How Many Solar Panels Do I Need?
Free UK calculator to estimate how many solar panels your home needs. Uses MCS-aligned irradiance, average British household consumption (2,700 kWh/yr), and 400 W panel specs.
How Many Solar Panels Do I Need?
How to use this calculator
Enter five figures and the tool returns the system size in kWp, panel count, required roof area, and estimated annual production:
- Monthly electricity use (kWh) — average from your last 12 months of bills. UK domestic average is 225 kWh/month per Ofgem TDCV 2026 (medium consumption household).
- Peak sun hours per day — UK averages 2.4 (Glasgow) to 3.0 (Cornwall), with 2.6 a workable national average. Energy Saving Trust publishes regional figures.
- Panel wattage (W) — 400 W is typical for UK residential MCS-listed panels in 2026.
- System efficiency (%) — leave at 78% for a typical pitched-roof install with a single string inverter, BS 7671-compliant DC isolators, and minimal shading.
- Offset target (%) — 100% covers your annual consumption. Going beyond rarely pays under SEG — you earn pence per exported kWh, far less than the 27p+/kWh Ofgem cap on imports.
The formula
required_kWp = (annual_kWh × offset / 100) ÷ (peak_sun_hours × 365 × derate)
panel_count = ceil(required_W ÷ panel_wattage)
roof_area_m² = panel_count × panel_area × 1.08 (mounting clearance)Worked example for a Manchester semi-detached home:
- Monthly use: 290 kWh → annual 3,480 kWh
- Peak sun hours: 2.5 (north-west England average)
- Derate: 78%
- Required: 3,480 ÷ (2.5 × 365 × 0.78) = 4.89 kWp
- At 400 W per panel: ceil(4,890 ÷ 400) = 13 panels
- Roof area: 13 × 2.1 m² = 27 m²
- Annual production: 5.2 kWp × 2.5 × 365 × 0.78 = 3,704 kWh
Reference table by household profile
Using 2.6 peak sun hours, 78% derate, 400 W panels, 100% offset:
| Monthly kWh | Annual kWh | Profile | System size | Panels | Roof area |
|---|---|---|---|---|---|
| 150 | 1,800 | Ofgem low (1-2 person flat) | 2.4 kWp | 6 | 13 m² |
| 225 | 2,700 | Ofgem medium (3-4 person semi) | 3.6 kWp | 9 | 19 m² |
| 350 | 4,200 | Ofgem high (4+ bed detached) | 5.7 kWp | 15 | 32 m² |
| 500 | 6,000 | Heat pump household | 8.1 kWp | 21 | 44 m² |
| 700 | 8,400 | All-electric + EV | 11.3 kWp | 29 | 61 m² |
Beyond 4 kWp, you cross the MCS threshold for DNO-approved G99 application (vs. simpler G98 notification), which can add 4–8 weeks to the installation timeline.
What changes the panel count
UK regional variation
Solar Energy UK and the MCS Performance Estimation Calculator put Cornwall at 1,050 kWh per kWp installed annually; central Scotland at 800 kWh/kWp; Greater London around 950 kWh/kWp. The further north you are, the more panels you need for the same offset. For Glasgow, multiply panel count by 1.18 versus the national average.
Roof orientation
The MCS Standard MIS 3002 publishes performance multipliers:
- South at 30–40° pitch: 1.00 (reference)
- South-east or south-west at 30°: 0.96
- East or west at 30°: 0.85
- North at 30°: 0.55 (rarely viable)
A pure east/west array therefore needs ~18% more panels than a south-facing one for the same annual yield.
Shading from chimneys and dormers
A single shaded panel on a UK string inverter can drag the entire string down 50%+. Module-level power electronics (SolarEdge optimisers, Enphase micro-inverters) are now standard on MCS-certified UK installs and limit losses to the shaded panel only.
Battery storage and time-of-use tariffs
If you’re pairing with a battery and using Octopus Go, Intelligent Octopus, or Cosy, sizing logic changes. Battery owners typically slightly oversize the array (10–15%) to charge the battery from solar in summer rather than from the grid, capturing the cheap-rate import to top up only in winter.
Common UK mistakes
- Sizing off a December bill. UK winter consumption is 70%+ higher than summer for most households. Always average across 12 months, ideally pulled directly from your supplier’s HH (half-hourly) data via Octopus, EDF, or the smart-meter portal.
- Ignoring permitted-development limits. PD rights cover most domestic solar in England under the 2008 amendments to the Town and Country Planning Order, but Conservation Areas and Listed Buildings need explicit planning consent. AONBs allow rear-roof installs only.
- Specifying without MCS. Without MCS certification you cannot claim SEG and your insurer may decline cover. Always commission via MCS-listed installers (Checkatrade and MyBuilder filter for this).
- Forgetting the DNO half. Anything above 3.68 kW per phase (3.68 kWp on single-phase, 11 kWp on three-phase) requires G99 pre-approval from your distribution network operator.
How this calculator compares to a real MCS quote
An MCS-accredited installer runs the MCS Standard Performance Estimation Method (MIS 3002 Annex F) for your exact roof — measured azimuth, pitch, shading from PV-Sol or PVGIS, and panel-level losses. This calculator uses Ofgem TDCV averages and PVWatts default losses, so expect ±12% variance versus a site survey. Use it for budgeting and as a fit check before booking a Checkatrade or MyBuilder installer visit.
Sources
- Ofgem Typical Domestic Consumption Values 2026 — average household kWh by tier
- MCS Performance Estimation Calculator — kWp-to-kWh conversion by region
- Energy Saving Trust — regional solar yield data
- Solar Energy UK Annual Report 2026 — installed capacity and panel-mix trends
- Checkatrade Solar Cost Guide 2026 — typical UK system pricing
- BS 7671:2018+A2:2022 IET Wiring Regulations — electrical install standards