Battery Size Calculator
How big a battery do you actually need? Pick your setup (RV, boat, home or off grid), add what you run and for how long, and this calculator works out the lithium battery capacity to suit. Buy too small and you're flat by morning. Buy too big and you've spent money on capacity you'll never use.
What are you powering?
Pick the closest match — this sets sensible defaults for the next steps.
These figures are an estimate to help you choose, not a guarantee. Real results depend on things outside our control, including the weather and shading, how you actually use your appliances, temperature, the age of the battery, and how the system is installed.
How it works
- Your setup. Pick RV or campervan, boat, off grid tiny home or bach, or home backup. Vans and boats also choose a system voltage: 12V for most vans and boats, 24V for bigger vessels and motorhomes. Homes and off grid houses are sized on 48V.
- Your loads. Tick what you run. Each appliance's watts multiplied by the hours you use it each day gives watt-hours per day. If you'd rather not itemise, pick a quick estimate instead.
- Inverter. For vans and boats, tell us what runs at 230V so we can size the inverter and allow for its losses.
- Charging. Add your solar and, for vans and boats, charging from the engine. Whatever comes back in each day is less the battery has to carry.
- Days without charging. How long you want to last with no sun, shore power or driving.
- Usable capacity. LiFePO4 can use most of its rated capacity, unlike lead acid, which shouldn't go much below half. We still keep 20% in reserve and allow for losses, then match you to a battery.
Sizing tips for NZ
- Winter solar is the limit off grid. Size for June and July, not summer.
- Fridges run all day. A 12V compressor fridge is often the biggest load in a van, even though it's small.
- Inverter loads add up fast. A kettle or microwave on an inverter uses more in five minutes than LED lights do all night.
- Heated batteries for the South Island. LiFePO4 can't be charged below 0°C; SOK heated models warm themselves first.
Common questions
It depends on your fridge, lights and inverter use. A typical van with a compressor fridge and LED lights does well on 100 to 200Ah at 12V; the calculator gives you your number.
Usually yes. You'll often need less capacity because more of a lithium battery is usable, but check your charger and alternator setup suit lithium.
Two to three days is a common starting point. Pair it with a generator input for long grey spells.
No. We size and supply systems and work with installers around NZ. Get in touch and we'll put you onto one near you.
Still have questions?
Our team is here to help you choose the right battery and parts.