12V basics for caravan owners

Most of what runs in a caravan away from a powered site runs on 12 volts: lights, the water pump, the fridge, fans, USB chargers. Understanding four units and six components is enough to read a spec sheet, talk to an auto electrician, and work out whether your setup will last the weekend.

Four units

Volts (V) are electrical pressure. Caravan house systems are nominally 12V; lithium batteries sit a little higher, around 12.8–13.4V most of the time.

Amps (A) are the rate of flow. A water pump might draw 5A while it’s running; a 2,000W inverter running a kettle can pull well over 150A.

Watts (W) are power: volts × amps. A 12V fan drawing 2A uses about 24W.

Watt-hours (Wh) and amp-hours (Ah) are amounts of energy. Run that 24W fan for 5 hours and you’ve used 120Wh. Batteries are usually sold in amp-hours, so convert: Wh = Ah × volts. A 200Ah lithium battery at 12.8V holds about 2,560Wh.

Watt-hours are the easier unit to plan with, because they work the same for 12V and 230V appliances. That’s what the off-grid calculator uses.

The six parts of a typical system

House battery

Separate from the tow vehicle’s starting battery, it powers everything in the van. Two types are common:

Lithium (LiFePO4) batteries are lighter, keep their voltage steady as they discharge, charge quickly, and can be used down to about 10% without harm. They cost more up front but last many more cycles.
AGM (absorbed glass mat) and gel batteries are heavier and cheaper. To protect their life, plan on using only about half their rated capacity. A 100Ah AGM gives roughly 600Wh you can actually use; a 100Ah lithium gives about 1,150Wh.

Mains charger

Charges the battery when you’re plugged into a powered site or a generator. Often built into a combined unit with the DC-DC charger and solar controller.

DC-DC charger

Charges the house battery from the tow vehicle’s alternator while you drive. Modern vehicles with “smart” alternators lower their voltage to save fuel, which is why a plain wire from the car rarely charges a van battery properly. A 25A DC-DC charger adds roughly 300Wh for every hour of driving.

Solar controller

Sits between the panels and the battery and stops the battery overcharging. MPPT (maximum power point tracking) controllers get more out of the panels than the older PWM type, especially in cool weather or with portable blankets on a long cable.

Inverter

Turns 12V DC into 230V AC (120V in the US) so you can run household appliances: a laptop charger, a coffee machine, a hair dryer. Inverters are rated in watts; the rating must be higher than whatever you plug in. They lose around 10–15% as heat, and many draw 10–20W just from being switched on, so turn it off when you’re not using it.

Battery monitor

Usually a shunt on the negative cable and a small screen inside. It counts every amp in and out and shows the battery’s state of charge as a percentage. Voltage alone is a poor guide with lithium, because it barely changes until the battery is nearly empty. If your van has only a voltmeter, a monitor is the most useful upgrade you can make.

Fuses, cables and plugs

Every circuit should have a fuse or circuit breaker close to the battery, sized to protect the cable. Cables have to be thick enough for the current and the length: a long, thin cable to a fridge or DC-DC charger loses voltage and the appliance underperforms. Anderson plugs are the grey connectors used for portable solar, fridges and the car-to-van charging cable.

What you can and can’t do yourself

Extra-low-voltage 12V work is something many owners do themselves: adding a USB socket, fitting a fan, connecting a solar blanket. Anything connected to 230V, including the inverter’s AC output wiring and power points, must be installed by a licensed electrician in Australia and New Zealand. If you’re unsure which side of that line a job sits, ask an auto electrician or RV specialist.

Next steps

Read How to size batteries and solar to work out what your trips need, then plug your own gear into the off-grid calculator.