Technical
How much roof space do I need for commercial solar?
A commercial solar install needs roughly 6-7 m² of usable roof per kW. A 100 kW system fills 600-650 m². For SMEs, that's typically 50-70% of a light-industrial unit roof, 80-100% of an office roof, or one full bay of a large warehouse. Pitched roofs, shaded roofs, and roofs with skylights or HVAC plant lose effective area — usable area is rarely the full roof footprint.
A commercial solar install in 2026 needs roughly 6-7 m² of usable roof per kW of installed capacity. A 100 kW system therefore needs 600-650 m² of usable roof area. For a typical UK SME, that translates to: 50-70% of a 1990s steel-portal light-industrial unit roof (1,000-1,500 m² total), 80-100% of a typical office building roof (300-700 m² total), or roughly one full bay of a large distribution warehouse (single bay 400-800 m²). Pitched roofs, roofs with skylights, roofs with rooftop HVAC plant, shaded roofs, and roofs with limited setback to edges all reduce effective usable area below the gross footprint.
How to calculate usable roof area
Start with your gross roof area and subtract:
- Edge setback: 50 cm minimum from roof edges (CDM safety)
- Penetrations: skylights, smoke vents, AC units, satellite dishes
- Shaded zones: anywhere shaded for more than 2 hours per day in summer (mostly north-facing slopes)
- Walkways: maintenance access aisles required for arrays >40 kW
- Structural exclusion zones: areas where a structural engineer flags load capacity issues
Typical reduction from gross to usable: 25-40%. A 1,000 m² gross roof is often 600-750 m² usable.
Roof area to system size lookup
| Usable roof area | Typical system size | Annual generation |
|---|---|---|
| 100 m² | 16 kW | 14,500 kWh |
| 200 m² | 32 kW | 29,000 kWh |
| 300 m² | 48 kW | 43,500 kWh |
| 500 m² | 80 kW | 72,500 kWh |
| 750 m² | 120 kW | 109,000 kWh |
| 1,000 m² | 165 kW | 150,000 kWh |
| 1,500 m² | 250 kW | 227,500 kWh |
| 2,500 m² | 400 kW | 365,000 kWh |
| 5,000 m² | 800 kW | 730,000 kWh |
Roof type efficiency
Different roof types have different effective panel densities:
- Standing-seam metal (modern profile): 95-100% gross utilisation. Clip-fix mounting, no penetration, easy install.
- Trapezoidal metal: 90-95%. Penetrating self-tapping fixings or seam clamps.
- Concrete tile: 75-85%. Hooks under tiles. Tile breakage risk during install.
- Slate: 70-85%. Slate hooks. Conservation issues common.
- Asphalt-membrane flat roof: 70-80%. Ballasted or mechanically fixed frames; need wind uplift checks.
- Single-ply membrane (TPO/EPDM): 70-80%. Ballasted frames preferred to avoid penetration.
- Asbestos cement sheet: not directly mountable. Over-clad with metal panel system first (£25-£60/m² over-clad cost) before installing PV.
What if your roof is too small?
Several options:
- Smaller system: install what fits. Maximises generation for available area.
- Carport solar: convert car park to solar carport. £1,300-£1,600/kW (50% premium over rooftop). Gives EV charging integration.
- Ground-mount: use yard or unused land. Above 9 m² needs planning permission. £750-£1,000/kW.
- Adjacent building: if your business owns or leases nearby buildings (e.g. multi-let estate), install across multiple roofs and aggregate to single supply.
- Off-site PPA / Corporate PPA: a third party builds elsewhere and supplies via the grid under sleeved arrangement.
When roof space isn’t the binding constraint
For most SMEs, roof space isn’t actually the binding constraint — annual electricity consumption is. A small office might have 400 m² of roof but only consume 30,000 kWh/year. A 30 kW system (180 m²) covers their consumption; the remaining 220 m² is wasted unless you’re prepared to export 60%+ at low SEG rates.
The economic optimum is usually a system sized at 60-80% of annual consumption, not at the maximum the roof can take.
Common misconceptions about roof area
“More panels = more savings” — only true if you self-consume them. Above the consumption line, additional panels just export at low SEG rates. Marginal economics deteriorate quickly.
“East-west roofs are useless” — wrong. East-west splits achieve about 90% of south-facing yield. Sometimes preferred because they spread generation across the day, lifting self-consumption.
“Pitched roofs are cheaper than flat” — varies. Standing-seam metal flat roof is the cheapest install per kW. Pitched tile is the most expensive due to mounting complexity.
“All flat roofs need ballast” — wrong. Modern flat-roof systems use mechanical fixing through the membrane (with manufacturer-approved sealing). Ballast is one option, not the only one.
Next steps
For a roof survey and sizing model, request a feasibility study. See system sizing FAQ, generation per panel, cost guide, and grants and funding.
Related questions
How much energy does a commercial solar panel produce?
A typical 540 W commercial solar panel in the UK produces 480-580 kWh per year, with the variation driven by location (south coast vs Scottish highlands), orientation, pitch, and shading. South-facing 35-40 degree pitch in the Midlands is the benchmark — about 530 kWh/panel/year. The whole system produces 850-1,050 kWh per kW of nameplate annually.
What size solar system does a business need?
Most UK SMEs need a solar system sized at 60-80% of annual electricity consumption to maximise self-consumption while staying within roof and capex constraints. For a business spending £30,000/year on electricity, that's typically a 60-75 kW system. Sizing should be driven by half-hourly meter data, not roof area or rules of thumb.
What size solar system does my business need? (sizing methodology)
To size a commercial solar system, start with your annual kWh consumption from your bills, target 60-80% of that as annual generation, then check against roof area (6-7 m²/kW), electrical capacity (G98 vs G99 threshold), and budget. The right size is the smallest of these four ceilings. Insist on PVSyst modelling from half-hourly meter data, not generic estimates.