Solar Panel Vmp and Voc requirements

Always a fun rabbit hole to fall down, this one 😄

So I've been nerding out on this lately while planning an expansion to my garden office array. Got a Victron SmartSolar 100/30 and wanted to squeeze a bit more out of it before upgrading.

The bit I keep seeing people get muddled on:

• Voc = absolute max open circuit voltage — never exceed the controller's rated input (100V on mine). Remember to account for cold temps too, panels get spicier in winter
• Vmp = the voltage the panel actually works at under load — this needs to be high enough above your battery voltage for the MPPT to do its thing properly

The 5V above battery rule is a rough guide for startup, but realistically you want a decent headroom on Vmp, especially on a 12V or 24V system where the margins feel tighter.

What caught me out was not factoring in temperature coefficients properly. A panel that looks fine on paper can creep over your controller's Voc limit on a freezing January morning. Victron's MPPT calculator is brilliant for this — worth running your numbers through it before committing.

Anyone running panels in series on a 24V system — what Vmp are you actually seeing comfortably triggering the MPPT? Curious whether people are finding 35-40V gives enough headroom or if you're pushing higher.

Also does anyone know if Fogstar are planning any higher voltage panel options? Their pricing is hard to argue with 👀

Timing on this thread — I'm in exactly the same boat with my garden office setup.

One thing I kept tripping up on: the 100V is an absolute hard limit on the Victron SmartSolar, not a "try to stay under it" suggestion. Voc climbs on cold mornings, so I always run the numbers using the lowest realistic winter temp for my location.

Victron's MPPT calculator tool is genuinely useful for this — you plug in your panel specs and it'll flag if your string configuration is going to breach the controller's limits at temperature extremes.

Can I ask @LK_Solar — are you running a single string or considering series/parallel? That changes the Vmp headroom calculation quite a bit, and it's where I nearly made an expensive mistake with my own array.

Spent an embarrassing amount of time staring at my own SmartSolar docs before this clicked for me — the 100V limit is absolute, no wiggle room whatsoever. Voc gets nastiest on those bitter January mornings when panels are stone cold and you're not even watching.

What caught me out was forgetting to apply the temperature correction. Take your panel's Voc, find the temperature coefficient (usually buried in the spec sheet), then calculate for the coldest realistic temperature you'd expect at your location. For most UK spots I'd plan for around -10°C to be safe, maybe colder in Scotland.

My two Renogy 200W panels in series looked fine on paper until I ran the cold-temperature numbers — suddenly I was sitting very close to that ceiling. Ended up rewiring them in parallel rather than risk a fried MPPT.

The cold temperature derating is the one that catches people out most. Voc rises as panels get colder — in the UK, if you're sizing for a crisp winter morning, you need to apply a temperature correction factor.

Rough rule: for every 10°C below STC (25°C), Voc increases by roughly 3-4% depending on your panel's temperature coefficient (check the datasheet).

So if you're at -10°C (possible in parts of the UK), that's 35°C below STC — potentially 10%+ above your rated Voc. On a 100V controller that headroom disappears fast.

For the 100/30, I'd be targeting a max string Voc of around 85V at STC to give yourself comfortable margin. @LK_Solar — what panels are you looking at? Some have better temperature coefficients than others, and it genuinely matters here rather than being marketing fluff.

Really useful thread — got a SmartSolar 100/30 myself on the garden office and spent ages second-guessing this exact thing.

One thing worth flagging that I haven't seen mentioned: if you're also planning EV charging from the same array (as I am), it's worth modelling your string configurations carefully from the start rather than retrofitting later. I nearly wired mine for 24V nominal which would've severely limited expansion options.

@RayWatson81 the cold Voc point is critical — has anyone used the Victron MPPT Calculator spreadsheet for this? It accounts for temperature coefficients automatically and saved me a lot of manual arithmetic. You just plug in your panel datasheet figures and it spits out worst-case Voc.

Does Renogy publish temperature coefficients clearly on their datasheets? Trying to work out if their panels are worth considering for the expansion.

Been caught out by this myself — had a 3-panel string on my backup setup that was technically fine on paper but I hadn't accounted for a proper cold snap. Voc crept uncomfortably close to the 100V ceiling.

Worth using Victron's own MPPT Excel calculator if you haven't already — plug in your panel specs and worst-case temp for your location and it does the maths properly. Takes the guesswork out.

@RayWatson81 is spot on about UK winters — don't just use 0°C either, we do get -10°C patches in parts of Scotland and northern England that'll push Voc higher than you'd expect.