How much solar and battery to run an air conditioner in a off-grid cabin?
An air conditioner is the heaviest load most rigs ever ask of a battery. In a off-grid cabin in the UK it uses about 4000 Wh a day. Wattonomy sizes the battery (20.48 kWh), the solar (1245 W) and every cable around it to BS 7671, then hands you the wiring diagram and a shoppable parts list.
A real Wattonomy output — a UK off-grid cabin running an air conditioner
System
48 V
Battery
20.48 kWh
Solar
1245 W
Inverter
MP-II 48/3000
SmartSolar 150/351×
415W solar panel3×
Battery → inverter cable4×
Battery → inverter fuse1×
Wire & fuse sizes follow BS 7671 — each conductor carries its fuse and stays under a 3% voltage drop.
Built to BS 7671 standards
Gauge · fuse · volt-drop sized
Every spec sourced
Air-con is where solar dreams meet physics.
A rooftop air conditioner can pull more in an hour than your lights use in a day. It needs a large bank, a strong inverter and serious cable, sized honestly. Wattonomy sizes the bank, solar and every cable around it, to BS 7671.
What it takes to run an air conditioner
Real numbers from a sized off-grid cabin build for the UK, not rules of thumb.
Daily energy
About 4000 Wh a day for the air conditioner itself, the figure everything else is sized from.
Battery
About 20.48 kWh of LiFePO4 (roughly 427 Ah at 48V) to carry it through the night and a cloudy day.
Solar to replace it
About 1245 W of panel replaces a day’s use in fair sun; poor light or winter wants more, or a second charging source.
Inverter and surge
Sized to start the 1500 W surge cleanly: MultiPlus-II 48/3000, with cable to match, to BS 7671.
What you walk away with — free
A full system sized to run your air conditioner — battery, solar and inverter
A real wiring diagram with cable gauges and fuses
Cable and fuse sizing with the voltage drop checked, to BS 7671
A shoppable parts list, every line traceable to a standard or datasheet
Building it for real? Unlock the build binder.
Free gets you a safe, correctly-sized design. The build binder gets you the documents to build it right — and prove it's right.
Printable wiring diagram, yours to keep & tape inside the build
Full parts list, every line sourced
The volt-drop & fusing calculation trail — the "why" behind every spec
The build binder is a one-time unlock — no subscription, no auto-renew.
Questions
How many watts does an air conditioner use?
An air conditioner runs at about 1400 W and can surge to roughly 1500 W on start-up, using around 4000 Wh over a day. The inverter has to cover that surge, not just the running watts.
What size battery runs an air conditioner in a off-grid cabin?
For this build Wattonomy sizes about 20.48 kWh (roughly 427 Ah at 48V) of LiFePO4 — enough to carry the air conditioner plus your basics through the night and a cloudy day. Your exact number depends on how many days of backup you want.
How much solar do I need to run an air conditioner?
About 1245 W of panel replaces a day’s use in fair sun; poor light, winter or shade wants more, or a second charging source such as a DC-DC charger off the engine. The tool sizes it from your climate.
Is this sized to UK standards?
Yes. Every cable, fuse and busbar is sized to BS 7671 using mm², at the 12/24V DC and 230V AC typical of the UK systems. Nothing here is a rule of thumb.
The standards we build to
Plain version: these are the recognized rulebooks your design is sized against, so the numbers hold up to a surveyor, an inspector or an insurer.
BS 7671The UK IET Wiring Regulations for the 230 V side.
ISO 13297The international standard for small-craft electrical systems.
Wattonomy applies these standards in its calculations. It is not certified, sponsored or endorsed by ABYC, ISO, NFPA or Victron — it sizes your design to meet what they require, and shows the working.
Sized and checked to BS 7671. Last reviewed June 2026 — see the full methodology for the exact rules behind every number and how each design is validated.
Built by anABYC Certified Master Technician