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

I picked up a pair of "100W" monocrystalline panels off eBay a few weeks back — paid about £35 each, which seemed too good to be true. Wired them in parallel into a 20A PWM controller going into a 100Ah leisure battery. On a clear July afternoon, pointing them pretty much due south at roughly 35°, I was seeing a combined maximum of around 85W on the controller display. Not terrible, but I've seen people claim they barely get 60W from similar setups.

I know STC (Standard Test Conditions) ratings are done at 1000W/m² irradiance and 25°C cell temp, which basically never happens here. But I'm wondering how much of the shortfall is down to dodgy labelling on the budget panels versus just the UK climate being rubbish. Has anyone actually measured their panels properly with a clamp meter and compared to the rated spec?

Also curious whether stepping up to a proper MPPT controller would make a meaningful difference on a small setup like this, or whether it's overkill until I'm running more panels. I've seen figures like 10–30% efficiency gains thrown around but that seems like a wide range.

@PartnerConvert short answer — no, never, not even close.

Long story: I've got a mix of Renogy and some no-name eBay panels on my narrowboat. The Renogy 100W panels might scrape 85W on a perfect July day, aimed directly at the sun, measured at the panel terminals. The mystery eBay ones I tested last summer? Peak was around 62W under identical conditions.

The other villain in your setup nobody's mentioned yet is that PWM controller. You're leaving probably 15-20% of whatever the panels could give you on the table compared to even a basic MPPT. Victron make a 75/15 that'd pay for itself fairly quickly.

Rated output is measured at STC — 25°C cell temperature, 1000W/m² irradiance. In the UK, you'll rarely see both simultaneously. Factor in cheap cells and dodgy specs, and 60-70% of rated is a realistic expectation.

@PartnerConvert short answer — no, never. Not even close on those cheap ones.

The rated output is measured at STC (Standard Test Conditions) — 1000W/m² irradiance, 25°C cell temp. In the UK you're lucky to hit 600-700W/m² on a clear day, and real-world deductions stack up fast.

Also worth knowing — a lot of those eBay "100W" panels are genuinely more like 80W panels with optimistic labelling.

Your PWM controller isn't helping either. Victron or a decent MPPT like the Renogy Rover would squeeze noticeably more out of what you've got.

In my shepherd's hut setup I budget roughly 50-60W average daily output per "100W" panel across the year. Some cracking summer days you'll hit 80W+, but November through February is grim.

Manage expectations and you won't be disappointed.

Hey @PartnerConvert, to add a bit more practical context to what @Spider and @Lefty are saying —

Even setting aside the dodgy STC ratings on cheap panels, the UK climate works against you regardless. You're realistically looking at perhaps 3-4 peak sun hours on a decent summer day, maybe 1-2 in winter, and that's before cloud cover hammers things further.

The PWM controller isn't helping either — an MPPT would squeeze noticeably more out of those panels, particularly in low-light conditions which is basically most of the year here.

That said, £35 panels can still be decent value if you go in with realistic expectations. I'd suggest logging your actual output over a few weeks and working backwards from there. You'll probably find they're delivering somewhere between 60-75W peak on a genuinely clear day, which isn't terrible for the money.

@PartnerConvert worth mentioning something nobody's touched on yet — your PWM controller is costing you a fair chunk of output compared to MPPT. With a PWM unit, the panel voltage gets pulled down to match your battery voltage, so you're essentially throwing away a good portion of what the panel could produce. An MPPT controller would harvest considerably more, especially on those overcast days we get so frequently here in the UK where the panel voltage sits higher relative to current output.

Also check your wiring — undersized cable between panel and controller causes voltage drop that really adds up. I'd run at least 4mm² for any decent run length.

Realistically though, even sorted perfectly, expect maybe 50-65W peak from a £35 "100W" panel on a decent summer day. Manage your expectations and plan your system around that. 😄

Hey @PartnerConvert, just to add something practical — have you actually measured the panel voltage and current under load on a clear day around solar noon? A cheap multimeter and clamp meter will tell you a lot. I've found with budget panels the quoted wattage is often based on optimistic Isc/Voc figures that bear little resemblance to real MPP output. Also worth checking your connections and cable gauge — voltage drop in undersized cable quietly eats your gains before the power even reaches the controller. On a good British summer day you might realistically see 60-70W from a genuine 100W panel; from a £35 eBay special, I'd honestly expect somewhere in the 50-65W range at absolute best.

Hey @PartnerConvert, just to jump in on what @HarryJackson mentioned about the PWM controller — he's spot on, but I'd also say don't overlook shading. Even partial shade on one panel in a parallel setup can drag the other one down more than people expect. Also worth checking your cable run lengths and gauge — undersized cable causes voltage drop that quietly robs you of output before it even reaches the controller. On a tight budget build these small things stack up quickly. With cheap panels, a basic PWM, and UK irradiance levels (realistically 2-4 peak sun hours on a decent day), getting 60-70W combined from your pair on a good summer afternoon is probably a realistic target. Manage expectations accordingly and you'll be less frustrated!