Victron Autotransformer - small 120V inverter generator step up to 120V/240V? Too much power/capacity loss?

Been pondering this exact setup for the narrowboat actually. Running a Victron Autotransformer to step up from a small 120V inverter generator (thinking a Honda EU22i equivalent) to split-phase 240V for shore power compatibility when cruising near marinas.

My main concern is the efficiency losses stacking up — the generator already isn't running at peak efficiency at partial load, then you're adding transformer losses on top. Anyone worked out the real-world numbers on this?

A few things I'm unsure about:

• Capacity derating — I've read the Autotransformer can lose a significant chunk of usable capacity in step-up mode. Is 50% derate roughly accurate?
• Harmonic distortion — small inverter gens like the Honda are supposedly clean sine wave, but does the Autotransformer care about input quality in this configuration?
• Neutral bonding — this seems to get complicated quickly on a boat where earthing is already a minefield with hull corrosion risk

The alternative I keep coming back to is just biting the bullet and getting a proper 240V generator, but the weight and noise penalty on a narrowboat is genuinely painful.

Has anyone actually run this combination long-term rather than just theoretically? Curious whether the Victron kit handles it gracefully or whether it becomes a fault-finding nightmare. Also whether a Multiplus in the mix changes the calculus at all given it can handle shore input conditioning anyway.

@RustyNomad interesting one for a narrowboat setup. Worth noting the Autotransformer is really designed for the US split-phase 120/240V world — over here we're single-phase 230V so it's a bit of an odd fit unless you're running US-spec equipment.

If you're after proper 230V AC from a smaller generator on the cut, you'd be better served just grabbing a EU22i or similar that natively outputs 230V — Honda do the EU22i in a UK spec already.

The Autotransformer will work but you're taking an efficiency hit and adding complexity for not much gain on a narrowboat. Space and weight matter there. Keep it simple if you can.

What are you actually trying to power? That'll shape the answer a lot.

@RustyNomad slight snag — you're on a narrowboat in the UK, so unless you've somehow imported a North American 120V/240V split-phase shore power setup, the Victron Autotransformer is solving a problem you don't actually have. UK mains is single-phase 230V already, so a standard EU22i feeds straight into your Victron inverter-charger without any transformer gymnastics. The Autotransformer is really a North American headache remedy. If you're losing significant power through unnecessary conversion stages you're basically paying Honda for watts you're immediately throwing in the bin.

@RustyNomad @FogstarFan has hit the nail on the head here — the Victron Autotransformer is fundamentally a 120V↔240V split-phase device designed around North American 60Hz infrastructure. On a UK narrowboat you're dealing with single-phase 230V/50Hz, so the Autotransformer simply isn't the right tool.

If you genuinely need 240V on the cut, a standard UK inverter-generator like the Honda EU22i (which outputs 230V natively) is the straightforward answer — no step-up jiggery-pokery required. I ran a similar setup on my motorhome for two seasons before going full Victron Multiplus/Fogstar lithium, and the Honda just worked without complexity.

The efficiency losses from misapplying an autotransformer outside its design parameters would be significant anyway. What loads are you actually trying to run? That might open up simpler solutions entirely.

@ExFirefighter42 @FogstarFan right, so just to make sure I'm following correctly — even if I somehow sourced a 120V generator, the Victron Autotransformer isn't going to give me usable UK 240V single-phase, it's specifically doing that split-phase thing for North American systems?

Been scratching my head wondering if there's any clever workaround here for the narrowboat, but it sounds like I'd be barking up completely the wrong tree. What would you lot actually recommend for backup gen situations on a UK narrowboat then? Most of us are running 230V single-phase off the marina hookup anyway.

Is there a Victron product that actually makes sense for this use case, or is the answer just "buy a proper EU 230V generator and be done with it"? Genuinely curious whether the autotransformer has any application in a typical UK off-grid setup.