation has a compounding effect. At 3% annual inflation, a 24.5p tariff becomes 33.5p after 10 years and 45.8p after 20 years. Higher inflation makes solar increasingly attractive over time.

Panel degradation reduces generation by ~12% over 25 years at the standard 0.5% rate. This means year-25 savings are lower than year-1, but the effect is partially offset by rising tariff rates.

Net System Cost Calculation

The net cost is straightforward:

Net System Cost = Panel Installation Cost + Battery Cost (if selected) − Tax Credit or Grant

This is the break-even target for cumulative savings. In the UK, solar panels are currently VAT-free (0% until March 2027), which is already reflected in quoted installation prices. The tax credit field is for any additional grants or incentives.