Short answer: the VA rating is the bigger, rounder number on the box, and it isn't the watts the inverter can run. Victron's MultiPlus-II 12/3000 is 3000 VA, which is 2400 W at 25°C, and less again when it's hot. Size an inverter by watts at the temperature it'll actually live at, then make sure the battery, cable and fuse can feed it, because on 12 V that's a lot of current.
What's the difference between VA and watts?
Watts are the real power: what actually comes out of your battery as energy and heat. VA is "apparent power", voltage times current, and it's the number used for sizing wiring, breakers, inverters and generators.
The gap between the two is the power factor. For a heater or a toaster it's 1, so watts and VA are the same. For loads with motors, coils or electronics, the current and voltage get out of step and the real watts come in under the VA.
For everyday sums, Victron's Wiring Unlimited says 0.8 is fine as a power factor. That's exactly where the MultiPlus-II's numbers come from: 3000 VA x 0.8 = 2400 W. (That's our arithmetic, but it matches Victron's rating.)
Why does the rating drop when it's hot?
Victron don't rate it at one temperature. The MultiPlus-II datasheet gives continuous output at three, and it falls as it gets hotter:
| Model | VA rating | At 25°C | At 40°C | At 65°C | Peak |
|---|---|---|---|---|---|
| MultiPlus-II 12/3000 | 3000 VA | 2400 W | 2200 W | 1700 W | 5500 W |
| MultiPlus-II 12/5000 | 5000 VA | 4000 W | 3700 W | 3000 W | 9000 W |
A cupboard in a van parked in the northern sun isn't 25°C. If the inverter lives somewhere warm, plan on the 40°C figure.
Which loads are hard on an inverter?
Victron sort loads into three kinds:
- Resistive: heaters, toasters, hair dryers. Power factor 1, the easy ones.
- Inductive: things with coils, like motors and transformers: fridges, compressors, air conditioners, fluorescent lights.
- Capacitive: battery chargers and UPS units.
There's a fourth group to watch: non-linear loads such as LED lighting, dimmers, heat guns, rectifiers and some soft-starts. Victron warn these can overload an inverter sooner than the ratings suggest. The MultiPlus-II's VA figure is itself quoted for a non-linear load with a 3:1 crest factor.
So if your list includes a compressor or a lot of electronics, leave more headroom than the label watts suggest.
How much current does an inverter pull from a 12V battery?
A lot. Current is power divided by voltage, and Victron's own example is 2400 W / 12 V = 200 A. Run a 2400 W load through an inverter and it takes 2400 W from the battery, before the inverter's own losses. In real life it's a bit more.
Some sums on Victron's figures (ours, ignoring losses):
- MultiPlus-II 12/3000 flat out: 2400 W / 12 V = 200 A.
- MultiPlus-II 12/5000 flat out: 4000 W / 12 V = about 333 A.
- A 2000 W kettle (an example, not a product spec): 2000 / 12 = about 167 A for as long as it's boiling.
That's why big 230 V loads and small batteries don't mix, and why the battery's limits matter as much as the inverter's. Check the maximum continuous discharge of your battery's BMS before you pick the inverter.
What size cable and fuse does a big inverter need?
Victron give the numbers in the MultiPlus-II manual. Cable sizes are per terminal, for the run between battery and inverter:
| Model | Battery capacity | DC fuse | Cable, 0-5 m | Cable, 5-10 m |
|---|---|---|---|---|
| 12/3000/120 | 400-1200 Ah | 400 A | 2 x 50 mm² | 2 x 70 mm² |
| 12/5000/220 | 600-2400 Ah | 600 A | 2 x 95 mm² | 2 x 150 mm² |
The DC cables should be copper, rated to 90°C, and not run in a closed conduit. Follow local installation rules. Victron's own summary: to get the full capacity out of the inverter, you need enough battery, the right DC fuse and thick enough cable.
Note the battery sizes. Victron's recommended minimum for the 12/3000 is 400 Ah. If you were hoping to run one off a single 100 Ah battery, this is the moment to rethink. Our guides on cable sizing and fusing a 12V system go further, and fuses are grouped by type.
What does an inverter use when nothing's plugged in?
More than you'd think. Victron list the MultiPlus-II 12/3000's zero-load power as 13 W, 9 W in AES mode and 3 W in Search mode (the 5000 VA 12 V model is 15 W). Over a day:
- 13 W x 24 h = 312 Wh, about 26 Ah from a 12 V battery, for doing nothing.
- 9 W x 24 h = 216 Wh, about 18 Ah.
- 3 W x 24 h = 72 Wh, about 6 Ah.
Switch it off when you don't need 230 V, or use the low-power modes. Our sizing guide has more on hidden loads like this.
So how do I pick a size?
- List the 230 V things you'll run at the same time, with their watts from the label. A kettle and a toaster together is a very different job from a laptop charger.
- Add headroom for compressors, motors and electronics, using the load types above.
- Check the watts at the temperature it'll live in, not the VA on the box.
- Check the battery, cable and fuse can feed it, using the maker's table.
- Check what it costs you at idle, and whether you'll switch it off.
For most touring setups that's a laptop, phones, a few small appliances and maybe a coffee machine, which is a very different inverter from one meant to run a microwave and an air-conditioner. We've installed plenty of MultiPlus-IIs, and the load list is the place to start, not the biggest number on the shelf.
The MultiPlus-II 12/3000 and MultiPlus-II 12/5000 are in inverter/chargers; plain inverters are listed separately.
What about the 230 V wiring?
Plugging an appliance into the inverter's own outlet is one thing. Running fixed 230 V wiring through a van or caravan, to power points, a shore-power inlet or an inverter/charger, is a licensed electrician's job. Our guide to 230V in a van or caravan explains what the regulators say.