A serious solar proposal starts with your half-hourly meter data, a structural survey, and a DNO capacity check. But you can get a useful order-of-magnitude sizing with three numbers: roof area, annual electricity spend, and farm type.
Quick sizing rules
- 📐 Roof area to kWp: 6 sqm per kWp installed (modern 410W panels).
- 🔆 Annual generation: 950 kWh per kWp installed (UK average, south-facing 30° pitch).
- 📊 Self-consumption: 70% for dairy/poultry, 50% for arable, 40% for sheep/mixed (with battery: +20–30%).
- 💰 Cost: £600–£900 per kWp installed.
- 📈 Tariff offset: 25–32p per kWh (current rural commercial three-phase).
- ⏱️ Payback: 2–4 years post-FETF and AIA.
Worked examples by farm type
| Farm | Roof m² | System | kWh/yr | Saving/yr | Net cost | Payback |
|---|---|---|---|---|---|---|
| 200-head dairy | 450 | 75kW | 71,000 | £15,500 | £27,000 | 1.8 yr |
| Arable 800ac | 800 | 130kW | 123,000 | £22,000 | £47,000 | 2.1 yr |
| Free-range poultry (16k bird) | 500 | 80kW | 76,000 | £18,500 | £29,000 | 1.6 yr |
| Mixed 500ac | 350 | 55kW | 52,000 | £11,500 | £20,000 | 1.7 yr |
| Soft fruit + packhouse | 1,200 | 200kW | 190,000 | £42,000 | £72,000 | 1.7 yr |
Net cost = gross cost (£700/kWp avg) less capital grants (25–40%) less first-year AIA tax effect (assumes corporation tax). Savings assume 70% self-consumption at 28p/kWh + 30% export at 8p/kWh SEG.