One of the most common questions homeowners ask before quoting solar: "how many panels do I need?" The honest answer is "it depends" - on your electricity use, roof orientation, available space, and budget. But there's a methodical way to work it out, and most UK homes end up wanting between 8 and 15 panels. Here's the calculation, with worked examples for Cornwall.
The quick rule of thumb
For a typical UK home (no heat pump, no EV, normal occupancy):
- 1-bed flat or small home: 6-8 panels (2.4-3.2 kWp)
- 2-bed terrace or semi: 8-10 panels (3.2-4.5 kWp)
- 3-bed semi or detached: 10-13 panels (4-5.5 kWp)
- 4-bed detached: 12-16 panels (5-7 kWp)
- 5+ bedroom or all-electric home with heat pump: 15-25 panels (6.5-10+ kWp)
- Home with EV charging: add 4-6 panels (1.7-2.7 kWp) to base sizing
These are starting points - personal usage patterns and roof constraints push you up or down.
The proper method
Step 1: Know your electricity consumption
Pull 12 months of electricity bills. Sum the kWh used. Or get a quick estimate from your annual bill divided by your unit rate (around 27p/kWh in 2026).
- UK average household: 2,700 kWh/year
- 3-bed gas-heated home: 3,500 kWh/year
- 4-bed gas-heated home: 4,500 kWh/year
- All-electric with heat pump: 6,000-12,000 kWh/year
- EV charging (typical commuter): +2,500-4,000 kWh/year
Step 2: Understand Cornwall PV yield
Cornwall postcodes typically generate 950-1,050 kWh per kWp installed, per year - one of the higher UK regional yields. Coastal sites and south-facing roofs sit at the top of the range; inland sheltered or east/west sites lower.
To match 100% of annual consumption (in kWh terms):
- kWp needed = annual kWh consumption ÷ 1,000 (Cornwall yield)
- For 3,500 kWh: ~3.5 kWp
- For 6,000 kWh (heat pump): ~6 kWp
- For 9,000 kWh (heat pump + EV): ~9 kWp
Note: matching annual kWh doesn't mean self-sufficiency - you generate in summer, use in winter. Battery helps with daily shifting; seasonal storage is impractical at home scale.
Step 3: Choose panel wattage
Modern domestic panels in 2026 are 400-500W. Higher-wattage panels mean fewer panels for the same kWp:
| Panel wattage | Panels for 4 kWp | Panels for 6 kWp | Panels for 8 kWp |
|---|---|---|---|
| 400W | 10 panels | 15 panels | 20 panels |
| 420W | 10 panels | 15 panels | 19 panels |
| 450W | 9 panels | 14 panels | 18 panels |
| 500W | 8 panels | 12 panels | 16 panels |
Higher-wattage panels cost more per panel but save on roof space, install labour, and mounting hardware. Sweet spot for UK domestic in 2026 is around 430-450W.
Step 4: Check roof space
A modern 400-450W panel measures around 1.75 x 1.10m - roughly 1.9 m². Allow:
- 200mm clear from roof edge for wind loading and access
- 20mm gap between panels for thermal expansion
- One ridge clearance for ventilation
Rule of thumb: a south-facing roof slope of 5 x 4 m fits 8-10 modern panels comfortably. 6 x 4 m fits 10-12. 8 x 6 m fits 18-22.
Step 5: Consider orientation effect
| Orientation | Output vs ideal south (Cornwall) |
|---|---|
| South (180°) | 100% |
| SE/SW (135° or 225°) | ~95% |
| East/West (90° or 270°) | ~80-85% |
| NE/NW | ~65-70% |
| North | ~55-60% |
East/west split installations can actually be useful for self-consumption - generation spread across the day matches household demand better than a peaked midday south-only profile.
Worked example: 3-bed Truro semi, 4,200 kWh annual use
Roof: south-facing, 30° pitch, 5m wide x 4m high. Cornwall yield: ~1,000 kWh/kWp.
- To match 100% of annual consumption: 4,200 / 1,000 = 4.2 kWp needed
- Roof fits 10-12 modern panels (5 x 4m slope)
- 10 x 440W panels = 4.4 kWp - perfect fit
- Predicted Cornwall generation: 4,180-4,620 kWh/year
- Self-consumption without battery: 30-35% (1,300-1,500 kWh saved at 27p/kWh = £350-£400)
- Export at 15p SEG: 2,700-3,100 kWh × £0.15 = £405-£465/year
- Total annual benefit: £750-£870
Worked example: All-electric home with heat pump, 8,500 kWh annual use
Roof: south-east-facing, 35° pitch, 7 x 4m. Heat pump runs year-round.
