Panel installation and wiring
How to Choose Solar Panels for Your UK Home: The Key Specs Explained
Your installer will quote specific brands and panel models. Understanding the five specs that actually matter, and the two very different warranties, lets you compare quotes properly and avoid buying on headline efficiency alone.
1. Watt peak: the headline output figure
The watt peak (Wp) figure on a panel datasheet tells you its electrical output under Standard Test Conditions (STC): 1,000 W/m² of irradiance, an air mass of 1.5, and a cell temperature of exactly 25°C. Real UK conditions rarely hit these numbers, so your actual output will be lower, but Wp is still the standard measure for comparing panels and sizing a system.
For a typical UK household needing 3.5 to 4 kWp, expect 7 to 10 panels at modern output ratings of 400 W to 500 W each. Our guide to how many panels you need walks through the sizing calculation in full.
2. Efficiency: how much space your panels need
Panel efficiency, expressed as a percentage, is simply the proportion of incident sunlight that is converted to electricity. A 22% efficient 450 W panel is physically smaller than a 20% efficient 450 W panel. On a roof with limited usable space, higher efficiency lets you fit more kWp. On a spacious roof, it matters far less.
Mainstream monocrystalline panels in 2026 sit between about 20% and 23% efficiency. Premium N-type panels from Aiko and Longi reach 23 to 24%. The efficiency gap between mainstream and premium has narrowed significantly in the last two years, so it is rarely worth paying a large premium for a fractional efficiency gain on a roof that has plenty of space.
3. Temperature coefficient: performance on warmer days
Panels lose efficiency as they heat up above 25°C. The temperature coefficient, shown on the datasheet as a negative number in %/°C (or sometimes Pmax/%/°C), tells you how much output drops per degree. A panel with a coefficient of -0.29%/°C degrades less on a hot sunny day than one at -0.40%/°C.
In the UK, summer temperatures rarely push cells above 50 to 55°C, so the difference between a good and an average temperature coefficient is modest. It becomes more relevant if your panels face south on a dark roof in an area that gets genuine summer heat. HJT panels tend to have the best temperature coefficients (around -0.25%/°C), followed by N-type TOPCon (around -0.29 to -0.32%/°C), then P-type PERC (around -0.35 to -0.40%/°C).
4. Degradation rate and the two warranties
Panels degrade very slightly every year as the cells age. Typical figures for good-quality monocrystalline panels are around 2% in year one and 0.4 to 0.5% per year thereafter, bringing a panel to roughly 85 to 88% of its original output by year 25. N-type panels tend to degrade more slowly than P-type.
Two warranties cover a solar panel, and they cover very different things:
- Product warranty: Covers the physical panel against manufacturing defects. Ranges from 12 years on budget panels to 25 years on premium ones.
- Performance warranty: Guarantees that the panel will still produce at least a stated percentage of its rated output at a future date, typically 80 to 87% at year 25 or 30. This protects you against abnormally fast degradation.
A long performance warranty is only meaningful if the manufacturer still exists when you need to claim. Stick to brands with a European or UK trading entity and a track record of honouring warranty claims.
5. Panel technology: PERC, TOPCon, HJT and ABC explained
All mainstream residential panels in 2026 use monocrystalline silicon cells. The differences lie in how the cell is engineered:
- PERC (P-type): The outgoing mainstream standard. Efficient, well-proven, and increasingly cost-competitive. Still a sensible choice on a budget.
- TOPCon (N-type): The new mainstream. Lower degradation, marginally better low-light performance, and a slightly better temperature coefficient than PERC. Most Tier-1 manufacturers have shifted their volume production to TOPCon. Brands include JA Solar DeepBlue 4.0, Longi Hi-MO 6/7, and Solarwatt.
- HJT (Heterojunction): A thin amorphous silicon layer added to the cell gives HJT panels the best temperature coefficient and very low degradation. Premium price, but worthwhile on very constrained roofs or in warm locations. REC Alpha RX and Panasonic EverVolt are the main UK options.
- ABC (All Back Contact): The highest efficiency tier in 2026, used by Aiko and Longi in their top-end ranges. Both contacts are on the rear of the cell, removing front-side shading from busbars. The AIKO Neostar 3 and Longi Hi-MO S10 lead this category. Premium price, justified mainly on very space-constrained roofs.
For most UK rooftops and budgets, a Tier-1 TOPCon panel offers the best balance of output, longevity, and cost in 2026. The step up to HJT or ABC pays off most clearly when roof space is the binding constraint.
Frequently Asked Questions
Is N-type better than P-type for UK solar?
N-type panels (TOPCon and HJT) generally have lower degradation rates, better low-light performance, and smaller temperature coefficients than P-type PERC panels. In UK conditions with frequent overcast skies, the low-light advantage is real. For most budgets a Tier-1 TOPCon panel is the practical choice in 2026. P-type PERC panels are still competitive where cost is the primary consideration.
What efficiency should I look for in a UK solar panel?
Mainstream monocrystalline panels in 2026 range from around 20% to 23% efficiency. On a spacious roof, efficiency matters far less than cost per watt. On a constrained roof, a higher-efficiency panel lets you fit more kWp in the available space. Do not buy on efficiency alone: degradation rate and warranty terms often matter more over a 25-year panel lifetime.
How long do solar panel warranties last?
There are two separate warranties. The product warranty covers the panel against manufacturing defects, typically 12 to 25 years depending on the manufacturer. The performance warranty guarantees a minimum output percentage at a future date, typically 80 to 87% of rated output at year 25 or 30. Always check that the manufacturer has a European or UK presence: a 25-year warranty is only worth having if the company is still trading when you need to use it.