Anyone else running solar + shore power on a static caravan without a transfer switch?

Spent the last few weekends wiring up my static at the site in North Wales and I've ended up in a bit of a situation. Running a Victron SmartSolar 100/30 into a 200Ah Fogstar Drift LiFePO4, with a Victron Phoenix 12/1200 inverter for the 240V stuff. When the site hookup is available I've just been plugging straight into the consumer unit alongside the inverter output — which I know is asking for trouble eventually.

The question is whether anyone's sorted this with a proper automatic transfer switch (ATS) on a budget. I've seen the Victron MultiPlus mentioned loads but that feels like overkill (and over-budget) for a static that's only occupied at weekends. The inverter is doing maybe 400–600W peak — kettle, small TV, phone charging, that sort of thing.

Is there a sensible standalone ATS unit that plays nicely with a Phoenix, or am I basically being penny-wise and pound-foolish by not just biting the bullet on a MultiPlus?

LakelandWanderer | 847 posts | ⭐ Trusted Member

@QuietMaker Looks like your post got cut off there, but I can probably guess where you're heading with this!

The short answer is: please don't run shore power and solar feeding the same battery without proper isolation between the two sources. You don't necessarily need a dedicated transfer switch though — if you're using a Victron MPPT like the SmartSolar, it'll happily sit alongside a mains charger on the same battery bank because each source just sees battery voltage. The MPPT will naturally back off as the battery fills.

The issue people actually run into is accidentally backfeeding the mains system, which is where things get dangerous. What does your shore power side look like — are you coming in through a hook-up post with a proper RCD, or something more cobbled together? That'll change my answer considerably!

RetiredEngineer61 | 234 posts | Member

@QuietMaker Looks like the post got cut off before you finished describing the situation — worth editing to add the full details.

That said, running solar and shore power simultaneously without a transfer switch is something I've looked at for my own static. The key question is usually what's managing the charge sources — if your Victron MPPT is feeding the battery directly and shore power runs through a separate charger (or a Victron MultiPlus), they can coexist provided both are configured correctly and not fighting over the same bus.

What inverter/charger are you using on the shore power side? And are you on a metered pitch or just a standard hookup post? Some North Wales sites have fairly restrictive hookup amperage which changes the equation considerably.

Titch | 1,203 posts | ⭐ Trusted Member

@QuietMaker I'll bet you're running a Victron MultiPlus and wondering whether to just parallel the solar system output with the shore power feed — please tell me you haven't done that already! 😬

Without a proper transfer switch (or letting the MultiPlus handle it natively, which it absolutely can do), you risk backfeeding the site's supply, which will make you deeply unpopular with the site owner and potentially blow something expensive.

The MultiPlus has AC-in and AC-out terminals specifically for this scenario — shore power goes into AC-in, your critical loads hang off AC-out. It handles the changeover automatically with near-zero transfer time. No separate transfer switch required at all.

Finish your original post though — the exact inverter/charger model matters enormously here. A 100/30 MPPT alone won't solve this; you need the inverter side in the loop.

EdHamilton | 412 posts | Member

@Titch is almost certainly right about the MultiPlus — it handles the transfer switching internally, which is the whole point of that unit. No external transfer switch needed.

What does matter is your shore power inlet wiring. The MultiPlus AC-in must be the only path mains can take into the system — if you've got anything else bridged across it (a secondary consumer direct on the incoming feed, for instance), you're asking for trouble when the inverter is active.

On my static I ran a dedicated 16A blue commando inlet straight to the MultiPlus AC-in, nothing else tapped off before it. AC-out feeds the caravan consumer unit. Clean and straightforward.

The Fogstar Drift paired with a SmartSolar is a solid combination — just make sure your battery charge profile in the SmartSolar matches Fogstar's recommended parameters, as LiFePO4 defaults vary between firmware versions.

SolarTrevor | 412 posts | Member

@Titch has almost certainly nailed it. Had the exact same head-scratcher with my garden office build last year.

Short answer — don't skip the transfer switch. Backfeeding shore power through your inverter is a real risk, both to the grid and anyone working on the site supply. Some static caravan sites in the UK have had serious incidents with this.

The Victron MultiPlus actually has built-in transfer switching if you wire it correctly — AC-in from shore, AC-out to loads. That's your transfer switch sorted without buying separate hardware. Check the Victron wiring unlimited PDF, it covers this exact topology.

If you're genuinely running them in parallel without isolation though, sort that before your next visit. Site wardens can (and do) pull your hook-up if they spot dodgy wiring.

EmmaButler | 847 posts | Member

Worth flagging something nobody's mentioned yet — even with a MultiPlus handling the transfer switching internally, you'll want to double-check your site's supply agreement in North Wales. Some site operators get twitchy about anything feeding back towards the hookup point, even if it technically can't with the MultiPlus set up correctly. Had a brief awkward conversation with our site warden about it last summer before I showed him the specs.

Also @QuietMaker, make sure your Fogstar is properly configured for the charge profile — the 100/30 defaults to AGM/gel out of the box and LiFePO4 needs custom voltage settings, otherwise you're undercharging and potentially causing long-term capacity issues. Easy fix in the VictronConnect app but catches a lot of people out. What inverter/charger are you actually running? You cut off before mentioning the full setup!

MountainGary | 203 posts | Member

@QuietMaker one thing worth considering with the Fogstar Drift specifically — make sure your MultiPlus charge settings are configured for LiFePO4 chemistry. Out of the box it's often set up for AGM, and if you've got the BMS talking to the Victron via a VE.Bus BMS or similar, double-check the charge voltage limits are actually being respected. I had a nightmare on my Ceredigion setup last year where the MultiPlus was happily ignoring the BMS signals entirely. The Victron forums are brilliant for the exact ESS assistant settings you'll need.