How to size batteries and solar for off-grid camping

The question isn’t “how big a battery should I get?” It’s “how long do I want to stay away from power, and what will I run while I’m there?” Answer that and the battery and solar sizes follow.

You’ll need the basics from 12V basics for caravan owners, mostly the idea that watt-hours (Wh) measure energy.

Step 1: Add up what you use in a day

For each appliance: watts × hours a day = watt-hours a day. Typical figures:

Appliance	Typical watts	Typical hours a day	Wh a day
Compressor fridge (12V, average)	30	24	720
LED lights	15	4	60
Water pump	60	0.3	18
Phones and tablets	20	3	60
Roof fans	25	4	100
Diesel heater (running)	20	8	160
CPAP machine	40	8	320
Starlink Mini	30	6	180
Laptop (via inverter)	60	2	136
Coffee machine (via inverter)	1,300	0.1	148

For anything that runs through the inverter, divide by about 0.88 to allow for its losses. That’s why the laptop shows 136Wh, not 120Wh.

The fridge is almost always the biggest load. Its average draw rises a lot in hot weather and when it’s packed full of warm food, so use a summer figure if you travel north.

Step 2: Decide how many days without sun

Solar does most of the work on most days, but you need a buffer for overcast weather, shady campsites and the days you don’t drive. Two to three days is a common target for touring; one day is enough if you mostly move between powered sites.

Step 3: Size the battery

Battery Wh needed = daily use × days of buffer ÷ usable share.

The usable share is about 0.9 for lithium and 0.5 for AGM. Then convert to amp-hours by dividing by the battery’s voltage (12.8V for lithium, 12V for AGM).

Step 4: Size the solar

Solar watts needed = daily use ÷ (peak sun hours × 0.75).

Peak sun hours are the equivalent number of hours of full sun in a day: around 6 on clear summer days across much of Australia, 4–5 in spring and autumn, and 3 or less in a southern winter. The 0.75 covers heat, panel angle, dust, cable and controller losses.

Roof panels are always connected but often in shade. A portable panel or blanket you can put in the sun while the van sits under a tree is a useful extra.

Worked example

A couple touring in spring, wanting three nights off-grid without relying on sun:

Fridge 720, lights 60, pump 18, phones 60, fans 100, laptop 136. Daily use: about 1,100Wh.
Battery: 1,100 × 3 ÷ 0.9 = 3,667Wh, or 3,667 ÷ 12.8 = about 290Ah of lithium. A 300Ah battery fits.
Solar: 1,100 ÷ (4.5 × 0.75) = about 330W. A 400W roof setup covers a typical sunny day with some to spare.

With 300Ah of lithium and 400W of solar, this couple can stay indefinitely in sunny weather and about three days with no sun at all.

The same couple with a single 100Ah AGM battery would have about 600Wh usable, less than one day’s use. That’s the most common mismatch in older or entry-level vans.

Ways to need less

Pre-cool the fridge on mains power before you leave, and don’t overfill it.
Park for the sun if your panels are on the roof, or carry a portable panel.
Turn the inverter off when nothing’s plugged in; standby draw adds up to hundreds of Wh a day.
Charge while you drive with a DC-DC charger. Two hours with a 25A unit puts back about 600Wh.
Use gas for cooking and hot water, and the fridge on gas if it’s a three-way model.

Try it with your own gear

The off-grid calculator does these sums for you with your actual battery, solar and appliances, and shows how many days you’ll get in sun, overcast weather or with no charging at all.