What efficiency figures mean in practice for a UK installation.
What 'efficiency' actually means
Efficiency is the percentage of incoming sunlight converted to electricity. A 2.0 m² panel at 21.4% efficiency converts 21.4% of the light hitting it into usable electricity.
The reference condition is STC: 1,000 W/m² irradiance, 25°C cell temperature, AM1.5 spectrum — a laboratory condition that rarely occurs on a UK roof.
Important caveat
UK solar irradiance: what your roof actually receives
| Region | Annual irradiance (kWh/m²) | Daily average |
|---|---|---|
| South West England | 1,250–1,300 | 3.4–3.6 |
| South East / London | 1,100–1,200 | 3.0–3.3 |
| Midlands / Wales | 1,000–1,100 | 2.7–3.0 |
| North West (Lancashire) | 950–1,050 | 2.6–2.9 |
| Scotland | 850–950 | 2.3–2.6 |
Compare with southern Spain at 1,800–2,000 kWh/m²/yr. UK solar works — but efficiency figures calibrated for Spanish or Californian irradiance need recontextualising.
What efficiency means for real output
A 465W DMEGC at 21.4% on a south-facing Lancashire roof at 30° pitch can reasonably expect:
- Annual generation: ~400–450 kWh per panel
- System performance ratio: 78–82% (losses from temperature, wiring, inverter, soiling)
- Actual utilisation of STC rating: ~12–15% of peak hours at STC conditions
A 480W Aiko at 23.6% on the same roof: ~415–470 kWh/panel/yr. Difference: ~15–20 kWh per panel per year.
On a 10-panel system: 150–200 kWh/yr difference — ~£45–£60 at current prices. Meaningful over 25 years (£1,125–£1,500 discounted), but a fraction of the panel premium.
Want a generation estimate for your specific roof?
We use your roof's exact orientation, pitch and location to produce an accurate estimate — not a calculator average.
Low-light performance: the real differentiator
In UK conditions, the figure that matters most isn't peak efficiency — it's low-irradiance efficiency. How does the panel perform at 200 W/m² (overcast winter) vs 1,000 W/m² STC?
- PERC (older): typically 94–96% of rated at 200 W/m²
- TOPCon (DMEGC, Jinko): 96–98%
- HJT (REC): 97–99%
- ABC (Aiko): 97–99%
The UK-specific efficiency rule
Temperature coefficient and UK performance
- DMEGC 465W TOPCon: -0.34%/°C
- Aiko A-MAXima ABC: -0.24%/°C
- REC Alpha HJT: -0.26%/°C
On a hot UK summer day at 65°C (40°C above STC): DMEGC loses 13.6%, Aiko loses 9.6%. The difference is real — but a 65°C panel temperature is also when you're probably generating more than you can use.
What efficiency should actually influence
Panel efficiency is important in one scenario: constrained roof space. If planning, roof features, or shading limit you to a fixed number of panels, higher efficiency means more generation from the same area.
If you have adequate roof space, efficiency is a secondary consideration after:
- Degradation rate — annual decline over 25 years
- Low-irradiance performance — how it performs in UK winters
- Manufacturer financial strength — the entity backing the 25-year warranty
- Price — the cost difference between options
Frequently asked questions
What efficiency should I look for in a solar panel?
For UK domestic installations, any monocrystalline TOPCon or HJT panel above 21% from a Tier 1 manufacturer is a strong choice. The benefit of going from 21% to 23% is modest unless roof space is genuinely constrained. Focus on degradation rate and manufacturer stability.
Do solar panels work in winter in the UK?
Yes — UK winter generation is meaningful. A well-specified south-facing system produces ~15–20% of annual output in November–February. Cold, clear winter days can produce surprisingly good output.
How much does roof orientation affect output?
South-facing at 30–35° pitch is optimal. East or west at 30° produces ~80% of south-facing. East-west splits give excellent morning/evening profiles. North-facing falls to 50–60% and isn't recommended.
Does dirt reduce solar panel efficiency?
Yes, but less than most assume. Typical rain-cleaned soiling reduces output 1–3% annually. UK rainfall generally keeps panels clean. Manual cleaning is rarely economically justified for residential.
