The increase in energy production between the lowest and highest performing panels in year 1.


The increase in energy production between the lowest and highest performing panels over 20 years.


They’re all made in the same factory aren’t they?..

Definitely not, we’ve seen many different qualities of panel come through the UK market and there’s some you should definitely avoid, including some from very big household names.  There’s a lot of smoke and mirror tactics used to convince you that certain panels are the best.  We look at the whole market so here’s how to become a Solar PV detective.


There are several factors that affect the performance and durability of Solar panels, this is our guide to help you find the power you’re looking for.

Mono is better than Poly…right?

Mono crystalline panels will give a small amount of extra power per square meter, however there’s a lot of other factors that affect performance so a good quality Poly panel will generate more than a poor quality Mono.

Poly crystalline is fragments of silicon crystal melted together.  Poly is older technology, cheaper to produce and has a blue shade to it.  Because there are edges where the crystal fragments meet, there are small areas that are not perpendicular to the light hitting the panel, meaning that a small amount of efficiency is lost.

Mono crystalline is wafers cut from a single ‘ingot’ of silicon crystal.  This means that the surface of the crystal is flatter, meaning that the surface is perpendicular to the light hitting the panel.  This increases efficiency,  allowing you to have more power in a smaller area.

kWp does not equal kWh

Solar panels are sized in kWp (kilo watts peak) which refers to their performance under STC (Standard Test Conditions).  However you need kWh (kilo Watt hours = units of electric) as the more kWh your system generates the more Feed in Tariff you’ll be paid and the bigger the impact on your bills.  So what affects solar panel performance in conditions outside of STC?

Low light level performance: When the light level drops, better quality panels will maintain more power.  This is affected by the quality of the silicon and the clarity of the glass.

Temperature co-efficient: Everything electrical increases in resistance the hotter it gets.  Panels with a lower temperature co-efficient are affected less by increases in temperature.

Heat Capacity: If a solar panel can dissipate heat better it will give better results at high temperatures.  So even if a panel has a good temperature co-efficient, it may perform badly if it can’t dissipate heat effectively.

Maxeon technology

Invented and patented by Sunpower, Maxeon technology is the most advanced type of solar cell available today.  Sunpower’s entire range and BenQ’s Sunforte panels are the only panels on the market available with Maxeon technology.

Thanks to the explanation above from Sunpower, it’s easy to see why these high quality panels are more durable and generate much more energy over their lifetime.  It’s also easy to see why Sunpower give a 25 year full cost of replacement warranty and are top of the independent field testing carried out by Photon Magazine in Germany.

Sunpower are only available through their accredited installers.  There is rigorous scrutiny carried out on the installers before Sunpower will allow them to install their products.  All Sunpower panels must be registered through the Sunpower warranty system so they know exactly where each of their panels has been installed.  Sunpower also carry out a customer service check with every system owner to maintain the quality of their service.

A quick lesson about the difference between conventional and Maxeon technology

Learn more about the difference between conventional and Maxeon technology

Degradation – how long will they last

Over time, all solar panels will lose some of their power.  An average silicon crystalline panel will lose about 15% of it’s power over 25 years in typical UK climate.  However this can vary between panels and through installation methods.

Maxeon technology cells are the most durable, independent testing show’s they’ll lose just 2.5% of their power over 25 years (more about Maxeon technology below).  The diagram above shows the main causes of power loss.

Corrosion: Even panels manufactured to the highest quality are going to end up with some moisture getting in behind the glass and weather seals, with thin strips of aluminium across the front of the cells, it doesn’t take much corrosion to break the connection of such a fragile strip.

Interconnection breaks: Thermal expansion and contraction puts strain on the aluminium connections between cells, along with corrosion will eventually cause these connections to break, adding resistance to the conductivity in a panel, which decreases the amount of power a panel can generate.

Other degradation: The remaining issues that cause degradation are much smaller but still contribute to the overall loss of power from panels.  These are less prominent in better quality panels which have better quality components and sealants.

Warranties – There’s two of them!

Solar panels come with two different warranties, performance and product.  However these vary in clarity and can also be used to miss-sell panels.

Product: This warranty guarantees against any manufacturing defects, i.e. physical faults with the panels.  Usually 10-15 years long they will usually offer a replacement panel or the refund the cost of a replacement panel, except Sunpower who offer a 25 year product warranty which covers the full cost of replacement, including sending a replacement panel, labour and scaffold or other access requirements.

Performance: Sometimes called the degradation warranty, these warranties guarantee how much power a panel will retain after 25 years.  All MCS certified panels require a warranty that guarantees they will retain at least 80% of their power after 25 years.  Some manufacturers offer better warranties that guarantee more than 90% of their power after 10 years and 80% of their power after 25 years.  Check the data sheet of the panels you’re interested in for complete clarity of this issue.

Guaranteed for 25 years? Often we hear of solar sales people focusing on the 25 year performance warranty and neglecting to mention the product warranty.  This can also cause problems because if you have a degradation issue with a panel after the product warranty has run out the manufacturer may say, for example, that the degradation is caused by excess corrosion due to breaking down of the sealant, they may then say that this is a product fault and the product warranty has lapsed, leaving you with a low producing panel and no warranty covering the fault.