Connecting an off grid inverter to a house that is on grid

Been down this rabbit hole myself when I was planning my van build and briefly considered doing something similar for my mum's place.

The short answer is: tread very carefully.

The main concern everyone glosses over is islanding — if the grid goes down for maintenance and your inverter is still pushing power back into what the engineers assume is a dead line, someone could get seriously hurt. Grid-tied inverters have anti-islanding protection built in specifically for this reason, but a standard off-grid unit absolutely does not.

In the UK, you're also looking at G98/G99 compliance if you're exporting anything to the grid, which means notifying your DNO and using approved equipment. Most off-grid inverters won't qualify.

The safer route I've seen people take is keeping the systems completely isolated — using a proper transfer switch (Victron make decent automatic ones) so the off-grid system only powers your loads when the grid is absent, never simultaneously. No parallel connection, no grey areas.

There are hybrid inverter/chargers like the Victron MultiPlus range that are actually designed to do exactly what you're describing properly, with the right protections baked in. Costs more upfront but you're not gambling with safety or breaking regulations.

What's the actual goal here — backup power, reducing your bills, or something else? That changes the answer quite a bit.

Would love to hear what others have done, particularly anyone who's had conversations with their DNO about it. My experience is they vary wildly in how helpful they are.

Yeah the big one nobody mentions is G98/G99 compliance — if your inverter backfeeds onto the grid even for a split second, you're potentially in breach and the DNO can get involved. Not fun.

For the shepherd's hut I keep everything completely isolated from any grid supply. Two separate systems, no crossover point. Bit of a faff to set up but zero headaches.

If you must interconnect, an automatic transfer switch that physically cannot parallel the two supplies is the safer route. Victron do decent solutions for this but you'd want an electrician familiar with off-grid setups — not just any sparky — to sign it off.

@FrostySocket is absolutely right on G98/G99, but there's another layer worth flagging: islanding protection.

If there's a grid outage and your inverter keeps running, you've potentially got a live circuit that DNO engineers assume is dead. That's a serious safety issue, not just a compliance one.

The practical solution most people here end up with is a proper transfer switch (manual or automatic) that physically disconnects the grid side before your inverter comes online — no overlap possible. Victron Multiplus units handle this elegantly with their built-in transfer relay, and you can configure anti-islanding behaviour in VE.Configure.

I've got a Multiplus-II in my garden office setup doing exactly this — it monitors grid presence and only switches to inverter mode when grid is confirmed absent. Zero backfeed risk.

Whatever you do, loop in a qualified electrician familiar with BS 7671 and ideally one who understands off-grid systems specifically.

Good points from @FrostySocket and @DodgyRoamer. Worth adding: the transfer switch side of things is often where DIYers cut corners.

If you're running an off-grid inverter (say a Victron Multiplus) alongside a grid supply, you must have a proper changeover switch — mechanical interlock or an ATS — so the two sources can never be live simultaneously on the same circuit.

Had to think through this carefully on my boat setup. DNO won't care until something goes wrong, then suddenly everyone cares very much.

Also worth checking your inverter's spec sheet — some units have anti-islanding built in, some absolutely don't. Don't assume.

Keep it simple, keep it separated.

What @BatteryRay said about transfer switches really resonates — I went through exactly this when wiring up my garden office last year.

I ended up running a completely separate circuit from a dedicated consumer unit, physically isolated from the house mains. No crossover points whatsoever. My Victron Multiplus handles the office loads entirely, with Fogstar lithium batteries behind it.

The key thing nobody's mentioned yet: even if you engineer it perfectly, your home insurance and building regs situation gets murky the moment an off-grid inverter touches anything grid-adjacent without proper notification. My insurer asked some very pointed questions.

Separate circuit, separate earth arrangement, proper RCD protection on both sides — that's the only path I'd genuinely recommend without involving a qualified electrician who actually understands both worlds. They're rare, but they exist.

Good thread this. One thing nobody's mentioned yet — earthing arrangements can get really messy when you're bridging off-grid and on-grid systems.

Depending on how your inverter handles the neutral-earth bond, you can end up with conflicting bonding points when the grid's also present. Some Victron inverters let you configure this properly, but cheaper units often don't give you that control.

I learned this the hard way with my own setup — RCD kept nuisance tripping until I properly sorted out where my neutral-earth bond was sitting.

Worth getting someone with Part P competency to at least cast an eye over it, even if you're doing the bulk of the work yourself. DNO won't care about your inverter spec — they'll care about whether your installation is safe and compliant.