How this 12V battery calculator works
Every 12V setup comes down to one sum: how many amp-hours (Ah) you use each day, multiplied by how many days you want to go without charging. The calculator adds up each appliance (amps × hours per day × how often it actually runs), then adds a safety margin and adjusts for how much of the battery you can really use.
Charging while you travel changes the sum. If you top up with solar, driving, or both, some of each day's use is replaced as you go, so the battery mainly has to cover overnight and cloudy stretches rather than every day in full. Choosing "No charging" sizes the battery to cover every day off-grid on its own.
Duty cycle matters most for fridges. A compressor fridge doesn't run constantly. It cycles on and off to hold temperature, usually 30–40% of the time in mild weather and 50% or more on a 35°C day. If your fridge spec says 45W, that's about 3.75A while running, but only around 30–40Ah over a full day.
Lithium vs AGM. LiFePO4 lithium batteries can use 80–100% of their rated capacity and weigh about a third as much as AGM. AGM batteries last longest if you only use about half. That's why the same trip needs roughly twice the AGM amp-hours.
Solar and DC-DC charging stretch your days off-grid. If your panels put back what you use each day, the battery only has to cover cloudy days and nights. A DC-DC charger tops up the house battery from the vehicle while you drive, and it's sized here to put back one day's use in your typical drive time.
Common questions
What size lithium battery do I need for a 12V fridge?
A typical 40–60L compressor fridge draws about 4A while running and runs roughly a third of the time, so it uses around 30–40Ah a day in mild weather and more in summer. For a weekend away with no charging, a 100–120Ah lithium battery covers a fridge plus lights and phones. For three or more days, most people end up at 200Ah, or less if they have solar or driving top-up.
How do I work out amp-hours from watts?
Divide watts by 12 to get amps at 12V, then multiply by the hours the item runs each day. A 60W device running for 2 hours uses 60 ÷ 12 × 2 = 10Ah. If it runs through an inverter, add about 15% for inverter losses.
Why does AGM need a bigger battery than lithium?
You should only use about half of an AGM battery's rated capacity if you want it to last, while a LiFePO4 lithium battery can safely use 80–100%. So the same daily use needs roughly twice the amp-hours in AGM, and the AGM battery weighs about three times as much.
How much solar do I need to run a caravan fridge?
Take your daily watt-hours and divide by peak sun hours times system efficiency. Using 4.5 sun hours and 70% efficiency, a setup using 550Wh a day needs about 175W, so a 200W panel or blanket. In winter in southern Australia sun hours can drop to 2–3, so you'd need more panel or a top-up from driving.
What size DC-DC charger do I need?
Size it to put back a day's use in the time you usually drive. If you use 45Ah a day and drive about 2 hours, you need around 25A after losses, so a 25–30A charger. Bigger lithium banks often use 40–50A chargers, but check your alternator and cable size can handle it.
Do I really need to charge while I travel?
No — you can leave "Will you top up while away?" set to "No charging" and the battery is sized to cover every day fully from stored charge. Most vanners with solar or regular driving can get away with a smaller, cheaper battery because some of each day's use is replaced as they go.
What are peak sun hours?
Peak sun hours are the number of hours per day the sun's energy equals 1,000W per square metre. Across much of Australia it's about 5–7 in summer and 2.5–4.5 in winter, depending on how far south you are. We default to 4.5 as a middle-of-the-road figure you can change.
Assumptions we use
- 12V nominal for converting watts to amps (LiFePO4 sits around 12.8–13.3V, so real current is a little lower, which is slightly conservative).
- Inverter loads lose about 15% in conversion (85% efficiency).
- Solar: 4.5 peak sun hours and 70% overall system efficiency by default. Change these to suit where and when you travel.
- DC-DC: about 90% charger efficiency. Common sizes are 20, 25, 30, 40 and 50A.
- Charging modes: "Driving" counts DC-DC input over your driving hours, "Solar" counts panel input over peak sun hours, and "Solar + driving" adds both. Charging reduces the days the battery has to cover alone, but we still size for a couple of days of night/cloud buffer.
- Batteries are rounded up to common sizes: 100, 120, 200 and 300Ah, then in 100Ah steps for multi-battery banks.
- Product prices are approximate and were checked on 17 October 2026.
Guide only. RigMaths isn't an electrician. Always follow the manufacturer's instructions and get high-current 12V and any 240V work done by a licensed professional.
Need a more precise solar size by town and month? Try the Solar Panel & Blanket Sizer.