Choosing a van inverter starts with the appliances you will run at the same time: add up their power, add a margin, then check that the inverter’s peak rating can cope with anything that has a motor. A pure sine wave model avoids most compatibility problems, and its idle draw matters as much as its size if it stays on all day. This guide walks through the sizing step by step, using Victron Energy’s datasheets as the reference.
1. Add up the power of everything you will use at once, then add 20 %, the margin recommended by REDARC.
2. Check the peak: anything with a motor or compressor can draw several times its running power as it starts.
3. Work out the current on the 12 V side, because it sets the cable size, the fuse and the minimum battery capacity.
List the appliances before sizing a van inverter
The sizing starts with an honest list of what you will actually plug into the 230 V sockets in your van, and above all what will run at the same time. A laptop and a phone charger are no challenge at all. A hairdryer or a kettle changes the scale completely.
For a sense of scale, the “What uses watt?” leaflet from the Centre for Sustainable Energy, published by the charity National Energy Action, lists typical power ratings for household appliances. These are guide figures, and the rating plate on your own appliance always takes precedence.
| Appliance | Typical power (CSE leaflet) |
|---|---|
| Smartphone charging | 2.5 to 5 W |
| Laptop | 20 to 65 W |
| Fridge | 40 to 120 W |
| Microwave | 600 to 1,500 W |
| Toaster | 800 to 1,500 W |
| Hairdryer | 2,000 W |
| Kettle | 3,000 W |
The table already shows the first trade-off. Charging gear sits at a few tens of watts, while a household kettle on its own asks for more power than most vans can deliver. Before you size a van inverter, decide which power-hungry appliances you keep and which you replace with a 12 V version or a gas hob.
Working out the size of a van inverter
Australian manufacturer REDARC sets out the method in its guide to inverters for van life: add up the power ratings of your appliances to make sure they do not exceed the inverter rating, then add 20 % on top of that base requirement. The build site Explorist.life puts it another way, saying the inverter should be slightly bigger than the most power-hungry device you plan to run.
Here is an example to swap for your own list: a 65 W laptop and two 5 W phones, all charging together. That adds up to 75 W, and the 20 % margin takes it to 90 W, so a small inverter is plenty. Add an 800 W microwave at the same time and the need jumps to 875 W, then 1,050 W with the margin, which puts you in a completely different class of inverter.
Watch the unit on the box as well. Many inverters are sold by their VA rating, which differs from their output in watts. Victron’s Phoenix datasheet gives both: the 12/800 model, for instance, is rated at 800 VA and 650 W of continuous power at 25 °C. The watt figure is the one to compare with your calculation. The same datasheet lists versions fitted with UK (BS 1363) sockets, which matters for a British build: your van inverter can then take standard three-pin plugs.
Motor start-up and the peak rating of a van inverter
Some appliances draw far more as they start than when they run. REDARC points out that any appliance with a motor, pump or compressor will generally have a higher peak draw than the figure listed on it. American manufacturer Samlex America puts numbers on this in its inverter FAQ: motors, compressors and pumps need a starting surge of around 5 to 7 times their running power, with factors of 5 for a fridge, 4 for an air compressor, 3 for a pump. Resistive appliances such as a toaster or an iron start at their running power instead.
This is where a van inverter’s peak rating comes in, listed separately on every datasheet. Samlex adds one caveat: if the surge lasts longer than 5 seconds, only the continuous rating should be considered, so an appliance with a slow start gets sized on continuous power.
Pure sine wave or modified sine wave for a van inverter
Inverters come in two families. Pure sine wave models produce current close to what comes out of a mains socket. Modified sine wave models cost less but produce a stepped waveform. REDARC says these cheaper inverters can often lead to appliances overheating, and that pure sine wave inverters are almost always recommended.
Samlex lists the devices that may misbehave on a modified sine wave: laser printers, clocks on microwaves that stop keeping time, dimmers, electronic sewing machines, chargers for power tool batteries and shavers, with a higher heating effect in motors.
In a van, where most of what you plug in is electronics and tool batteries, that list is reason enough to choose a pure sine wave model, even with the price gap.
What a van inverter draws when idle
A van inverter that is switched on uses power even with nothing plugged in. In a van running on solar, that idle draw eats into your autonomy. The Victron Phoenix VE.Direct inverter datasheet gives the figures model by model.
| 12 V model | Continuous at 25 °C | Peak | Max efficiency | Idle | Idle in ECO mode |
|---|---|---|---|---|---|
| Phoenix 12/250 | 200 W | 400 W | 87 % | 4.2 W | 0.8 W |
| Phoenix 12/500 | 400 W | 900 W | 90 % | 6 W | 1 W |
| Phoenix 12/800 | 650 W | 1,500 W | 90 % | 6.5 W | 1 W |
| Phoenix 12/1200 | 1,000 W | 2,200 W | 91 % | 10 W | 1 W |
ECO mode puts the inverter on standby and, by default, wakes it every 2.5 seconds to look for a load, with a minimum load of 15 W according to the same datasheet. A device that draws less, such as a small charger, may not wake it. The datasheet also derates output by 1.25 % per degree above 40 °C, worth remembering if the inverter lives in a poorly ventilated locker.
The impact is easy to work out. An inverter drawing 6.5 W at idle and left on for 24 hours uses 156 Wh a day before it has powered anything. Switch it off when you are not using it, or rely on ECO mode if your appliances are above the threshold.
Cable, fuse and battery for a van inverter
On the battery side, a van inverter pulls a lot of current. Victron’s Wiring Unlimited guide gives the basic formula, current equals power divided by voltage, noting that it ignores inverter losses. Factoring those in with a simple sum, a 650 W model at 90 % efficiency draws about 650 divided by (12 × 0.90), or 60 A. Victron also points out that an inverter connected to a battery can draw a maximum current equal to its peak rating.
The Phoenix manual lists cable sizes and minimum battery capacity for each van inverter model. It states that if the DC cable is longer than 1.5 m, an additional fuse or DC circuit breaker must be fitted close to the battery.
| 12 V model | Cable up to 1.5 m | Cable 1.5 to 3 m | Minimum battery |
|---|---|---|---|
| Phoenix 12/250 | 4 mm² | 6 mm² | 30 Ah |
| Phoenix 12/500 | 6 mm² | 10 mm² | 60 Ah |
| Phoenix 12/800 | 16 mm² | 25 mm² | 100 Ah |
| Phoenix 12/1200 | 25 mm² | 35 mm² | 150 Ah |
REDARC offers its own rule of thumb: every 100 Ah of battery will power a 1,000 W inverter. The inverter circuit fits into the rest of the system like any other load, with its own fuse on the busbar, following the order set out in our campervan wiring diagram guide. If you have not chosen your battery yet, our guide on how to size a van battery bank lets you build the inverter’s consumption into the calculation.
Your van inverter by type of use
| Use | What to aim for |
|---|---|
| Charging a laptop and phones | A small pure sine wave inverter, around the size of a Phoenix 12/250 or 12/500, switched off when idle |
| Occasional light appliances | Continuous power matched to the hungriest appliance plus 20 %, peak checked for motors |
| Microwave or power tools | A model with 1,000 W continuous or more, cable and fuse from the manual, battery sized to match |
Start from your list of appliances running together, add the 20 % margin, check the peak for anything with a motor, then pick a pure sine wave model whose continuous rating in watts covers that total. Finish with the cable, fuse and battery capacity the manufacturer specifies for the model you choose.