How we test solar panels

To be able to recommend solar panel brands, we put popular solar panels through a series of in-depth tests to make sure they live up to their claims of maximum power and efficiency. 

If your roof has the right conditions, installing solar panels can be a good way to generate your own electricity and reduce your household's energy bills. But are all solar panel brands the same? We've tested panels from popular brands to be able to compare them.

Which? buys every solar panel we test, and we do not accept free samples for any of our full lab tests, so you can have complete confidence in our recommendations. The Which? overall test score is calculated purely on the measurements and ratings supplied by our independent testing at our specialist labs, meaning there is no room for unconscious bias when compiling the overall ratings. 

Below, you can see the details of our in-depth testing.

Or head to our guide to the best solar panel brands to see the results.

Why our solar panel tests are different

Many solar panel recommendations you'll find online are based on the stated information provided on the data sheets (or spec sheets) that brands supply with their solar panels. But not all panels live up to these claims. Which? goes one step further by testing the panels independently, so you can be sure the solar panels you buy can deliver what they say.

How is the Which? solar panel score calculated?

The main objective of our solar panel test is to determine their performance and quality. The test is divided into two main phases: 

• a laboratory evaluation test of the new solar panel modules under standard lab conditions;
• a second evaluation after real direct exposure to the sun.

A range of tests are repeated in both conditions, by experts who have been analyzing these products for over 10 years. Scroll down this page to see the full details of these.

To make sure our testing is current, we keep an eye on the changes and advances in solar panel technology. The efficiency of solar panels is constantly increasing, as are their power and size. We adapt our testing and analysis to make sure they are appropriate for the market at any given time.

Weightings and star ratings 

A Which? solar panel test score is made up of results from dozens of individual tests and checks. 

Key factors include power output and efficiency, as well as the presence of defects. We simulate energy production based on a number of typical UK daylight scenarios, taken from stats for cities up and down the UK.

To keep things simple, the most important scores are shown as star ratings out of five on the solar panel brand pages.

The most important test results have a greater impact on a solar panels overall score.

• Determination of maximum power (37.5%)
• Energy production (37.5%))
• Defects (25%)

Why don’t we have individual solar panel Best Buys?

In the UK, solar panels are sold via installers rather than direct to consumers. That means that it's unlikely that you would be in a position to choose between specific models as a domestic consumer. 

Instead, we rate brands based on a sample of their products, and give our Which? Best Brand recommendation to those that consistently meet the criteria, so that you can feel confident you are choosing a brand with a proven track record. 

Solar panel technology is being improved all the time, so the specific models available may not exactly correspond with those we looked at in our test lab.

What do our solar panel tests include? 

Below are the key testing categories and how we evaluate each one.

Our analysis begins with a visual inspection of the products to ensure that there are no detectable structural problems that could affect the performance or safety.

Determination of maximum power

Key question: Is the solar panel efficient and will it degrade over time?

We determined the efficiency and actual power of the PV panels, comparing them with that on the label, both of new photovoltaic systems and after exposure to the sun, to evaluate any possible variation and performance deterioration. 

There are many tests which contribute to this star rating, and all of these help us uncover if the panel will do what it claims:

• power stabilization with solar radiation exposure of 25 kWh/m 2
• determination of maximum overall production performance
• nominal efficiency
• performance decline after sun exposure
• power output
• power exposure with an external solar radiation exposure of 60 kWh/m2 
• determination of maximum power generation
• electroluminescence
• current leakage due to humidity

Energy production

Key question: How well do the panels work in UK locations? 

After analysing the panels performance, with this information and that of the analysed inverters we carry out several energy production simulations to know the real production of the panels for UK specific scenarios.

We use the following locations as a representative selection of the United Kingdom.

• Belfast
• Cardiff
• Edinburgh
• London
• Manchester.

Defects

PV panels are fragile and require careful packaging, handling and transportation to avoid micro cracks or damage that can compromise the operation of the panel which, in turn, could affect the whole system. 

It’s difficult to see with the naked eye if a panel has been damaged or cracked during transportation, so we conduct an initial inspection, called an electroluminescence inspection. A photograph is taken of each panel after a current equal to the model’s ‘Isc’ (short-circuit current) is injected. The image is captured with a specific exposure time to identify any defects.

Safety

The wet leakage current test involves submerging the panel in a small pool to check for any current leakage points. None of the panels tested showed any problems, and are all rated safe, so for this reason we do not use this as a weighting factor in our final test results.