Solar panel output depends on panel rating, sunlight hours, and system efficiency. Knowing how to calculate expected generation helps you size a system correctly and estimate savings.
Key Metrics
- Peak power (Wp): Output under standard test conditions (1,000 W/m², 25°C)
- Peak sun hours (PSH): Hours per day when irradiance averages 1,000 W/m²
- System efficiency: Accounts for inverter losses, shading, temperature, wiring
Daily Output Formula
Daily output (kWh) = Panel power (kW) × Peak sun hours × System efficiency
Example: 10 × 400W panels in London (2.8 PSH average), 80% efficiency:
Panel power = 10 × 0.4 = 4 kW
Daily output = 4 × 2.8 × 0.80 = 8.96 kWh
Annual output = 8.96 × 365 = 3,270 kWh
Peak Sun Hours by Location
| Location | Annual PSH (daily avg) |
|---|---|
| London, UK | 2.5–3.2 |
| Edinburgh, UK | 2.2–2.8 |
| Madrid, Spain | 4.5–5.5 |
| Dubai, UAE | 5.5–6.5 |
| Sydney, Australia | 4.8–5.6 |
| New York, USA | 3.5–4.2 |
| Los Angeles, USA | 5.0–5.8 |
System Losses (Why Not 100% Efficient?)
| Loss type | Typical deduction |
|---|---|
| Inverter efficiency | 3–5% |
| Temperature effect | 3–8% (panels lose ~0.4%/°C above 25°C) |
| Shading/soiling | 1–5% |
| Wiring and connections | 1–2% |
| Total system efficiency | 75–85% |
Sizing a System for Your Needs
Step 1: Find your annual electricity consumption from your bills (e.g. 3,500 kWh/year)
Step 2: Estimate system output per kWp installed:
Annual yield ≈ kWp × PSH × 365 × efficiency
In London: 1 kWp × 2.8 × 365 × 0.80 = 819 kWh/year per kWp
Step 3: Size your system:
System size (kWp) = Annual demand ÷ Annual yield per kWp
= 3,500 ÷ 819 = 4.3 kWp → round to 4.5 kWp (≈11–12 panels)
Financial Return
Example: 4.5 kWp system, UK:
- Cost: approx £6,000–£8,000 installed
- Annual generation: ~3,700 kWh
- Self-consumed @ 24.5p: £480/year
- Exported to grid (Smart Export Guarantee): £80/year
- Total annual saving: ~£560
- Payback period: ~12–14 years
Battery Storage
Adding a battery lets you use more of your generated power at night. A 5 kWh battery typically increases self-consumption from 30% to 50–60%, adding ~£150–200/year in savings but adding £2,000–£3,000 to system cost.