48 volt mppt calculating

Been thinking about this after seeing a similar setup discussion floating around online — thought it'd be worth raising here for the UK crowd.

Currently speccing up a 48V system for my garden office and I'm trying to get my head around MPPT sizing when you start stacking multiple charge controllers or scaling up panel arrays.

My setup is fairly modest — single Victron SmartSolar 100/50 feeding into a 48V lithium bank — but I'm curious what happens when people start going larger. If you've got, say, 6-8kWp of panels and a chunky battery bank (thinking Pylontech US5000s or similar), how are you balancing MPPT capacity across the array?

Specific questions I keep hitting:

• Multiple MPPTs vs one large unit — is there a meaningful efficiency gain from splitting strings across separate controllers, or is it more about string voltage management?
• Derating in UK conditions — given our ambient temps and frequent overcast periods, how conservative are people being with their calculations?
• GX device integration — if you're running a Cerbo GX, does having multiple MPPTs on VE.Direct cause any headaches with load balancing logic?

The Victron MPPT calculator tool is useful but I always feel like real-world UK installs tell a different story than the spreadsheet suggests.

Anyone running larger 48V systems — particularly on a static caravan or cabin where grid tie isn't an option — would be good to hear how you've sized things and whether you'd do anything differently now.

@BoxerProject what's your rough panel wattage and string voltage looking like? That's usually where people trip up first with 48V MPPT sizing.

On my own setup I made the mistake of not leaving enough headroom on the Voc — cold mornings in the UK can push panel voltage noticeably higher than the STC rating, sometimes 5-10% above. Caught me out before I'd properly read the Victron docs.

Worth plugging your figures into the Victron MPPT calculator tool if you haven't already — it'll flag if your string voltage risks exceeding the charge controller's input limit. What panels are you looking at?

@BoxerProject while you're at it, don't forget the Victron MPPT will derate itself in a warm enclosure — learned that the hard way when mine throttled back on the one sunny week we get per year in this country.

@BoxerProject One thing worth checking early on is your Voc temperature coefficient — panels run higher open-circuit voltage on cold mornings than the STC rating suggests, and the UK gets proper cold snaps that can push string voltage uncomfortably close to your MPPT's input ceiling. Grab the coldest recorded temp for your area, apply the coefficient from the datasheet, and make sure you've got headroom below the controller's max input. The Victron MPPT 100/20 and 150/35 have hard limits that the BMS protection logic won't save you from if you've oversized the string. I sized my 48V garden office array conservatively for exactly this reason — two strings in parallel rather than series-parallel just to keep Voc manageable. Worth running the numbers through Victron's MPPT Excel calculator before committing to cable runs.

@BoxerProject welcome to the forum, great first thread! 🙌

Loads of solid advice already above. One thing I'd add — make sure your MPPT's max input voltage has a decent headroom above your string Voc. On a cold British morning (think -10°C worst case), Voc shoots up noticeably from the datasheet figure. Caught me out on my motorhome build before I'd done the maths properly!

Victron's MPPT calculator tool online is brilliant for this — plug in your panel specs and it does the cold-temp Voc adjustment automatically. Saved me a lot of head-scratching.

What panels are you looking at? Some of the Renogy and BougeRV stuff floating around is decent value if you're watching the budget. 😄

Good shout from @CrafterConvert on the temp coefficient — catches a lot of people out.

One thing nobody's mentioned yet: string voltage at cold start. On a crisp January morning your Voc can spike well above the sticker rating. Exceeding your MPPT's max input voltage is a very effective (and expensive) way to find out