Does anyone actually get the rated watts from cheap 100W panels in the UK?

I picked up a pair of 100W monocrystalline panels off Amazon for about £55 each a few months back — no-name brand, but the specs looked reasonable on paper. Voc around 22V, Isc 5.8A, the usual sort of thing. Stuck them on the roof of my camper in a basic series/parallel setup feeding into a Victron 75/15 MPPT.

Problem is, on what I'd call a decent sunny day here in Yorkshire I'm barely seeing 130–140W combined out of the two panels. I know we're not Spain, but surely two 100W panels should be pulling more than that on a clear July afternoon? I've checked the wiring, connections seem solid, and the Victron app isn't flagging anything obvious.

Is this just the reality of budget panels, or is there something else I should be looking at? Wondering if the STC ratings on cheap panels are just completely optimistic and if anyone's done any proper side-by-side comparisons with decent branded panels like Renogy or Rich Solar.

SilverCaptain | 847 posts

@Trevor1986 Short answer: almost never, especially here in the UK. Those panel ratings are measured at Standard Test Conditions — 1000W/m² irradiance, 25°C cell temperature, which basically describes a Mediterranean summer's day, not a grey afternoon in Staffordshire.

Realistically you'll see perhaps 70-80W peak from a genuinely decent 100W panel on a clear British summer day, and considerably less the rest of the time. With no-name panels the rated figures can also be optimistic to begin with — some are binned slightly below spec at the factory.

Don't be too disheartened though. For £55 each they're still reasonable value if you size your system around actual expected output rather than the label. What are you trying to run off them? That'll help work out whether you've got enough capacity.

ExFarmer | 234 posts

My narrowboat panels haven't hit rated watts since about 2021, and that was a Tuesday in July when I accidentally parked in Spain.

SOC_Nerd | 1,203 posts

@Trevor1986 Even name-brand panels rarely hit rated watts in the UK — STC (Standard Test Conditions) is 1000W/m² irradiance at 25°C cell temp, which basically doesn't exist here. You're lucky to see 600-700W/m² on a clear summer day, and your cells will be hotter than 25°C anyway, which reduces output further.

The cheap Amazon panels are also frequently optimistic with their labelling. I've tested a few with my Victron SmartSolar and they're typically 85-90W peak under ideal conditions, not 100W.

That said — does it actually matter? Size your system around realistic yield, not nameplate. I plan around 50-60% of rated capacity averaged across the year for my setup.

What charge controller are you running? That'll tell you more about actual harvest than the panel spec sheet ever will.