Plug-in solar yield and payback calculator

A plug-in solar kit pays for itself when the money you save on your electricity bill adds up to what you paid for it. The calculator above does that sum in four steps: it estimates the annual yield from panel power and location, works out how much of it you use at home, values that at your electricity price, adds any export payment and divides the purchase cost by the result. Every field is an example value. Replace each one with your own figure.

What each field means

Panel power (Wp)

The combined rated output of your panels in watt-peak, as printed on the data sheet. A kit with two 400 Wp panels has 800 Wp. If your panels are rated well above the inverter cap of 800 W, the simple calculation overstates the yield a little, because the inverter clips some midday output. See the 800 W limit for how clipping works.

Specific yield (kWh per kWp per year)

How many kilowatt-hours one kilowatt-peak of panels produces in a year at your location, orientation and tilt. This is the field where most guesswork goes wrong. Instead of using a national average, look up your own address in PVGIS, the free photovoltaic calculator of the European Commission’s Joint Research Centre. Enter your location, choose the mounting angle and the direction the panels face, and PVGIS returns a yearly yield.

Three things change the specific yield noticeably:

• Orientation. South-facing panels in the northern hemisphere produce the most over the year. East or west faces produce less in total but spread output over the morning or afternoon, which can raise self-consumption.
• Tilt. Panels hanging vertically on a balcony railing produce less in summer than tilted ones, but relatively more in winter when the sun is low.
• Shading. Trees, neighbouring buildings or the balcony above can cost more than any other factor. PVGIS can account for the horizon but not for a parasol or a neighbour’s balcony. Reduce the value if part of your panels is shaded for hours.

Read more about placement in our guide to mounting options.

Share used in the home (%)

The part of the yield you consume directly instead of exporting. This is usually the most important assumption, because self-consumed electricity saves the full retail price while exported electricity often earns nothing. The share is higher if:

• you or someone else is at home during the day,
• appliances such as a fridge, freezer, router or heat pump water heater run constantly,
• you shift the washing machine or dishwasher into sunny hours.

The share is lower with large panels and a small daytime load. A battery raises it; see plug-in solar with a battery and the cheaper alternative of zero export with a smart meter reader.

Electricity price per kWh

What you pay per kilowatt-hour on your current tariff, including taxes. Use the unit rate from your bill, not an average that includes standing charges. If you are on a time-of-use or dynamic tariff, use the rate that applies during daylight hours.

Payment per exported kWh

Leave this at zero unless you receive payment for exports. Whether you do depends on your country and supplier; the trade-off is explained in feed-in or self-consumption.

Purchase cost

The total you pay for the kit, mounting hardware, delivery and any accessories. Include a meter replacement or electrician visit if you need one.

A worked example

With the example values in the calculator (all assumptions, not data for any real location):

Step	Calculation	Result
Annual yield	0.8 kWp × 850 kWh/kWp	680 kWh
Used at home	680 kWh × 40 %	272 kWh
Saving	272 kWh × 0.30 per kWh	81.60 per year
Payback	400 ÷ 81.60	about 4.9 years

Change one value at a time to see what matters. Raising the self-consumption share from 40 to 60 per cent shortens the payback far more than adding panel capacity does. Doubling the purchase cost doubles the payback time.

What the calculation leaves out

• Degradation. Panels lose a small share of output each year according to their manufacturers’ warranties.
• Price changes. Higher electricity prices shorten payback, lower ones lengthen it.
• Failures and replacements. Inverters typically carry shorter warranties than panels.
• Taxes and fees. In some countries, registration or meter changes cost money.

The result is a rough guide for your decision, not a guaranteed return. If the payback is long under realistic assumptions, a different location for the panels or a smaller, cheaper kit may make more sense than a bigger one.