Short answer: for campervan inverter size, find the biggest 230 V thing you'll run (or the total of things you'll run together), add about 25% for start-up surges, and check the inverter's continuous rating at a warm temperature, not its peak. Then check the battery side: a 1,500 W load on 12 V pulls about 139 A, which needs 35 mm² cable and a 175 A fuse.
An inverter turns your 12 V battery power into 230 V, so you can run things with a normal UK plug. It's also the single biggest load most van builds have. We've found the question isn't really "what size inverter", it's "what am I asking my battery and cables to do", and the answer to that decides whether the rest of the system copes.
Do you need an inverter?
Start by listing what you'd plug in. A lot of van kit runs on 12 V directly: compressor fridges, LED lights, water pumps, roof fans, diesel heaters, USB charging, and many laptops with a 12 V or USB-C charger. Running those straight off 12 V avoids the inverter's losses. Roam's maths assumes an inverter is 90% efficient, so every 100 Wh you use at the plug costs about 111 Wh from the battery.
An inverter earns its place for things that only come in 230 V: a kettle, toaster, air fryer, induction hob, coffee machine, hair dryer or power tool. If those are on your list, read on. If not, you may be able to skip it entirely.
How to work out campervan inverter size
- Find the watts on each 230 V item's label. Typical figures in Roam's planner are 1,000 W for a low-watt kettle, 800 W for a toaster, 1,200 W for a coffee machine and 1,500 W for an air fryer or induction hob. Use your own labels.
- Decide what runs at the same time. Kettle and toaster together is 1,800 W; one after the other, the biggest is 1,000 W. Roam's planner only adds up things you mark as running together, and otherwise uses the single biggest item.
- Add headroom. Some things draw a surge for a moment when they start. Roam multiplies the peak by 1.25, then rounds up to a common size: 600, 1,000, 1,500, 2,000 or 3,000 W.
| Biggest 230 V load | × 1.25 | Roam's suggested size |
|---|---|---|
| Laptop and small chargers, 150 W | 188 W | 600 W |
| Low-watt kettle, 1,000 W | 1,250 W | 1,500 W |
| Air fryer, 1,500 W | 1,875 W | 2,000 W |
| Kettle and toaster together, 1,800 W | 2,250 W | 3,000 W |
Those are starting points. Things with motors or compressors can surge much more than 25% at start-up, so check the appliance's manual, and check the inverter's surge figure against it.
Reading the ratings: continuous, peak and heat
Inverter names often quote the biggest number they can. Look for three figures on the datasheet.
VA versus watts. Victron's Phoenix Inverter Smart 12/1600 is named for 1,600 VA, but its datasheet gives continuous output as 1,300 W at 25 °C. Size against the watts.
Temperature. The same datasheet gives the 12/1600 as 1,200 W at 40 °C and 800 W at 65 °C. A van in summer sun, with the inverter tucked into a cupboard, can get warm, so give it ventilation and don't plan to run it at its 25 °C limit all day.
Peak. The 12/1600 lists a peak of 3,000 W, for short surges, not for running. The 12/3000 gives 2,400 W continuous at 25 °C, 2,200 W at 40 °C and 6,000 W peak.
Efficiency is on there too: the datasheet gives a maximum of 92% for the 12 V models, a little higher at 24 V and 48 V. Maximum efficiency is the best case, so the 90% Roam assumes is a fair everyday figure.
What it pulls from the battery
This is where inverters catch people out. The current on the 12 V side is the 230 V watts divided by the battery voltage and the efficiency:
| Load | 12 V system | 24 V system |
|---|---|---|
| 1,000 W | about 93 A | about 46 A |
| 1,500 W | about 139 A | about 69 A |
| 2,000 W | about 185 A | about 93 A |
Your battery has to be able to supply that. Check its maximum continuous discharge current on the datasheet, and if the inverter could ask for more, either choose a smaller inverter, add battery capacity, or move to 24 V. Energy matters too: an air fryer at 1,500 W for 15 minutes uses 375 Wh at the plug, about 417 Wh from the battery. Put it in the solar and battery calculator to see what that does to your daily budget, and our air fryer recipes if you're tempted.
Cable and fuse
High current means thick cable, even over a short run, because the cable has to safely carry the fuse that protects it. Using the same maths as our cable size calculator, with a 1 m run, 3% drop, 90% efficiency and 105 °C cable:
| Inverter load | Current | Fuse | Cable |
|---|---|---|---|
| 1,000 W at 12 V | 93 A | 125 A MIDI | 16 mm² |
| 1,500 W at 12 V | 139 A | 175 A MIDI | 35 mm² |
| 2,000 W at 12 V | 185 A | 250 A MEGA | 50 mm² |
| 2,000 W at 24 V | 93 A | 125 A MIDI | 16 mm² |
Size the cable and fuse for the inverter's continuous rating rather than what you think you'll use, as someone will eventually plug in something bigger. Keep the run short, mount the inverter close to the busbars, and fit its fuse at the busbar end. Victron's Wiring Unlimited suggests keeping the positive and negative cables of high current runs, like battery to inverter, close together. Inverter manuals usually give their own recommended cable and fuse, so follow those where they're given. The wiring diagram guide shows where it sits.
Inverter or inverter-charger?
An inverter-charger combines an inverter, a mains battery charger and a transfer switch. On hook-up it passes campsite power through to your sockets and charges the battery; off hook-up it switches to the inverter. Victron's MultiPlus-II 12/3000/120-32, for example, has a 32 A transfer switch and charges at up to 120 A. Its PowerAssist feature can top up a weak campsite supply from the battery when you briefly need more power than the bollard gives.
The alternative is a separate inverter and a separate mains charger, which is simpler and can be cheaper. Either way, anything wired into the van's 230 V circuits falls under BS 7671 Section 721 for caravans and motor caravans, so have a qualified electrician install or inspect and test that side.
Roam's Build your electrics tool suggests an inverter size from your appliance list, then sizes its cable and fuse and adds them to your shopping list. It's in the app now, in early access. The whole planning order is on the campervan electrics page.
Questions
What size inverter do I need for a kettle in a campervan?
For a 1,000 W low-watt kettle, a 1,500 W inverter on Roam's rule (the load plus 25%, rounded up). The kettle alone pulls about 93 A on 12 V, but cable the inverter for its own rating: for 1,500 W on a 1 m run, that's 35 mm² and a 175 A fuse. A standard 3 kW kettle is a much bigger ask.
Can I run a 2000 W inverter on 12 V?
You can, but at full load it draws about 185 A from the battery, needing 50 mm² cable and a 250 A fuse on a 1 m run by our calculator, and a battery rated to supply that. At 24 V the current halves.
Is a bigger inverter always better?
No. A bigger inverter needs thicker cable, bigger fuses and a battery that can supply more current, and it costs more. Size it for what you'll actually run.
Should the inverter be on all the time?
Inverters use some power just being on, even with nothing plugged in. That figure is on the datasheet. If you only use 230 V now and then, switch it off between uses.