Gera Tools

Solar Panel Payback Calculator

Calculate solar PV break-even, savings, and CO2 offset over 25 years

Enter system size, location yield, electricity tariff, export rate, and install cost to compute annual generation, bill savings, export income, payback period, and lifetime CO2 avoided over 25 years. For homeowners and solar installers. It runs free in your browser on Gera Tools, with nothing uploaded.

Last updated Source: Gera Tools

What is specific yield and what value should I use?

Specific yield is how many kilowatt-hours each kilowatt-peak of panels generates per year, depending on latitude, orientation, and shading. Northern UK is around 850 kWh/kWp, southern UK around 1000, southern Europe 1300 to 1600. Use a local solar map figure for accuracy.

A rooftop solar system is a long-lived investment, so the questions that matter are how much it generates, how much money it saves each year, and how long it takes to pay back. This calculator combines your system size, regional yield, and tariffs to answer all three, plus the lifetime carbon it avoids.

How it works

Generation and value are built up from your inputs:

annual generation = system kWp × specific yield (kWh/kWp/yr)
self-consumed kWh  = generation × self-consumption %
exported kWh       = generation − self-consumed kWh
bill savings       = self-consumed kWh × import price
export income      = exported kWh × export rate
annual benefit     = bill savings + export income
payback (years)    = install cost / annual benefit
lifetime CO2 (kg)  = generation × grid factor × 25

Self-consumed energy is worth the full import price you avoid paying, while exported surplus only earns the lower export rate, which is why the self-consumption ratio strongly affects the result.

The two inputs that move the result most

Of all the inputs, specific yield and self-consumption ratio have the largest effect on payback, yet they are often estimated poorly:

Specific yield depends on latitude, roof orientation, shading, and local climate. Typical planning figures for the UK are around 850 kWh/kWp in northern regions and 1,050 kWh/kWp in southern regions for a well-oriented south-facing roof. A west-facing roof loses roughly 15–20% from a south-facing baseline. Use a local solar resource map or installer quote rather than a national average.

Self-consumption ratio depends on when the household uses electricity. A home with working adults who are out all day typically self-consumes 25–35% of generation without a battery. A home with daytime occupancy or an EV charged midday may reach 50–65%. Adding battery storage can push self-consumption above 80% for many households.

Because self-consumed energy avoids a full import tariff while exported energy earns a lower rate, raising self-consumption from 30% to 60% can meaningfully shorten payback — sometimes by several years.

Worked example

A 4 kWp system in southern England:

Annual generation = 4 kWp × 950 kWh/kWp = 3,800 kWh
Self-consumed     = 3,800 × 40%   = 1,520 kWh
Exported          = 3,800 − 1,520 = 2,280 kWh
Bill savings      = 1,520 × £0.28 = £426
Export income     = 2,280 × £0.15 = £342
Annual benefit    = £426 + £342   = £768
Payback           = £6,000 / £768 ≈ 7.8 years
Lifetime CO2      = 3,800 × 0.207 × 25 ≈ 19,665 kg  (≈ 20 tonnes)

Adding a battery and raising self-consumption to 70%:

Bill savings = 2,660 × £0.28 = £745
Export income = 1,140 × £0.15 = £171
Annual benefit = £916
Payback = £8,500 / £916 ≈ 9.3 years

The battery case has a longer payback here even though annual savings are higher, because the battery adds upfront cost. Whether it makes sense depends on battery lifespan, future tariff trends, and the value the household places on energy independence.

The CO2 grid emission factor used in this tool is a planning estimate. Grids are decarbonizing over time, so the actual lifetime CO2 offset will be less than a static factor implies — but it remains a useful comparison figure.