New Solar Panel Sizer Tool for EG4, Sigenergy, Sol-Ark

Interesting to see tools like this popping up, though I'll admit my first instinct is to question how well it translates to a UK context. Most of these sizering calculators seem to be built around American insolation figures and assume you're running Sol-Ark or EG4 kit — neither of which I've ever touched in my van build.

That said, the underlying maths is the maths. Peak sun hours aside, if you're inputting your own load figures and the tool is spitting out sensible array-to-battery ratios, it's still a useful sanity check regardless of inverter brand. I did something similar manually when I spec'd out my Victron MPPT + Fogstar lithium setup — lots of spreadsheet faff that a decent tool would have saved me.

A few things I'd want to scrutinise before trusting any sizer blindly:

• Does it account for partial shading losses? Critical if you're on a van or irregular roof layout
• What charge controller assumptions is it making? PWM vs MPPT makes a meaningful difference to usable yield
• Can you adjust for UK irradiance data (roughly 2.5–4 peak sun hours depending on season and region)?

If it's purely optimised for those three inverter brands with US assumptions baked in, it's limited usefulness for most of us here. But if the logic is sound and the inputs are flexible, worth bookmarking.

Has anyone actually run their existing setup through it and compared the output to real-world performance figures? That's the real test. Would be curious whether anyone using Sigenergy specifically has found it accurate — that kit seems to be gaining traction lately.

@BoxerDream my thoughts exactly. EG4 and Sol-Ark are barely relevant here — most of us are speccing around Victron or Growatt kit anyway, and the irradiance assumptions in these tools are almost certainly based on somewhere like Arizona, not the Scottish Highlands or even southern England.

I ran a similar US-based calculator for my cabin setup last year and it was telling me I'd be fine with half the panel capacity I actually needed. Peak sun hours in the UK are a completely different story, especially October through February.

Would be more useful if someone built something around PVGIS data (EU tool, actually covers the UK properly) combined with realistic UK-specific battery sizing. Until then I'd treat any of these American tools as a rough starting point at best — then double the panel figure and work from there.

@RelayNomad and @BoxerDream — agreed on the brand irrelevance, but the deeper problem is the irradiance assumptions baked into these tools. They're almost certainly using US regional peak sun hour data, which means anyone plugging in their actual location is getting figures that are optimistic by a considerable margin.

Running a 48V system across my narrowboat and garden office here in Somerset, I use PVGIS religiously — it's EU-developed, covers GB properly, and accounts for our famously "character-building" winter insolation.

The panel sizing methodology itself isn't wrong, it's just the input data that's garbage for our purposes. If the tool lets you override peak sun hours manually, it's salvageable. Stick in your PVGIS monthly averages and suddenly it becomes vaguely useful regardless of whether it's defaulting to EG4 or Victron kit.

Worth checking before dismissing it entirely.

@SomersetNomad hits the nail on the head with irradiance — running a shepherd's hut off a 400W array here in Wales and the difference between what a US-built calculator spits out versus actual winter yield is almost comical. We're talking 1.5-2 peak sun hours on a bad December day, not the 4+ these tools seem to assume.

The brand issue matters too but it's almost secondary — even if you swap EG4 for Victron in your head, the underlying solar resource data makes the output near useless between October and February.

Worth cross-referencing with PVGIS if anyone's actually trying to size properly. It uses real satellite irradiance data for UK locations and takes tilt/orientation into account. Not as slick as a web app but at least the numbers reflect what actually happens on a grey Welsh hillside.

@ValleyTony mate, a 400W array in Wales is basically just an expensive weathervane — my Fogstar-backed setup taught me the hard way that UK battery sizing should do the heavy lifting your panels can't be bothered to do in November.