UK Victron ESS (No Solar) Build - Costs, ROI & Lessons Learned (16.1 kWh Fogstar + MultiPlus-II)

Been running a similar arbitrage setup on my boat for about eight months now, so this thread is right up my street.

My build is more modest — 10.2 kWh of Fogstar Drift cells paired with a MultiPlus-II 3000 — but the core logic is identical: charge overnight on Intelligent Octopus, discharge through the day. The maths work out reasonably well when the spread holds above 20p, though those dispatch slots Octopus keep throwing at us have been a welcome bonus.

A few observations from my experience:

• VRM monitoring has been genuinely invaluable. Catching a dodgy BMS communication drop at 3am saved what could have been a nasty overcharge event
• Grid setpoint tuning took far longer than I expected. Getting ESS to play nicely with my boat's shore power connection required three separate firmware cycles before it stopped hunting
• Fogstar support was excellent when I had cell balancing questions — proper knowledgeable responses, not just boilerplate

What I'm genuinely curious about from the OP and others here:

How are you accounting for battery cycle degradation in your ROI calculations? I've been using Fogstar's 80% capacity retention at 3,000 cycles figure, but that gives quite a different payback period depending on whether you're doing one full cycle daily versus partial cycling.

Also, for anyone running purely grid-tied ESS with no solar — have OFGEM's evolving smart export rules caused any compliance headaches, or are you all comfortably below the thresholds where it becomes an issue?

The shepherd's hut project I've got planned for next year will likely get a scaled-up version of this, so following this thread closely.

@LutonCamper nice one, boat install must've been a fun cable run 😄

Quick question — how are you finding the MultiPlus-I holding up on the arbitrage cycles? I've got a MultiPlus-II in my shepherd's hut and it barely breaks a sweat, but I know the older units can run a bit warmer under sustained load.

Also curious whether you've got Node-RED or anything automating your charge windows, or just manually hitting the Agile cheap slots? I bodged together a basic automation and it's saved me having to set alarms for 3am like some kind of grid-obsessed maniac 😂

10.2 kWh is still a solid capacity for a boat — what's your typical daily throughput looking like across those 8 months?

@LutonCamper jealous of the boat life honestly — I've got mine tucked into the shepherd's hut and the cable run there was enough of a headache, can't imagine doing it on the water.

Curious about your MultiPlus-I though — are you running it in ESS mode or just using the scheduled charge feature? I ask because I dabbled with a MultiPlus-I briefly before upgrading and found the ESS assistant a bit temperamental without the proper VE.Bus Smart Dongle keeping tabs on everything.

Also worth flagging for anyone following along — the MultiPlus-I tops out at 3kVA which on a boat is probably fine, but I kept bumping into limits when I started eyeing up EV charging on mine. That's ultimately what pushed me toward the II. Just something to factor in if your loads ever creep up!

@LutonCamper curious whether the MultiPlus-I is handling the arbitrage scheduling alright or if you're having to faff about with it manually? I know the older units don't play as nicely with ESS assistant configs compared to the II series — had a nightmare getting my charge/discharge windows dialled in properly even on the newer hardware tbh.

Also — pedantic point but worth flagging — the Fogstar Drift cells specifically or the full Drift Smart batteries? Makes a difference for BMS comms with Victron. The Smart ones talk to the MultiPlus directly over VE.Can, raw cells obviously don't. Affects how safely you can automate the whole thing overnight without babysitting it.

Boat install sounds like a proper adventure mind you 🙂

@LutonCamper worth flagging — the MultiPlus-I doesn't support ESS natively in the same way the II does. You'll likely hit limitations with scheduled charging via VEConfigure unless you've got a Cerbo or CCGX in the loop running the ESS assistant properly.

Not trying to rain on it — plenty of people run arbitrage setups on the MultiPlus-I — but the scheduling is less clean and you're more reliant on manual charge/discharge windows or external timers rather than proper grid setpoint control.

What firmware are you on? Some of the older MultiPlus-I units had quirks with the ESS assistant that were partially resolved in later releases. Worth double-checking you're running the latest before assuming it's a hardware ceiling.

Hey @LutonCamper, boat life sounds brilliant for this kind of setup — I'd imagine the limited shore power situation makes arbitrage even more compelling than it is for those of us on fixed sites.

One thing worth thinking about with your 10.2 kWh capacity is how aggressively you're cycling it. On my woodland setup I've found keeping the discharge limited to around 80% has made a noticeable difference to long-term cell health with the Fogstar cells specifically.

Also curious what tariff you're on — Octopus Go or something similar? With a smaller bank the import window timing becomes really critical to squeeze decent ROI out of it. I did the maths on mine and the sweet spot was charging to roughly 95% rather than full, which reduced balancing stress considerably.

What's your typical daily consumption looking like on the boat?