Almost every question about car charging ends at the same place: how much power is available in the first place. The answer is smaller than most accessories imply, and it is set by parts of the car nobody thinks about.
The fuse is the ceiling
A 12V accessory socket sits on a circuit protected by a fuse, typically 10 A or 15 A. At a nominal 12 to 14 volts, that gives roughly:
- 10 A fuse: about 120 to 140 W available.
- 15 A fuse: about 180 to 210 W available.
- 20 A, found on some vans and 4x4s: about 240 to 280 W.
Those are theoretical maxima at the fuse. Real available power is lower, because conversion losses inside the accessory and voltage drop along the wiring both take a share.
Never fit a larger fuse to make something work. The fuse protects the wiring, and the wiring is not upgraded by changing it. A fuse that keeps blowing means the load is wrong for that circuit.
Two sockets often share one fuse
This surprises people with a front and a boot socket, or a front and rear pair. If both are on the same circuit, a compressor in the boot and a charger in the front are drawing from the same budget.
Your car’s handbook or the fuse box diagram will show it. It is worth checking before assuming you have twice the capacity.
What this means for chargers
A 140 W USB-C charger is at or beyond the limit of a 10 A socket once losses are counted. It will still work — because your devices never request that much — but the headline figure was never achievable through that socket.
For phones and tablets, 30 to 45 W total is genuinely sufficient. For a USB-C laptop while driving, 65 to 100 W is realistic on a 15 A circuit, and you should not expect to run anything else demanding at the same time.
Multi-port chargers divide a budget. A charger advertised at 65 W total will typically deliver something like 45 W plus 20 W when two ports are in use, and that split is the number worth checking before buying.
What this means for inverters
This is where the ceiling bites hardest. A 12V-to-230V inverter plugged into an accessory socket cannot exceed the socket’s limit, whatever the box says.
- A 150 W socket-powered inverter is realistic.
- A 300 W inverter plugged into a cigarette lighter socket will blow the fuse under real load.
- Anything above roughly 200 W needs direct connection to the battery with appropriately sized cable and its own fuse.
Kettles, hairdryers and anything with a heating element are outside this range entirely. Heating elements are the single most common reason people discover their inverter’s limits.
Ignition-switched or permanently live
Sockets fall into two groups and it changes what you can do:
- Ignition-switched: power stops with the engine. Safer for the battery, useless for a cool box or a dash cam parking mode.
- Permanently live: convenient, and capable of flattening the battery overnight.
Test with something that has an indicator light. If you need permanent power for parking-mode recording or a cool box, the correct solution is a hardwire kit with a low-voltage cut-off, not leaving an accessory in a live socket.
Voltage sag is normal
Voltage at the socket is not constant. It sits near 12.4 V with the engine off, rises to about 14 V while the alternator is charging, and dips sharply during cranking. Accessories are built for this, but very cheap ones behave badly at the extremes — resetting during start-up, or refusing to charge at low voltage.
The short version
Work out your fuse rating, subtract for losses, and treat the result as your budget. Buy accessories that fit inside it rather than accessories whose headline number exceeds it, and hardwire anything that needs more than the socket can honestly give.