- To match 100% annual: 8,500 / 1,000 = 8.5 kWp
- Roof fits 14-16 panels (7 x 4m slope)
- 15 x 440W panels = 6.6 kWp - undersized
- 15 x 500W panels = 7.5 kWp - closer but still under
- Reality: roof can't match annual demand. Install 7-7.5 kWp and accept seasonal mismatch (overproduce summer, underproduce winter)
Common mistakes
- Sizing to total roof rather than to need - "fill the roof" sometimes makes sense (cheap incremental cost per panel) but only if you actually use or export the generation.
- Ignoring inverter capacity - a 5kW string inverter on a 6.5kW array throttles generation during peak hours. Match inverter to array or accept clipping.
- Forgetting future demand - if you're planning a heat pump or EV in the next 5 years, size for the future, not now.
- Over-sizing without battery or use - a 10kW array with no battery on a low-consumption home exports 80%+ at 15p while you import at 27p. Diminishing returns kick in.
- Ignoring DNO restrictions - over 3.68kW per phase needs G99 application, which DNO can refuse or limit. Confirm before designing.
Battery sizing alongside panel count
If you're adding a battery, common sizing rules:
- Battery capacity ≈ 1.5-2 x daily winter usage - covers evening peak demand on most days
- For 4 kWp solar: 5-7 kWh battery
- For 6 kWp solar: 7-10 kWh battery
- For 8+ kWp solar: 10-15 kWh battery
Don't oversize battery - cycling efficiency, warranty cycles, and ROI all favour right-sized batteries used hard rather than oversized ones used lightly.
Cornwall-specific sizing tips
- Steep slate roofs - Cornish roofs are often steeper (40-45°) which is closer to winter-optimal angle. Slightly improves December generation.
- Coastal location - higher irradiance, less inland fog. Often justifies smaller systems for same generation as inland Devon.
- Holiday let usage - summer peak occupancy aligns with peak generation, so even modest systems cover summer demand. Off-season export pays back through SEG.
- Off-grid rural property - very different sizing rules apply (battery-dominated, must cover December independently).
Need a Cornwall-specific sizing assessment? Submit your postcode and we'll match you with installers who'll do a proper sizing calculation based on your bills and roof.
Frequently asked questions
How many solar panels does a typical UK home need?
Most UK homes need 8-15 panels (3.5-6.5 kWp). 3-bed semi: 10-13 panels. 4-bed detached: 12-16. All-electric with heat pump and EV: 15-25 panels. Match annual generation to annual consumption where possible.
How much roof space do I need for solar panels?
About 1.9 m² per modern panel (400-450W). A 4 kWp system needs around 19-21 m². A 6 m x 4 m roof slope fits 10-12 panels comfortably.
Should I install more panels than I need?
Sometimes - if you plan a heat pump or EV in future, sizing larger now is cheaper than retrofitting. But over-sizing without use means exporting at 15p while importing at 27p; the maths favours right-sizing.
What size system for a 3-bed Cornwall home?
Typically 4-5 kWp (10-13 panels). Annual generation 4,000-5,000 kWh in Cornwall - matches or slightly exceeds typical 3-bed consumption.
What's the maximum I can install without DNO restrictions?
Single-phase: 3.68 kW per phase before G99 application. Three-phase: 11 kW. Above these limits the DNO can require export limitation or upgrade contributions in constrained networks.
Are higher-wattage panels better?
Better if roof space is limited (fewer panels for same kWp). Same efficiency per area as lower-wattage equivalents. Cost per kWp is similar. Choose for fit and budget.
Does east-west split work as well as south?
About 80-85% of south yield, BUT generation spread across the day means more matches household demand. Total annual export can be lower (less self-consumption gap), so financial outcome is often similar.
How do I include a heat pump in sizing?
Add the heat pump's annual electricity use (typically 4,000-6,000 kWh for a 9kW heat pump) to your existing baseline. Size solar to cover the combined total. Solar + heat pump pair extremely well financially.
What if my roof is too small for the system I want?
Higher-wattage panels (450-500W) get more kWp into the same space. Garage roof, outbuilding, or ground-mount add additional capacity. For some homes the answer is "you can't have what you'd ideally want" - and that's an honest answer.