Before getting quotes from installers, knowing the approximate size of system you need puts you in a much stronger position. An oversized system wastes money; an undersized one leaves generation potential — and savings — on the table. This guide walks through the calculations step by step.
Step 1: Find Your Annual Electricity Usage
Your electricity bill should state your annual consumption in kWh. If not, look for a 12-month total or multiply your average monthly kWh by 12. UK household averages by property type in 2026:
| Property Type | Average Annual kWh | Typical Occupants |
|---|---|---|
| 1-bed flat | 1,500–2,200 | 1 |
| 2-bed house | 2,200–3,000 | 2 |
| 3-bed house | 3,000–4,500 | 2–4 |
| 4-bed house | 4,000–6,000 | 3–5 |
| 5-bed+ house | 5,500–8,000+ | 4–6+ |
If you have an EV, add 2,000–4,000 kWh/year depending on mileage. If you have an air source heat pump, add 2,500–5,000 kWh/year.
Step 2: Calculate Your Ideal System Size
The simple formula: Annual kWh ÷ 850 = System size in kW
The 850 figure represents a conservative estimate of kWh generated per kW of installed capacity per year in the UK, accounting for average irradiance, typical shading losses, and inverter efficiency.
Examples:
- 3,500 kWh/year ÷ 850 = 4.1kW system
- 5,000 kWh/year ÷ 850 = 5.9kW system
- 2,500 kWh/year ÷ 850 = 2.9kW system
In practice, installers recommend sizing your system to cover 70–80% of your annual demand — you’ll always need some grid electricity (evenings, winter peaks), and oversizing beyond your roof’s capacity or your export capability is wasteful.
Step 3: Check Your Roof Space
Modern 400–450W panels measure approximately 1.7m × 1.1m (1.87m²). Here’s how many panels different roof areas can accommodate:
| Usable Roof Area | Max Panels (420W) | Max System Size |
|---|---|---|
| 15m² | 8 panels | 3.4kW |
| 20m² | 10 panels | 4.2kW |
| 25m² | 13 panels | 5.5kW |
| 30m² | 16 panels | 6.7kW |
| 40m² | 21 panels | 8.8kW |
Remember to account for ridge tiles, skylights, chimneys, and the 30cm clearance typically required from roof edges for fire safety and access.
Step 4: System Size by Property Type
This combines steps 1–3 with typical UK roof sizes:
| Property | Typical System | Panels (420W) | Est. Annual Generation | Est. Annual Saving |
|---|---|---|---|---|
| 2-bed terrace | 3kW | 7–8 | 2,550 kWh | £500–£700 |
| 3-bed semi | 4kW | 10 | 3,400 kWh | £700–£900 |
| 3-bed detached | 4–5kW | 10–13 | 3,400–4,250 kWh | £800–£1,100 |
| 4-bed detached | 5–6kW | 13–15 | 4,250–5,100 kWh | £1,000–£1,300 |
| 5-bed detached | 6–8kW | 15–20 | 5,100–6,800 kWh | £1,200–£1,700 |
Step 5: Inverter Sizing
Your inverter should be sized at 80–100% of your panel array’s peak DC output. A 4kW panel array needs a 3.6–4.0kW inverter. Slightly undersizing the inverter (clipping) is common practice and only loses 1–2% annual generation while reducing inverter cost.
Frequently Asked Questions
Q: Can I calculate my savings before getting a quote?
A: Yes — a rough estimate: (system kW × 850 kWh) × 0.5 (self-consumption rate) × 28p/kWh + (system kW × 850 × 0.5) × 10p/kWh (export). For a 4kW system: (3,400 × 0.5 × £0.28) + (3,400 × 0.5 × £0.10) = £476 + £170 = £646/year.
Q: Does roof pitch affect how many panels I can fit?
A: Pitch doesn’t affect the number of panels directly — it affects the spacing between rows if multiple rows are needed. Single-row arrays on a standard 35–45° pitch need no row spacing adjustments.
Q: Is it worth going bigger than I need to accommodate future demand?
A: Yes, if your roof allows. Adding an EV or heat pump later is common, and installing a larger system now costs proportionally less than expanding later. A 5kW system on a 3-bed home is often a smart future-proofing choice.
Q: What if my roof faces east-west rather than south?
A: An east-west split array (panels on both sides of the roof) can actually provide more consistent generation throughout the day than a purely south-facing array, though peak generation is lower. Installers can model both options for comparison.
Q: Can I use an online solar calculator?
A: Yes — the Energy Saving Trust’s solar calculator and PVGis (EU tool) are both reliable. Input your postcode, roof orientation, and pitch for a localised generation estimate. These estimates tend to be slightly conservative.