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Free calculator

Use this calculator to

  • Work out how many panels and what array size in kWp cover your annual usage
  • Adjust peak sun hours for roof orientation and UK location
  • Model system losses from inverters, shading and soiling
  • Split generation into self-consumed and exported kWh at your own rates

Solar Panel System Sizing Calculator

Estimate the number of panels and system size needed to meet your electricity demand.

kWh/yr

UK average is ~3,100 kWh/year for a 3-bed home. Check your energy bill.

hr/day

UK average: ~3.0–3.5 hr/day. South-facing roof = 3.8, north = 2.5.

W
%

Accounts for inverter loss, shading, soiling. Typical: 75–85%.

%

% of solar energy used directly (not exported). Typical without battery: 30–50%.

p/kWh
p/kWh
System Estimate

Formulas

  • Daily energy needed = Annual kWh / 365
  • System kW = Daily kWh / (Peak sun hrs × Efficiency)
  • Number of panels = System kW × 1000 / Panel wattage
  • Annual generation = System kW × Peak sun hrs × 365 × Efficiency
  • Annual saving = (Self-consumed kWh × tariff) + (Exported kWh × export rate)

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Common scenarios

Select one to run it in the calculator above.

For business

Why this matters for businesses

A commercial solar decision starts with a single number: the kWp array you can realistically install against your roof or land, set against the kWh you actually consume. The default mistake is to size the array to annual consumption divided by 950 kWh/kWp and call it done. UK irradiance is lower than that headline in the North, the load is rarely coincident with generation in a manufacturing setting, and a 1 MWp on paper might only displace 60% to 70% of daytime demand once cloud cover, weekend patterns and shading are layered in. Better to start with monitored half-hourly demand, then layer the irradiance curve.

The other commercial input is your purchase route. A landlord-funded array with a PPA shifts the question from capex payback to the strike rate negotiated against your wholesale import price, which means the array sizing has a direct line into your supply contract review. A self-funded array with a REGO-backed supply contract is a different conversation again, because the carbon and renewable claims need to be reconciled to avoid double-counting between your on-site MWh and your supplier's REGO certificates. Sizing decisions that ignore the contract structure routinely create downstream SECR and reporting headaches.

Purely Energy does not install solar, but we work with the contract side of every major UK supplier and we run Purely Insights monitoring across thousands of sites. The clients who consistently extract the strongest IRR from an on-site array are the ones who size against monitored consumption data, not nameplate billing data, and who plan the half-hourly profile match before signing the EPC contract. Use this calculator as the first-pass sanity check, then have a competent solar designer model irradiance, shading and inverter losses against verified consumption before committing.

Common questions

How many solar panels does my building need?

Divide annual usage by 365 to get daily kWh, then by peak sun hours and system efficiency to get the array size in kWp, and finally by panel wattage for the count. A home using 3,500 kWh a year with 3.5 sun hours and 80 percent efficiency needs about 3.4 kWp, which is nine 400 W panels. Commercial roofs scale the same way: 50,000 kWh a year needs roughly a hundred panels.

What are peak sun hours in the UK?

Peak sun hours compress the day's variable sunlight into equivalent hours of full-strength irradiance at 1 kW per square metre. The UK averages around 3.0 to 3.5 a day across the year, higher on the south coast, lower in northern Scotland. Orientation matters too: a south-facing roof at a 30 to 40 degree pitch performs best, with east or west facing arrays giving up roughly 15 percent.

What system efficiency figure should I use?

75 to 85 percent is the realistic band. The losses stack up from the inverter, cable runs, panel temperature on warm days, soiling, and any shading from chimneys, trees or parapets. 80 percent is a sensible default for an unshaded pitched roof. If part of the array is shaded for part of the day, model it separately or specify optimisers, because shading hits whole strings disproportionately.

What is self-consumption and why does it drive the savings?

Self-consumption is the share of generated energy you use on site rather than export. Each self-consumed kWh offsets a unit bought at your full supply rate, while each exported kWh earns only the Smart Export Guarantee rate, which is usually much lower. Homes without storage typically self-consume 30 to 50 percent; daytime-heavy businesses can reach 80 percent or more, which is why commercial rooftop solar pays back faster.

How much roof space does the system need?

Allow roughly 2 square metres per panel including mounting clearances: a 400 W module measures about 1.7 by 1.1 metres. Nine or ten panels for a typical home therefore need around 20 square metres of unshaded roof. Commercial flat roofs need more per kWp because ballasted frames are tilted and spaced in rows to avoid self-shading, typically 7 to 10 square metres per kWp.

Related calculators

The conversions that procurement and engineering use alongside this one.

Solar Panel System Sizing Calculator | Purely Energy