Solar Output Calculator: kWh per Year at Your Postcode

How Much Electricity Would Solar Panels Generate on Your Roof?

A 4.275 kWp solar system generates about 4,136 kWh a year on a south-facing roof at 35 degrees with no shade in postcode area B, at 967.6 kWh per kWp. The figure is the calculator's result for a postcode in area B (our calculation from PVGIS 5.3, 2005 to 2023); with no postcode, the card above uses a stand-in figure and says so. kWp (kilowatt peak) is the system's rated output under standard test conditions. Your postcode, roof direction, tilt and shade move that figure, and the calculator above works out yours.

This output calculator is one of a set, listed with more solar calculators. An output figure is an average for a place, and your own roof changes what the same system costs and saves; the cost and savings calculator on the home page works out both.

The estimate is in kWh a year, the unit your bill counts. The solar power calculator costs nothing to use and asks for no email or phone number before the result. A postcode is enough; no full address is needed. Four things you enter decide the figure.

What You Need to Run the Solar Output Calculator

You need four things: your postcode, your system size in kWp, your roof direction and pitch, and how much shade the roof gets. The table lists what the calculator asks, why each answer changes the output and where to find it.

Your yearly use does not change what a size you enter generates; it changes what the output is worth to you. With "Not sure: size it for me", it also sets the size. Check the output for your roof

System Size: kWp In, kWh Out

kWp measures what the panels can deliver at one moment under standard test conditions; kWh measures the electricity they generate over time. System size in kWp is the number of panels times the wattage of one panel, divided by 1,000.

kWp = number of panels x panel wattage / 1,000

Ten panels of 475 W each, our default wattage (our rule, which you can change in the calculator), make a system of 4.75 kWp. A search for a "kW calculator" or a "power calculator" is answered here in kWh a year, because kWh is what a bill counts. kWh is also the unit for a battery's size, and kWp belongs to the panels only. kWp goes in and kWh comes out: the step between them is the formula.

How Do You Calculate Solar Panel Output?

Yearly output (kWh) = system size (kWp) x kWh per kWp x shade factor. The kWh per kWp value is what each kWp makes in a year at your postcode, roof direction and pitch; the shade factor runs from 1 (no shade) down to 0. The calculation has three steps.

1. Find your system size in kWp.
2. Look up the kWh per kWp for your postcode area, roof direction and pitch.
3. Multiply the two, then multiply by the shade factor.

A 4.275 kWp south-facing roof at 35 degrees with no shade in postcode area B gives 4.275 x 967.6 x 1 = 4,136 kWh a year. MCS generation estimates use the same sum: kWp x Kk x shade factor (MCS standard MIS 3002), where Kk is the MCS name for the kWh per kWp value. Our kWh per kWp values come from PVGIS 5.3, not from the MCS tables, so the two estimates share a formula and can differ in the yield they feed it. A formula that uses one regional number misses the effect of direction and pitch, which is why the calculator asks for both.

Specific Yield and the Monthly Profile

Specific yield is the kWh one kWp generates in a year at a given postcode, direction and pitch; on this page we call it kWh per kWp. Postcode area B has a specific yield of 967.6 kWh per kWp for a south-facing roof at 35 degrees.

The calculator holds one yearly value for each of 121 UK postcode areas, worked out with PVGIS 5.3, the solar tool of the European Commission Joint Research Centre. A second table holds the share of that value each of eight directions keeps at three pitches. System losses of 14% are inside the values. The calculator holds yearly values only, so it shows no month-by-month profile. Because kWh per kWp changes with place, direction and pitch, those inputs move your result.

How Location, Direction, Tilt and Shade Change Your Result

A roof facing east generates 79.5% of what the same roof generates facing south, and a roof facing west 78.9%, at a 35 degree pitch (our calculation from PVGIS 5.3). Location, pitch and shade move the figure too. The first table shows what each input does.

Roof direction and its effect on output are the subject of our guide to the best orientation for solar panels. Tilt and roof pitch are covered in our guide to the best tilt for your roof pitch. The second table puts the same roof in four postcode areas.

The calculator uses your own postcode area, not the nearest row of this table. The yearly total also hides a swing between summer and winter.

