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Solar Panel Output Calculator (UK)

Free UK solar panel output calculator. Estimate daily, monthly, and 25-year kWh production from your array, sun hours, and system efficiency — plus pound savings.

Solar Panel Output Calculator

Daily output
8.11 kWh
Monthly output
247 kWh
Annual output
2,961 kWh
Year-1 bill savings
£799
25-year output
69,746 kWh
25-year bill savings
£31,055

How to use this calculator

Enter six numbers and the calculator returns daily, monthly, annual, and 25-year energy output, plus pound savings:

  1. Array size (W) — total panel wattage. A 4 kW system is 4,000 W (sum of STC nameplates).
  2. Peak sun hours per day — UK ranges 2.2 (Scotland) to 2.8 (South Coast). Default 2.6 is the England/Wales average.
  3. System efficiency (%) — leave at 78%. This is MCS MIS 3002 standard.
  4. Panel age (years) — 0 for new install, 10 for ten-year-old system.
  5. Electricity rate (£/kWh) — Ofgem cap for April 2026 is 27p/kWh standard rate.
  6. Annual rate escalation (%) — UK retail electricity has risen 5–8%/yr in the 2020s. 4% is a balanced default.

The formula

daily_kWh    = array_W × peak_sun_hours × derate / 1000
annual_kWh   = daily_kWh × 365
lifetime_kWh = sum over 25 years of annual_kWh × (1 - degradation)^year

A worked example for a 4 kW MCS-certified system in Surrey:

  • 4,000 W × 2.7 sun-hr × 0.78 = 8.4 kWh per day
  • 8.4 × 30.4 = 256 kWh per month
  • 8.4 × 365 = 3,074 kWh per year
  • At 27p/kWh: £830 in year-1 bill savings
  • Lifetime (25 years, 0.5% degradation, 4% rate escalation): about £36,000 in savings

Output reference table by system size

Using 2.6 peak sun hours, 78% derate, 0-year age, £0.27/kWh, and 4% annual rate escalation:

System sizeDaily kWhMonthly kWhAnnual kWh25-yr kWh25-yr savings
3 kW6.11852,22252,100£24,600
4 kW8.12472,96369,500£32,800
5 kW10.13083,70486,900£41,000
6 kW12.23704,445104,200£49,200
8 kW16.24935,927139,000£65,600
10 kW20.36177,408173,700£82,000

The pattern is linear in array size — a 6 kW system produces twice what a 3 kW does under identical UK conditions.

What changes the UK output

Region

Peak sun hours dominates. Cornwall (2.9 PSH) and the South Coast give 12–15% more annual production than the Lake District or Scottish Highlands (2.2 PSH). PVGIS-SARAH2 satellite data is the standard reference and matches MCS MIS 3002 calculations within 3%.

Tilt and orientation

A south-facing roof at 35° tilt is the UK reference case. East-west splits cost 10–12% per year. Flat roofs benefit from 10–15° ballast frames. North-facing roofs are not viable in the UK — the loss is 25–35% versus south. See the solar panel tilt calculator for your specific roof.

Shading

Chimneys, neighbouring buildings, and mature trees are the biggest UK output killers. A single shaded panel in a 10-panel string can cut overall production by 30–50% if the system has only a central string inverter. SolarEdge HD-Wave or Enphase IQ8 micro-inverters limit losses to the shaded panel only. Pay the £400–600 premium if your roof has any shading.

Temperature

Hot panels lose 0.3–0.4% per °C above 25°C cell temperature. UK summers reach 35°C cell temperature on dark roofs — about a 4% loss versus rated. Far less of an issue than US Sun Belt installs but still worth modelling for accurate annual estimates.

Soiling

UK rain washes panels effectively. Soiling losses average 1–2% per year, far less than the 4–6% seen in arid climates. Specialist cleaning is rarely cost-effective; a once-every-3-years rinse is sufficient. The solar panel cleaning cost calculator shows when it pays back.

