How Efficient Are Solar Panels?

Last Updated on February 25, 2025

Solar panels are becoming an increasingly popular way for UK homeowners and landlords to reduce energy bills and lower their carbon footprint. But how efficient are solar panels, and what factors affect their performance? 

In this guide, we’ll break down solar panel efficiency, how it is measured, and what you can do to maximise the benefits of your solar PV system.

What Is Solar Panel Efficiency?

Solar panel efficiency refers to the percentage of sunlight that a solar panel can convert into usable electricity. The higher the efficiency, the more power a panel can generate from the same amount of sunlight.

How Is Solar Panel Efficiency Measured?

Solar panel efficiency is calculated using the following formula:

Efficiency (%) = (Power Output / Solar Energy Input) × 100

For example, if a solar panel receives 1,000 watts per square metre of sunlight and produces 200 watts of electricity, its efficiency would be 20%.

Average Efficiency of Solar Panels in the UK

The efficiency of solar panels in the UK varies depending on the type of panel and environmental conditions. On average:

• Monocrystalline solar panels: 18% – 22% efficiency
• Polycrystalline solar panels: 15% – 18% efficiency
• Thin-film solar panels: 10% – 12% efficiency

Monocrystalline panels are the most efficient and are ideal for homeowners looking to generate the maximum amount of electricity from limited roof space.

Factors Affecting Solar Panel Efficiency

Several factors influence how well solar panels perform in the UK climate:

1. Sunlight Availability

Although the UK has fewer hours of direct sunlight than sunnier regions, solar panels still generate electricity in winter and on cloudy days. Modern panels are designed to capture diffuse sunlight, ensuring consistent performance even in overcast conditions.

2. Angle and Orientation

For optimal efficiency, solar panels should be installed at a 30–40-degree tilt and face south. East- or west-facing panels will still generate electricity but at a slightly lower rate.

3. Temperature and Weather Conditions

Solar panels actually perform better in cooler temperatures. Excessive heat can reduce efficiency, making the UK’s mild climate ideal for solar PV systems.

4. Shading and Obstructions

Shadows from trees, buildings, or chimneys can block sunlight and lower panel efficiency. Installing micro-inverters or power optimisers can help mitigate shading issues.Read out guide on how shading affects solar panels. 

5. Panel Quality and Degradation

Wondering how long solar panels last? Higher-quality panels last longer and maintain efficiency better over time. Most panels degrade at a rate of 0.5% per year, meaning a 90% efficiency retention after 20 years.

How Efficient Are Solar Panels for UK Homes?

Solar panels are highly efficient for UK homeowners and landlords, providing significant savings on electricity bills. A typical 4kW solar PV system can generate:

• 3,500 – 4,200 kWh per year, covering a substantial portion of a household’s electricity needs.
• A reduction of up to £500 per year on energy costs, depending on tariff rates and self-consumption levels.
• A payback period of 6–10 years, making them a cost-effective long-term investment.

How Solar Panels Perform in Harsh Weather

While solar panels are built to withstand various environmental conditions, extreme weather can impact their efficiency. Let’s explore how different weather conditions affect performance.

1. Snow

Light snowfall has minimal impact on solar panels, as the panels’ dark surface absorbs sunlight and helps melt snow quickly. However, thick snow coverage can temporarily block sunlight, reducing efficiency to 0% until cleared. 

Tilted solar panels shed snow more effectively than flat-mounted ones, and self-cleaning coatings can enhance performance.

2. Wind

Wind does not directly impact solar efficiency, but strong gusts can create dust accumulation or cause physical damage if panels are not securely mounted. 

However, moderate wind (10–15 mph) can cool down panels, increasing their efficiency by 1%–2% compared to still, hot conditions.

3. Cloudy Weather

Solar panels still generate electricity on cloudy days, but efficiency drops due to reduced sunlight penetration. On overcast days, panels typically operate at 10%–25% of their maximum capacity. 

High-efficiency panels designed for low-light conditions can retain 40%–60% efficiency under light cloud cover.

