Anyone else found inverter placement makes a massive difference to efficiency?

Been helping a mate fit out his Transit camper and we spent ages debating where to mount the Victron Phoenix 12/1200. Ended up going under the bed about 600mm from the 200Ah Fogstar lithium — cable run is short and tidy but the ventilation is rubbish and it's been hitting thermal cutout on sustained loads over about 800W.

Moved it temporarily to a locker near the side door and the difference was noticeable straight away. Same loads, no cutout, and the idle draw seemed slightly lower too (though that might be wishful thinking). Cabinet was still enclosed but at least had a proper vent to outside air.

My static caravan setup has the inverter in a dedicated vented enclosure and I've never had heat issues there, so I should've known better from the start. Is there a rule of thumb people use for minimum airspace or CFM when you're boxing these things in? Wondering if a small 12V fan on a thermal switch would sort the Transit install without having to relocate again.

WFS_Camper | Posts: 847

@WD40Wizard 600mm is decent — every bit counts with high current draws. One thing worth mentioning that often gets overlooked: ventilation around the inverter matters almost as much as cable length. I had mine tucked in a tight cupboard initially and it was thermally throttling under load without me realising. Moved it somewhere with proper airflow and picked up noticeable efficiency gains, plus the fan cycles far less frequently now.

Also worth checking the Victron app if you haven't already — the temperature compensation data is genuinely useful for spotting whether placement is causing issues. Did you go with busbar connections or direct to battery terminals?

WattNick | Posts: 312

When I was setting up my garden office system I agonised over exactly this. Ended up with my Victron MultiPlus sitting practically on top of the battery cabinet — maybe 200mm of 70mm² cable. The difference in voltage sag under load compared to my earlier test position (about 1.5m away with undersized cable) was genuinely eye-opening on the Victron app.

@WD40Wizard one thing people overlook in van builds specifically is ventilation around the inverter itself — stuffed under a bed with restricted airflow and it'll throttle back thermally before cable losses even become the main story. Victron's thermal derating is quite aggressive once it gets toasty. Might be worth checking the fan exhaust has somewhere useful to vent before signing off on that position.

GeorgeJohnson83 | Posts: 47

Doing a van conversion at the moment and this is something I've been wrestling with. My inverter ended up about 900mm from the battery — mainly because of awkward layout constraints — and I'm genuinely wondering if I made a mistake.

Question for the thread: does the cable gauge compensate for the extra distance, or is there a point where even 70mm² just can't overcome the voltage drop on high-draw appliances? I'm running a 1000W inverter and occasionally wanting to use a travel kettle.

Also — has anyone found the heat from the inverter affects nearby Fogstar cells noticeably? I've got limited airflow under the bed platform and that's possibly a bigger concern than cable length for me.

SparkySparky | Posts: 1,203

My static caravan setup taught me this lesson the hard way. Originally had the inverter mounted at the far end of the battery compartment — about 1.2 metres of cable run on cheap CCA stuff. The voltage drop under load was embarrassing.

Repositioned everything so the Victron Multiplus sits literally adjacent to the Fogstar cells, proper tinned copper throughout, and the difference under kettle load was measurable on the BMS display.

@GeorgeJohnson83 in a van you've got even less forgiveness than a caravan build — thermal management matters too. Under-bed placement sounds ideal but make sure there's airflow; Victron units do throttle when they cook.

The other thing nobody mentions — fusing as close to the battery terminal as physically possible. Cable length between battery and fuse is where the real danger lives, regardless of where the inverter ends up.

DodgyGrafter | Posts: 847

Good shout @WD40Wizard. Cable length and cross-section is where most people come unstuck. Even 600mm can cost you if the cable's undersized — voltage drop at full load on a 12V system is brutal compared to 24V.

Worth checking what you're actually losing with a multimeter at the inverter terminals under load versus at the battery terminals. Wouldn't surprise me if you've got 0.3-0.5V drop depending on what gauge you ran.

Also don't overlook ventilation around the Phoenix — they throttle back when they get warm, which people often mistake for a battery or cable issue. Under-bed spaces can get surprisingly stuffy.

@GeorgeJohnson83 if you're still planning yours, go 24V if you can — halves your current, makes the whole cable sizing headache much more forgiving.

T6Life | Posts: 312

Great thread this. One thing worth adding — thermal management of the inverter location is just as important as cable runs. Under beds in Transits can get surprisingly warm in summer, especially if you've got foam insulation trapping heat around the unit. The Phoenix 12/1200 will start derating above 25°C ambient and you'll notice it on high draws like a kettle or hair straighteners.

I drilled a couple of 50mm holes with a spade bit either side of mine and fitted mesh grilles — made a noticeable difference. Also worth leaving a decent air gap between the inverter and any surrounding woodwork rather than boxing it in tightly. @GeorgeJohnson83 if you're mid-build now, factor that in before you fix your cabinet panels permanently — much easier to sort it at this stage than retrofit later!

Shunt_Guy | Posts: 2,156

Dead right about placement mattering. One thing I'd add that's often overlooked — inverter orientation. The Phoenix 12/1200 really wants to be vertical for proper convection cooling through those fins. @T6Life touches on thermal management, and it's worth noting that even a slight tilt can reduce cooling efficiency noticeably under sustained load. Under-bed mounting can be tricky for this reason; you want airflow around the unit, not just near it. Cut some vent holes with a grommet either side if it's enclosed. 600mm cable run sounds sensible — just make sure you've fused as close to the battery terminals as physically possible.