Inverter Size Calculator
By Frank Zale · Last updated
Size the inverter for a camper van, RV or caravan, and see the DC current and fuse that come with it. Those two numbers matter more than the inverter price, because they decide your cable.
Add up only what runs at the same time. A 1,500 W kettle and a 900 W microwave used one after the other need 1,500 W, not 2,400 W. Motors and compressors draw a brief surge well above their rating, so leave headroom if you run a fridge compressor or a power tool.
Inverter size
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- Continuous need
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- DC current drawn
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- Battery cable fuse
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Show the math
Size to the moment, not the day
An inverter is sized by the heaviest instant, not by daily energy use. What matters is the largest set of AC appliances that will ever run together. For most vans that is one kettle, one induction ring, or one microwave, because nobody runs all three at once.
The 1.25 multiplier gives headroom for the fact that appliance ratings are nominal and inverters derate when hot. Motors and compressors also draw a brief surge several times their running figure, so a quality inverter quotes a separate surge rating: check that it covers anything with a motor in it.
The number that costs money
A 2,000 W inverter at 12V pulls around 185 A from the battery under full load. That is a serious current: it needs very thick cable, a large fuse inches from the battery terminal, and a battery bank capable of delivering it without sagging. Doubling the inverter roughly doubles all three costs.
This is the strongest argument for running 12V wherever possible. A 12V compressor fridge uses far less energy than an AC fridge on an inverter, because it skips the conversion entirely.
Once you have a size, work out the cable with the wire size calculator, or read what size inverter for a camper for the full reasoning.
Frequently asked questions
＋ What size inverter do I need for a camper van?
Take your largest simultaneous AC load and multiply by 1.25. A build whose heaviest moment is a 1,000 W kettle needs a 1,500 W inverter. Most van builds land between 1,000 W and 2,000 W; go larger only if you run an induction hob or power tools.
＋ Do I add up all my appliances?
No, only the ones running at the same time. A 1,500 W kettle and a 900 W microwave used one after the other need 1,500 W, not 2,400 W. Adding everything together is the most common reason people buy an inverter twice the size they need.
＋ How much current does an inverter draw from the battery?
Divide inverter watts by battery voltage, then allow for about 90% efficiency. A 1,000 W inverter at 12V draws roughly 93 A continuously, which is why it needs thick cable and a fuse close to the battery terminal.
＋ Do I even need an inverter?
Only if you run 120V or 230V appliances. Fridges, fans, lights, water pumps and most device charging can all run on 12V DC, which avoids the conversion loss entirely. If everything you use is 12V, skip the inverter and save both money and energy.