Use of Inverter as a substitute for main grid

Been running a similar setup on my narrowboat for a couple of years now, so this topic is close to home (literally).

I've got a Victron Phoenix 12/1200 handling most of my onboard AC loads, fed by a 600W solar array through a Victron MPPT. Works an absolute treat for the kettle, laptop, and the odd power tool when I'm doing maintenance.

The question I keep coming back to is whether you can genuinely treat a battery-inverter setup as a proper grid substitute for a domestic property — not just supplementary, but the whole thing.

My gut says it depends enormously on your load profile. A narrowboat is relatively forgiving because I know exactly what's running and when. A house is a different beast — tumble dryers, electric showers, overnight heating... the battery bank you'd need starts getting eye-watering pretty quickly.

What I have noticed is that a good true sinewave inverter (the Victron Phoenix range earns its money here) handles sensitive electronics far better than I expected. No issues with my TV, router, or induction hob on lower settings.

A few things I'd be curious to hear from others on:

• Are any of you running Fogstar Drift or similar LiFePO4 banks as your sole domestic power source?
• Has anyone managed to size a system that gets through a full UK winter without needing grid top-up?
• How are you handling Part P compliance if you're connecting any of this to fixed wiring?

The regulatory side feels like the murkiest water to navigate, especially for anything beyond a static caravan or boat context. Interested where others have landed on this.

@ExChippie72 the narrowboat life really does teach you inverter discipline fast, doesn't it.

My shepherd's hut runs a Victron MultiPlus 12/1600 and the moment that became my only AC source — no hook-up, no generator — everything changed. Suddenly I was tracking every kettle boil and hair dryer like a hawk.

The bit people don't appreciate until they're living it: it's not just about the inverter's rated wattage, it's the battery's continuous discharge rate that quietly runs the show. I learned that the hard way when my original lead-acid bank would sag horribly under load spikes.

Switched to a Fogstar Drift 100Ah lithium and it was like the inverter finally breathed properly. Stable voltage, no mystery shutdowns.

For emergency backup use, the Phoenix you're running is absolutely solid — that series just quietly gets on with it.

Worth adding — biggest lesson from my motorhome and cabin builds is don't undersize your inverter "just in case." Ran a Victron Phoenix 24/3000 in the motorhome and never regretted the headroom. Surge capacity matters more than most people realise when you've got a fridge compressor or pump kicking in.

Also worth checking your battery C-rate can actually back the inverter up properly. No point having a beefy inverter if your Fogstar Drift cells are screaming at the draw.

One thing people overlook — idle consumption. The Phoenix is decent but it's still ticking away 24/7. Factor that into your daily budget or use the energy save mode if your loads allow it.

@DownsCruiser absolutely right on not undersizing — but I'd add the flip side caveat: don't massively oversize either, particularly if you're running light loads frequently. A 3kVA inverter ticking over at 50W for EV charging trickle sessions draws surprisingly painful idle current.

For my cabin setup I landed on a Victron Multiplus-II 48/3000 which hits a genuinely useful sweet spot — the PowerAssist feature means it can pull from the grid and batteries simultaneously when my EV spikes demand, rather than forcing me to overspec the whole system for a momentary load.

Key metric people ignore: no-load consumption. Check the datasheet. Some cheaper units are absolute vampires at idle — I've seen budget inverters burning 15-20W doing absolutely nothing useful whatsoever.

Victron's figures here are genuinely competitive, which is partly why they dominate serious off-grid builds rather than just the marketing brochures.

Really useful thread for me — currently spec'ing out a system for a shepherds hut build and this inverter sizing question is exactly what I'm wrestling with.

@DownsCruiser and @DailySolar — so where does that leave us practically? Is there a rough rule of thumb for the "sweet spot"? I've seen people suggest sizing the inverter at roughly 1.5–2× your typical continuous load to account for surge without going silly.

Also wondering whether a Victron Multiplus makes more sense than a pure inverter given it handles the transfer switching too — anyone running one in a static off-grid setup rather than a boat? Seems like overkill for a small hut but the seamless changeover to a backup genny is tempting.

What battery capacity are people pairing with these? I'm looking at Fogstar Drift 12V 100Ah lifepo4 cells.

@AlexJones for a shepherd's hut the surge rating matters more than people realise — any small compressor fridge will spike hard on startup. Caught me out with my garden office setup before I upgraded.

Also worth thinking about idle consumption if you're battery-dependent. Some inverters sit there sipping 20W+ doing nothing. Victron's ECO mode helps a lot, but not every load plays nicely with it.

For a hut that size I'd probably look at a Multiplus-C 12/800 if budget allows — gives you the pass-through charging option later if you ever wire in a hook-up point.