Picked up a 200W panel off Facebook Marketplace - worth rewiring the shed for it?

Mate, I've been down this exact rabbit hole. Found a battered 200W panel on Marketplace for £25 last spring — bloke had it leaning against his garage for three years, zero idea what it was.

Here's the thing though: before you commit to rewiring anything, I'd stick a multimeter on it first. Open-circuit voltage, short-circuit current — takes five minutes and tells you whether the cells have actually degraded or whether you've genuinely landed a bargain. Marketplace panels are a complete lottery. Some are cracking finds, others are delaminated paperweights.

For my shepherd's hut I ran a simple dedicated circuit rather than integrating it into the existing shed wiring. Kept everything separate and avoided the headache of working around whatever mysteries lurk behind old shed boards. A basic Victron SmartSolar MPPT (even the 75/15 if your loads are modest) paired with that 200W panel would give you a genuinely useful little system — enough for lighting, phone charging, maybe a small inverter.

The rewiring question really depends what the shed's for. If you're running mains appliances off the grid connection, mixing DC solar into that gets complicated quickly. But a standalone solar circuit? Honestly, it's an afternoon's work and a trip to Screwfix.

What controller are you planning to pair it with? And do you know the panel specs — is there a label still readable on the back?

Anyone else picked up panels off Marketplace recently? Curious whether prices are creeping up or if there are still deals to be had out there.

WayneHamilton56 | 847 posts | ⭐ Trusted Contributor

@BreezyMechanic Cracking find at that price! Before you commit to rewiring anything though, definitely test that panel properly first. Three years leaning against a garage could mean delamination or cracked cells that aren't obvious to the eye. Grab a multimeter on a decent sunny day and check your open-circuit voltage and short-circuit current against the panel's spec label — if the figures are well off, some cells are likely dead.

Also worth considering whether 200W alone justifies a full rewire. What are you actually powering in the shed? If it's just lighting and phone charging, a simple plug-and-play setup with a small charge controller and a leisure battery might suit you better without touching existing wiring at all. Keep costs down until you know the panel performs reliably. 👍

EmmaPowell74 | 312 posts | 🔧 DIY Enthusiast

@BreezyMechanic Brilliant score for £25! One thing worth doing before you spend a penny on rewiring — get a multimeter on it in direct sunlight and check the open-circuit voltage. A healthy 200W panel should read somewhere around 45-48V typically. If it's reading significantly lower, the cells may be badly degraded and the maths won't stack up for rewiring costs.

Also worth checking the junction box on the back for any moisture ingress or scorching — three years propped against a garage in UK weather is asking for trouble in that department. A tube of silicone sealant fixes minor issues cheaply enough.

If it tests well though, absolutely worth it. My first system started with a single scavenged panel and grew from there. Sometimes the budget route is the best learning experience you'll have! 😄

LindaCross66 | 156 posts | 🌱 Off-Grid Learner

@BreezyMechanic Ooh nice one! I picked up something similar last year and the thing people don't mention enough is the wiring run length from panel to wherever your charge controller ends up sitting. I lost a fair bit of efficiency because my run was too long and I'd used undersized cable — rookie error! Worth calculating your voltage drop before you commit to a particular setup position for the shed. Also, if the junction box on the back of the panel looks dodgy or the seal has gone, that's worth sorting before anything else, moisture gets in and causes no end of bother. Shouldn't cost much to reseal properly. Good luck with it — £25 for 200W is genuinely a steal if it tests out okay! 🙂

SolarJunkie | 2,341 posts | ☀️ Veteran Member

@BreezyMechanic 200W for £25 is either brilliant or a headache, and you won't know which until you actually load-test it properly — not just chuck a multimeter on the terminals in your garden.

Voc and Isc figures in open circuit tell you almost nothing about real degradation. Get it under genuine load and measure actual wattage output against the rated spec. Three years outdoors with no maintenance, I'd realistically expect 140-160W on a good day if the cells aren't delaminated.

Before rewiring anything — check the junction box on the rear. If the silicone seal has cracked and moisture got in, you've potentially got corroded bypass diodes. Replacement diodes are pennies but it's faff you don't want mid-project.

For a shed, 200W properly working is absolutely worth rewiring. 200W half-dead isn't.