Why the 78% derate matches MCS

MCS Standard MIS 3002 uses 0.78 as the system performance factor for SAP calculations, which is the same default as NREL PVWatts v6. The components:

  • Inverter efficiency: 96–97% for modern UK-listed string and micro-inverters
  • AC and DC wiring losses: 2%
  • Soiling: 1.5% annual average for UK rooftops
  • Mismatch and connection losses: 2%
  • Light-induced degradation (LID): 1.5% over the panel lifetime
  • Temperature derating: ~5% effective annual average (less than US)

Multiplied: about 0.78. UK installations with optimisers, oversized inverters, and clean South Coast conditions can hit 82%; older central-inverter systems on shaded roofs may sit at 72–74%.

Common UK mistakes

  • Using US peak sun hours: 4.8 PSH might be Florida — the UK is 2.4–2.8. This single error overstates production by 60–80%.
  • Ignoring the SEG export rate: Smart Export Guarantee (SEG) tariffs from Octopus, OVO, and E.ON pay 5p–15p per kWh exported. If you can’t use a kWh on-site, exporting it is half the value of self-consumption — but it’s not zero.
  • Forgetting battery economics: A 5 kWh battery captures 60–70% of midday excess that would otherwise export at 5p/kWh and shifts it to evening peak at 27p/kWh. The solar panel ROI calculator handles this.
  • Quoting from US sites: SunPower, Tesla Solar, and EnergySage data is US-only. UK households should rely on MCS-certified installers using PVGIS or Skelion modelling.

Sources

Frequently asked questions

How much electricity does a 4 kW solar array produce per year in the UK?
About 3,400–3,800 kWh per year for a south-facing roof at 35° tilt, depending on whether you're in southern England (3,800+) or Scotland (2,900–3,200). The MCS Standard MIS 3002 reference SAP calculation uses 950 kWh per kWp/year for a horizontal panel and adjusts up to ~1,050 kWh/kWp for optimal tilt and orientation. Energy Saving Trust's 2026 figures put a 4 kW system at 2,700–3,800 kWh depending on region.
What are typical peak sun hours in the UK?
The UK averages 2.4–2.8 peak sun hours per day annually. Cornwall and the South Coast hit 2.8–3.0; the Midlands and Wales average 2.5–2.6; Scotland and Northern Ireland sit at 2.2–2.4. Met Office solar irradiance data and the Photovoltaic Geographical Information System (PVGIS) are the authoritative UK sources. Don't use US-style 4–5 PSH numbers for UK calculations — they overstate output by 60–80%.
Why does my UK solar system produce less than the rated kWp suggests?
STC ratings assume 1,000 W/m² irradiance and 25°C cell temperature. UK winters see 200–400 W/m² for most daylight hours, and even summer midday peaks rarely exceed 850 W/m². Combined with inverter losses (3%), DC/AC wiring (2%), soiling (3%), and mismatch (2%), real-world output is 75–80% of nameplate. The MCS MIS 3002 calculation uses 0.78 as a system performance factor, identical to NREL's residential default.
How does shade affect UK solar output?
UK rooftops often have chimneys, dormer windows, and neighbouring trees that throw shadow on at least part of the array for hours each day. With a string inverter, a single shaded panel can cost 30–50% of string output. SolarEdge optimisers or Enphase micro-inverters limit this to just the shaded panel. MCS-certified installers should use Hellios or Skelion shade modelling on every quote — ask for the printout.
What is the UK 0% VAT rate on solar installations?
Since April 2022, residential solar PV installations have been zero-rated for VAT in the UK (extended through March 2027 in the 2024 Spring Budget). This applies to panels, inverters, batteries, and labour for a single-property domestic install. Commercial installs still pay 20% VAT, and any system over 1 MW is excluded. The 0% rate has cut typical 4 kW system costs from ~£8,000 (incl. VAT) to ~£6,500–7,500 today.

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