--- title: "Solar Panel Efficiency in the UK: What the Numbers Really Mean | Alliant" description: "What solar panel efficiency percentages really mean for UK homes — real-world output, how UK irradiance affects performance, and what figure actually matters." lang: en json-ld: | [ { "@context": "https://schema.org", "@graph": [ { "@type": [ "Organization", "LocalBusiness", "ProfessionalService" ], "@id": "https://alliantenergy.co.uk/#organization", "name": "Alliant Energy Ltd", "alternateName": "Alliant Energy", "legalName": "Alliant Energy Ltd", "url": "https://alliantenergy.co.uk", "logo": { "@type": "ImageObject", "url": "https://alliantenergy.co.uk/logo.png", "width": 320, "height": 80 }, "image": "https://alliantenergy.co.uk/og-default.jpg", "description": "MCS-registered renewable energy specialists installing solar PV, battery storage, EV charging and air source heat pumps for UK homes and businesses across the North West. ISO 9001, 14001 and 45001 certified. 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than 21%. The actual output difference on your roof is far more nuanced. This guide cuts through the marketing.", "image": "https://alliantenergy.co.uk/assets/technology-BJJXrh0l.jpg", "author": { "@type": "Person", "name": "Alliant Energy Team", "url": "https://alliantenergy.co.uk/about" }, "reviewedBy": { "@type": "Person", "name": "MCS Registered Engineer", "jobTitle": "MCS Registered Engineer" }, "publisher": { "@type": "Organization", "name": "Alliant Energy", "url": "https://alliantenergy.co.uk", "logo": { "@type": "ImageObject", "url": "https://alliantenergy.co.uk/logo.png" } }, "datePublished": "2026-01-15", "dateModified": "2026-05-19" }, { "@context": "https://schema.org", "@type": "FAQPage", "mainEntity": [ { "@type": "Question", "name": "What efficiency should I look for in a solar panel?", "acceptedAnswer": { "@type": "Answer", "text": "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." } }, { "@type": "Question", "name": "Do solar panels work in winter in the UK?", "acceptedAnswer": { "@type": "Answer", "text": "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." } }, { "@type": "Question", "name": "How much does roof orientation affect output?", "acceptedAnswer": { "@type": "Answer", "text": "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." } }, { "@type": "Question", "name": "Does dirt reduce solar panel efficiency?", "acceptedAnswer": { "@type": "Answer", "text": "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." } } ] } ] --- [![Alliant Energy — Renewable Energy Solutions](/__l5e/assets-v1/3bd22195-cc6e-43e6-b5ff-5a808ce292d3/alliantenergyltd-2.svg)](/) [Home](/)[About](/about-us) [Services](/services) [Guides](/guides)[Case Studies](/case-study)[![Levelise](/__l5e/assets-v1/59fd362e-361a-47d7-ad1f-65e9beeb5289/levelise-logo.webp)](/guides/levelise-hub-explained) [01282 501589](tel:01282501589)[Get a Quote →](/quote) Spoke guide · Technology # Solar Panel Efficiency in the UK: What the Numbers Really Mean A panel rated 23% sounds dramatically better than 21%. The actual output difference on your roof is far more nuanced. This guide cuts through the marketing. By **Alliant Energy Team**· reviewed by MCS Registered Engineer Last updated 19 May 2026 [Get a Generation Estimate](/quote) [Book a Roof Survey](/request-callback) On this page - [What 'efficiency' means](#meaning) - [UK irradiance](#irradiance) - [Real output](#real-output) - [Low-light performance](#low-light) - [Temperature coefficient](#temperature) - [What efficiency should influence](#what-matters) - [FAQ](#faq) Ready to act? Free quote in one working day. [Get a Quote →](/quote) 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 A 23% efficiency panel doesn't generate 23% of all available sunlight — it generates 23% under STC. In typical UK conditions, both 21% and 23% panels produce significantly less than rated. The gap between them remains constant, but neither hits its quoted figure. ## 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. [Get a Free Generation Estimate](/quote) [Request a Callback](/request-callback) ## 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 For UK roofs, choose panels with strong low-irradiance performance. Any modern TOPCon from a Tier 1 manufacturer will outperform the same-wattage PERC panel in UK conditions — even if peak efficiency looks identical. ## 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. Three ways to get started ## Ready to find out what solar saves you? Get a tailored quote in one working day. No obligation. No hard sell. Speak to a renewable energy engineer — not a salesperson — at a time that suits you. [Get a Quote → ](/quote) [Request a Callback →](/request-callback) [Download Brochure →](/brochure) Accreditations ## Certified, registered, and insured. Every time. MCS certification isn't a box-ticking exercise — it qualifies your system for Smart Export Guarantee payments and government grants. Our installers are also NICEIC-approved and TrustMark-registered, and every install is fully insured. ![Citation ISO Certification: ISO 9001 quality management, ISO 14001 environmental management and ISO 45001 occupational health and safety](/__l5e/assets-v1/180183cd-e6fd-4b0a-984c-83e6a41390d7/accreditations-iso.jpg) ![Alliant Energy — Renewable Energy Solutions](/assets/logo-IFrgP5Dc.png) Renewable energy & energy-saving specialists. MCS-registered solar PV, battery storage, heat pumps and EV charging across the UK. #### Quick Links - [Home](/) - [About Us](/about-us) - [Services](/services) - [Case Studies](/case-study) - [Areas We Cover](/solar-panel-quotes-north-west) - [Contact](/contact) - [Privacy Policy](/privacy) - [Terms & Conditions](/terms) #### Services - [Solar for Homes](/services/solar) - [Commercial Solar](/services/commercial-solar) - [Battery Storage](/services/battery-storage) - [Air Source Heat Pumps](/services/heat-pumps) - [EV Charging](/services/ev-charging) - [Energy Saving Advice](/services/energy-saving-advice) #### Contact - [01282 501589](tel:01282501589) - [info@alliantenergy.co.uk](mailto:info@alliantenergy.co.uk) - Unit 214 Northlight Industries, Pendle Road, Brierfield, Nelson BB9 5FL [Get a Quote](/quote)[Brochure](/brochure) Alliant Energy Ltd is an Introducer Appointed Representative (Financial Services Register No. 1020779) of Phoenix Financial Consultants Limited (Phoenix). Phoenix is a credit broker, not a lender. Phoenix is authorised and regulated by the Financial Conduct Authority (FRN: 539195), and offers finance from its panel of lenders. All finance subject to status and credit checks. © 2026 Alliant Energy Ltd. All rights reserved. [Privacy Policy](/privacy)[Terms & Conditions](/terms) MCS Registered · TrustMark · NICEIC [Chat with AI](/chat) [Call back](/request-callback) [Get a Quote](/quote) [Get a Brochure](/brochure) [Get a Quote](/quote) [Chat with AI](/chat)