4. Hail

Modern solar panels are hail-resistant, tested to withstand hailstones up to 25mm (1 inch) in diameter at speeds of 50 mph. However, severe hailstorms can cause microcracks, leading to a long-term efficiency loss of 5%–10% if not properly maintained.

5. Ice

Unlike snow, ice formation can trap moisture and create surface obstructions, reducing efficiency by up to 30%. Most panels feature anti-reflective coatings that prevent ice accumulation, but periodic maintenance is required in freezing climates.

6. UV Degradation

Prolonged exposure to ultraviolet (UV) radiation can degrade the protective layers on a solar panel’s surface, a process known as Potential Induced Degradation (PID). 

This can result in a loss of 1%–3% efficiency per year, but high-quality panels with UV-resistant coatings reduce this effect.

Solar Panel Durability: Testing for Long-Term Performance

To ensure long-term reliability, the best solar panels undergo rigorous testing under extreme conditions. Here’s a breakdown of key durability factors:

Chemical Residue

Airborne pollutants and chemicals (such as sulphur dioxide from industrial areas) can cause panel degradation over time. Panels are tested for chemical corrosion resistance, ensuring they retain at least 90% efficiency after 20 years in polluted environments.

Damp Heat Testing

Solar panels are exposed to 85°C (185°F) temperatures and 85% humidity for 1,000 hours to simulate tropical conditions. High-quality panels lose less than 5% efficiency, while low-quality panels can degrade by 10% or more under prolonged exposure.

Insulation Resistance

This test measures a panel’s ability to withstand electrical leakage in wet conditions. Panels with poor insulation resistance may lose 2%–5% efficiency per year due to moisture penetration. IP67/IP68-rated panels offer superior waterproofing.

Thermal Cycling

Solar panels expand and contract due to temperature fluctuations. Thermal cycling tests simulate -40°C to 85°C temperature swings to check structural integrity. Panels that pass these tests maintain at least 80% efficiency after 25 years.

Maximising Your Solar Panel Efficiency

To get the most out of your solar PV system:

• Invest in high-efficiency panels – Monocrystalline panels provide better long-term returns.
• Use a solar battery storage system – Store excess energy for use at night or during peak demand.
• Keep panels clean and well-maintained – Regular cleaning ensures maximum sunlight absorption.
• Monitor performance with a smart meter – Track usage and optimise energy consumption.

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FAQs

Will solar panels ever be 100% efficient?

Currently, it is highly unlikely that solar panels will ever reach 100% efficiency due to fundamental laws of physics, specifically the Shockley-Queisser limit, which caps single-junction solar cell efficiency at around 33.7%. 

Some experimental multi-junction cells have achieved over 47% efficiency in lab conditions, but factors like heat loss and material limitations prevent full conversion of sunlight into electricity. 

However, ongoing advancements in quantum dot technology and perovskite solar cells may push efficiency closer to its theoretical maximum in the future.

Will solar panels ever reach 50% efficiency?

While conventional silicon-based solar panels are unlikely to reach 50% efficiency, researchers have already developed multi-junction solar cells that have exceeded 50% efficiency in controlled lab environments. 

These cells use multiple layers of materials to capture different wavelengths of sunlight more effectively. However, these technologies are currently expensive and primarily used in space applications. 

As research continues, we may see commercial solar panels approach 40%–50% efficiency in the coming decades.

Why are solar panels only 20% efficient?

Most residential solar panels today have efficiencies ranging from 18% to 22% due to several factors:

1. Heat loss – Some of the sun’s energy is converted into heat rather than electricity.
2. Material limitations – Traditional silicon-based solar cells can only convert certain wavelengths of sunlight into usable energy.
3. Reflection and transmission losses – Some sunlight is reflected off the panel or passes through without being absorbed.
4. Electrical resistance – Energy is lost as it travels through the system’s wiring and components.

Despite these limitations, efficiency is steadily improving with new materials like perovskite coatings and bifacial solar cells, which capture sunlight from both sides of the panel.