Estimate how long your battery bank might run an air conditioner off-grid. Most roof-top 240V units draw a lot more than people expect once you factor in the inverter.
Work out your AC runtime
Running watts vary a lot by brand, ambient temperature and how hard the compressor is working — startup surge is also much higher than running draw, so check your unit and inverter can handle it. For a full day of AC off-grid, most setups need a generator or mains power rather than battery alone. See Off-Grid Caravan Power, Water and Communications.
Air conditioning in a caravan, explained
New to this? Here's the plain-English version of how caravan air conditioners actually work, what they cost you in power, and what's realistic off-grid.
How caravan air conditioners work
Most caravan roof-top air conditioners are 240V units — basically the same technology as a home split-system, squeezed into a roof pod. They're designed to run off mains power (a powered site) or a generator. If you want to run one from your battery, the battery power has to go through an inverter first, which turns 12V or 24V DC into 240V AC — and that conversion isn't free, you lose a bit of energy along the way.
Some newer vans use a 12V or 24V DC compressor air conditioner instead. These skip the inverter altogether and are usually far more efficient off-grid, but they cost more up front and aren't common yet on older or entry-level caravans.
Why the wattage on the spec sheet isn't the whole story
The number on the compliance plate or spec sheet is the running draw — what the unit uses once it's settled into a steady cooling cycle. Starting the compressor pulls a lot more for a second or two (the "start-up surge"), sometimes two to three times the running figure. Your inverter has to handle that surge, not just the running number, or it'll trip out. Always check your inverter's surge rating against your air conditioner's start-up draw, not just its running watts.
Battery basics: what "usable capacity" actually means
A battery's usable capacity isn't the same as its total rated capacity. Most lithium (LiFePO4) batteries can be run down close to empty without damage, so usable capacity is close to what's printed on the box. Older AGM or lead-acid batteries should generally only be discharged to around 50%, so their usable capacity is roughly half the rated figure. Get this number wrong and every runtime estimate — from this calculator or anywhere else — will be wrong too.
Usable capacity is measured in watt-hours (Wh). If you only know your battery in amp-hours (Ah), multiply by your system voltage: a 200Ah, 12V battery bank is roughly 200 × 12 = 2,400Wh.
What to realistically expect
Running a 240V roof-top air conditioner off battery and inverter is one of the heaviest loads you can put on a caravan power system — expect a handful of hours from a typical house-battery-sized bank, not a full day. Ambient temperature, how well insulated your van is, how hard the compressor has to work, and your inverter's own efficiency all shift the number up or down from a simple calculation.
If you need air conditioning overnight, every night, off-grid, plan around a generator or mains power rather than battery alone — the numbers above explain why. Battery-only AC is more realistic for short bursts: cooling the van down before bed, or getting through the hottest part of a stopover.