Solar quotes vary wildly. One installer will come in at half the price of another, and both of them will claim their panels can power your home just fine. Usually, the difference comes down to what’s inside the panel. Cheap imports have flooded the UK market over the past few years, and most homeowners genuinely don’t know which specs to pay attention to when comparing quotes from different installers. Here’s what to check first.
Cell Efficiency Tells You More Than Brand Names
Put simply, cell efficiency measures what percentage of sunlight a panel can turn into usable electricity. The difference is stark. Premium panels from the best solar panel brands tend to land above 21%, with some pushing past 23%, while budget imports often fall between 17% and 19%.
On paper, a few percentage points don’t sound like much. But on a typical UK roof where space is limited and sunshine isn’t exactly abundant, those extra points of efficiency will determine how much electricity you actually get out of your system over its lifetime. That’s a big deal. If your installer can’t tell you the cell efficiency of the panels they’re quoting, think twice about using them.
Degradation Rates and What They Mean Over 25 Years
Every solar panel loses a small amount of output over time. The rate at which this happens is called the degradation rate, and it’s one of the clearest indicators of build quality you’ll find on any spec sheet. Decent panels will degrade by about 0.2% to 0.4% per year after year one. Cheaper ones can lose 0.7% or more.
After 25 years, the gap gets serious. A panel degrading at 0.25% per year will still be producing around 94% of its original output, while one degrading at 0.7% could drop to roughly 83%. That 11% difference will quietly eat into your energy bills, month after month, for years.
Temperature Coefficient: The Overlooked Spec
When panels heat up, they lose efficiency. The temperature coefficient tells you how much output drops per degree Celsius above 25°C, and a quality panel might come in at around -0.3% compared to -0.45% for a cheaper option.
Roof temperatures tell a different story from air temperatures. Most people assume UK summers aren’t hot enough for this to matter, but panel surfaces can easily hit 50°C or higher on a warm July day. A poor temperature coefficient will cut into your output at exactly the time you’d expect peak generation from your system, and over a full summer, those losses add up to real money.
Warranty Terms That Actually Mean Something
A 25-year performance warranty will sound reassuring when you first hear it. But dig into the details. Check whether the manufacturer guarantees a specific output level at year 25, and whether the product warranty covers defects for at least 12 to 15 years. Some budget brands will offer long performance warranties paired with just a 5-year product warranty, leaving you exposed if something fails early on.
Think long term here. Will the manufacturer still be around in 15 or 20 years to honour that warranty? Established brands with a genuine track record in the UK market will always be a safer bet than newcomers making promises they haven’t yet had to keep.
IEC Certifications: The Minimum You Should Expect
Any panel sold in the UK should carry IEC 61215 and IEC 61730 certifications. These cover safety and performance testing under controlled conditions, and they’re the absolute bare minimum you should expect from any panel an installer puts in front of you. If they’re missing, walk away.
Look beyond the basics too. Testing like IEC 62804, which covers potential-induced degradation (PID) resistance, will tell you whether a panel can handle the damp, changeable conditions that UK rooftops deal with year-round.
Don’t Let the Price Tag Do the Thinking
Price alone won’t tell you much. Some cheap panels will perform decently enough, while some expensive ones won’t live up to the marketing materials that got you interested in the first place. The specs that actually separate a good panel from a bad one are:
- Cell efficiency
- Degradation rate
- Temperature coefficient
- Warranty depth and certifications
Most homeowners who end up disappointed with their solar system didn’t get ripped off. They just accepted the cheapest quote without comparing what was actually on offer.
An installer who can’t tell you the degradation rate or the temperature coefficient of the panels they’re fitting either doesn’t know the product well enough or doesn’t want you asking. Either way, that’s your cue to get another quote. An hour spent comparing spec sheets now will save you from twenty years of underperformance you won’t notice until the energy bills tell you something’s wrong.