How Your Output Is Spread Across the Year

Your output is not spread evenly across the year: in winter, solar panels "generate less electricity than in summer". The reasons given are that "the days are shorter; it's more likely to be cloudy; the sunlight isn't as intense" (Energy Saving Trust, Solar power fact check). The same source says panels "still generate electricity on cloudy days".

The result card shows the yearly total and an average per day, which is the yearly figure divided by 365. A summer day generates more than that average and a winter day less, so the daily average is a poor guide to any one day. The calculator has no monthly figures. These are figures for an average year, and your real years vary.

How Close Will Your Real Output Be to the Estimate?

Expect your real output to land near the estimate in an average year and to move above or below it with the weather; the estimate is for a typical year, not a promise for any one year. A real year differs from the estimate for six reasons.

• A sunnier or duller year than the 2005 to 2023 average the estimate rests on
• Shade you did not enter, or new shade from growing trees
• Panel output falling with age, 0.5% a year in our 25-year figure
• Dirt on panels that are nearly flat: rain cleans panels "tilted at 15 degrees or more" (Energy Saving Trust, page updated 1 October 2026)
• Time the system is off for a fault or an inverter replacement
• The installer's design: panel layout and inverter size

If your installer gives you an MCS generation estimate, compare its kWh a year with ours for the same kWp, direction, pitch and shade, and ask about any big gap. The installer's survey sees your roof and we do not. Yearly kWh output per panel and per system, and ways to check real output after fitting, belong to our guide to solar panel output across the UK. Output is half of the decision; what it is worth decides the verdict.

What Your Result Means: Good Fit, Borderline or Poor Fit

Output alone does not decide the verdict: we compare what your panels would generate with what the system costs and what the electricity is worth to you, then call the result a Good fit, Borderline or a Poor fit. The table shows what follows each verdict.

On a north-facing roof the verdict is a Poor fit: the panels generate 54.2% of what a south-facing roof generates at 35 degrees (our calculation from PVGIS 5.3). The system may not pay back within the panels' expected life. Solar panels "should last 25 years or more" (Energy Saving Trust, page updated 1 October 2026). The verdict and the figures are the same whether or not you ask for a survey. The overall verdict on solar for a UK home is on our page about whether solar panels are worth it in the UK. This verdict rests on what the output saves.

What Is Your Solar Output Worth in Savings and Export?

Each kWh of your output that you use at home saves the unit rate you would have paid, 26.32p per kWh under the Ofgem price cap for October to December 2026; each kWh you export earns your export rate, 13p per kWh by default (our calculation from Ofgem figures).

The share you use at home depends on when you are in, and the card shows the split. A battery stores daytime surplus for the evening and is sized in kWh, not kWp; its price, size and payback are in our calculator for solar battery cost and payback.

Export payments come through the Smart Export Guarantee, which started on 1 January 2020 and covers Great Britain. For solar, the scheme asks for MCS certification or an equivalent and a meter that records exports. It is not the Feed-in Tariff, which closed to new applicants on 1 April 2019.

Payback is the installed cost divided by the yearly saving plus the export income. The card's cost line uses the DESNZ mean cost per kWp for your size band (£2,109 up to 4 kWp and £1,651 from 4 to 10 kWp, April 2025 to March 2026). Our rule at the band edge: the larger band has a lower mean per kW, so a system above 4 kWp is never priced below a 4 kWp system, which is why the card shows the same price as a 4 kWp system. The cost on the card comes from the calculator, rounded to the nearest £100. Home solar carries 0% VAT until 31 March 2027 and 5% from 1 April 2027 (HMRC VAT Notice 708/6), and the card shows the cost at both rates. Savings and the payback period in years, from a quote or from our estimate, are the job of our solar panel savings and payback calculator. A survey turns this estimate into a design and a quote.

Get a Free Survey to Check Your Output and Price

A survey checks your output and price on the roof itself, and it follows a Good fit or Borderline result in two steps.

1. You request a survey after a Good fit or Borderline result.
2. An installer visits your home and gives you a quote, with their own generation estimate.

You will get phone calls. That is how surveys are arranged. We are paid by Leads 2 Trade Limited when a homeowner we refer asks for a survey. You are never charged. It does not change our figures.

We do not refer tenants, people who are only researching, postcodes outside the installers' coverage, Poor fit results or households eligible only for funded schemes. The request form is not open on this site yet.

Solar Output Calculator Questions