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.

What is the self-consumption ratio?

It is the percentage of generated electricity you use directly in the home rather than exporting. Self-consumed energy saves you the full import price, while exported energy only earns the lower export rate. A typical home without a battery self-consumes 30 to 50 percent.

Why is payback shown as simple payback?

Simple payback divides the install cost by the first-year net benefit, ignoring tariff inflation, panel degradation, and the time value of money. It is the standard quick metric. Real payback is usually a little faster because energy prices tend to rise over time.

How is the CO2 offset estimated?

Annual generation is multiplied by a grid emission factor (the CO2 emitted per kWh by the displaced grid electricity) and then by 25 years. The default 0.207 kg CO2e per kWh reflects a typical UK grid average; cleaner grids displace less, dirtier grids displace more.

Does this account for panel degradation?

The headline payback uses first-year figures. Panels typically lose about 0.5 percent output per year, so 25-year lifetime generation is slightly lower than 25 times the first year. The lifetime CO2 figure uses first-year generation as a reasonable planning estimate.

Solar Panel Payback Calculator

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.

Example and tips

A 4 kWp system in southern England at 950 kWh/kWp generates about 3,800 kWh a year. At 40 percent self-consumption, a 28p import price, and a 15p export rate, that is roughly 760 of bill savings plus 342 of export income — about 1,100 a year. A 6,000 install pays back in around 5.5 years and avoids roughly 20 tonnes of CO2 over 25 years. Adding a battery raises self-consumption toward 70 percent, which shortens payback because saved import is worth more than export.