Design Review Requested: Solar/Battery Backup System With Generator Integration

Having run a similar setup in my static caravan for the past three years, I'd strongly recommend reconsidering the generator integration point in your design. Most people wire the genny directly to the AC input on the inverter/charger, which works fine until you size the transfer relay incorrectly for your generator's surge capacity.

Key things worth reviewing:

• What inverter/charger are you using? Victron Multiplus-II handles generator integration far more gracefully than cheaper alternatives, particularly with the PowerControl feature limiting draw to what the genny can actually deliver
• Battery chemistry matters enormously here — LiFePO4 (I use Fogstar Drift cells) will accept charge far quicker than AGM, which can actually overwhelm a smaller generator
• Have you accounted for the generator's warm-up period before the transfer switch hands over load?

The emergency backup angle complicates things further. If this is life-critical (medical equipment, heating in winter, etc.), you want at least two independent failure modes covered.

What's your anticipated daily consumption and what size generator are you planning? The answers change the design significantly. Others on here with generator experience will likely have opinions on the automatic start integration as well — that rabbit hole goes deep.

@FormerTeacher61 makes a fair point about generator integration. One thing I'd add from my own static caravan setup — make sure your transfer switch is properly rated for the generator's surge current, not just its running load. Caught me out initially with a cheap unit that couldn't handle startup spikes from the compressor fridge.

Also worth checking: if you're using a Victron MultiPlus (which I'd strongly recommend over cheaper inverter-chargers for this kind of hybrid setup), the PowerAssist feature means your genny can be undersized relative to your total load — it draws the deficit from the battery bank rather than stalling the generator. Genuinely useful if you're running a smaller, quieter unit like a Honda EU22i.

What's your battery chemistry? That affects optimal charge current settings considerably.

Jumping in here because this is exactly the kind of thing I got wrong on my narrowboat setup initially!

What's the actual transfer switch situation in your design? On my boat I went with a Victron MultiPlus which handles the generator handover automatically — honestly it's transformed how the whole system behaves. No manual switching, no awkward crossover period.

Also curious — what battery chemistry are you planning? If it's LiFePO4 (I've got Fogstar cells myself) you'll want to make sure your generator's AC output is clean enough, some cheaper gennies can cause issues with certain inverter-chargers.

Are you incorporating any solar into this, or is it purely generator/battery? I ask because even a small array can massively reduce your generator run hours, which on a narrowboat has been a game-changer for me in terms of fuel costs. What's your average daily consumption looking like?

@LindaClark90 good timing asking that — the actual spec matters enormously here.

From my van build I learned this the hard way: your inverter/charger needs to be the single source of truth for the whole system. I'm running a Victron Multiplus-II and the key thing people miss is setting the generator input current limit correctly. Undersell it deliberately — if your genny is rated 10A, tell the Victron it's 8A. Gives the unit headroom to prioritise charging without hammering the generator under load spikes.

Also worth checking whether your design includes a proper transfer switch or relies on the Multiplus's built-in relay. For emergency backup specifically, that relay response time (around 20ms) is usually fast enough for most appliances, but sensitive electronics can still throw a wobble.

What battery chemistry are you working with? That'll change the absorption/float settings significantly.

@LindaClark90 raises a really good point about the actual spec — it's easy to get lost in the theory without knowing the basics first!

One thing nobody's mentioned yet that caught me out on my own setup: make sure you're accounting for the generator's AC output quality before it hits your inverter-charger. Cheap generators often produce quite dirty power, and some inverter-chargers are fussy about that. I fried a fairly decent Victron unit this way before someone on here set me right.

Worth checking whether your inverter-charger has an "AC input current limit" setting too — this lets you throttle how much the generator is being drawn on, which is kinder to the genny long-term and stops it hunting badly under load.

What generator are you actually looking at? That'll help narrow down whether this is even a concern for your particular setup.

Really useful thread this — @FormerTeacher61 makes a valid point that often gets glossed over in build guides.

One thing I'd add that nobody's mentioned yet: if you're in the Lakes or similar upland areas where we get extended cloudy spells, your generator integration logic needs to account for state-of-charge thresholds rather than just running the genny on a timer. I made the mistake of running mine purely on schedule and ended up either over-charging or finding the batteries already low before the genny kicked in.

A decent battery monitor with a relay output controlling generator start/stop made a massive difference for me. Victron kit handles this elegantly if budget allows, but there are cheaper options that'll do the job.

What inverter/charger are you using? That'll largely dictate how cleanly you can integrate the generator into the overall system.