64KWH Smart Home Energy Trading System, with solar and extremely efficient heatpump

Fascinating thread concept — this is very close to what I've been nerding out over for the past eighteen months or so.

My background is commercial maritime, so like the OP I came to fixed installations via the marine world. Victron kit on boats teaches you to be ruthless about efficiency because every watt-hour matters when you're three days from a harbour. That mindset carries over nicely to a home system.

I've got a more modest setup than 64kWh — currently running a Fogstar Drift lithium bank on the garden office, paired with a Victron Multiplus-II and a small array on the south-facing roof. It handles the office loads comfortably year-round, but I've been eyeing whether I could tie the house in and do something smarter with grid trading via Octopus Agile.

The heat pump angle is what really interests me here. My neighbour runs a Vaillant arotherm+ and swears by running it hard overnight on cheap-rate electricity, then letting the house coast through the expensive peak window. Combining that thermal storage logic with a properly sized battery bank and solar feels like the next logical step — basically using your building fabric as an additional battery.

Has anyone actually modelled the economics on this for a UK climate specifically? We get such variable irradiance that the numbers must look quite different to a sunnier European install.

Also curious whether anyone's using Home Assistant with their Victron setup for the automation side — that seems to be where the real intelligence lives.

Mate, 64kWh is just my van's battery bank with extra steps.

@FormerMariner36 Welcome to the forum! Maritime background is genuinely brilliant preparation for this kind of system design — you lot understand load management and redundancy in a way that most domestic installers simply don't.

Curious what heat pump you're considering? The efficiency claims vary wildly and real-world COPs in a UK climate can disappoint if the system isn't properly sized. A few members here have had cracking results pairing Mitsubishi Ecodan units with good thermal mass, but it really depends on your building fabric.

The smart trading angle is interesting too — are you looking at Octopus Agile or similar? Genuinely compelling when you've got that battery capacity to play with, though the algorithm complexity to maximise it properly is a project in itself.

@MoorKev 😄 Fair point, though some of us are working with slightly less dramatic battery banks than you!

@FormerMariner36 64kWh is a decent chunk — what cells are you using? Curious whether you've gone down the Fogstar/EVE route or splashed on something more exotic.

The heatpump angle is where it gets interesting for me. Running mine off a Victron Multiplus-II setup and the interaction between the inverter's transfer switch and the heatpump's startup surge is... characterful. Had to detune the AC input sensitivity settings considerably before it stopped throwing wobblies.

Also — "energy trading system" sounds great on paper but the UK's Octopus Agile/Flux tariff situation changes constantly. What's your grid export arrangement? DNO approval for anything above 3.68kW single phase is a proper paperwork adventure.

Not trying to rain on it, genuinely interested. Just found the gap between "fascinating concept" and "actually working Tuesday morning in November" is where most of this stuff earns its keep.

@Curly38 beat me to the cell question. Also want to know the BMS situation — at 64kWh you're not messing about, and a budget BMS is how you end up with a very expensive bonfire.

@FormerMariner36 the heat pump integration is the genuinely interesting bit here. What's your Victron setup doing with the HP load during Agile pricing spikes? Because that's where most people's "smart" systems turn out to be not particularly smart — they'll happily run the compressor at 35p/kWh when they could've pre-heated the thermal mass at 7p.

Maritime background is useful precisely because you already understand load management under constraint, which is 90% of what home energy arbitrage actually is. The other 10% is arguing with your DNO about G99 compliance, which is considerably less romantic than navigating.

What inverter are you running? Multiplus-II I'd assume at that scale.