How big a house battery do you need?
A simple calculator for vans, RVs and boats. List what you run, choose how many days you want to go without charging, and get a battery size in watt-hours and amp-hours.
A "house battery" is the battery that runs your living space: the fridge, lights, fans, water pump and phone chargers. It is separate from the starter battery that cranks the engine. Sizing it is mostly arithmetic. The hard part is being honest about what you actually use each day.
This page walks through the method in four steps, then gives you a calculator that does the math. New to watts and watt-hours? Read power basics first.
Step 1: Add up your daily energy
For each device, multiply its power in watts by the hours it runs in a day. That gives watt-hours (Wh). Add them all up to get your daily energy use.
Two things trip people up:
- Fridges cycle. A compressor fridge does not run all 24 hours. Use the hours it actually runs, or better, the daily energy figure from the maker's manual if it gives one.
- Labels show maximums. The watts printed on a charger or appliance are usually its maximum draw. Real use is often lower. A plug-in or inline power meter gives a truer number.
Step 2: Choose your days of autonomy
"Days of autonomy" means how many days you want to run on the battery alone, with no sun, no driving and no shore power. One day is fine if you drive or plug in daily. Two or three days gives a cushion for cloudy spells or parking for a while. More days means a bigger, heavier, more expensive battery, so pick what matches how you travel.
Step 3: Only part of the battery is usable
You should not drain every battery to zero. How deep you can go depends on the chemistry. Battle Born, a lithium battery maker, says that with AGM (a sealed lead-acid type) you "generally don't want to go much deeper than ~50%" on a regular basis, while with lithium you can use 80 to 100% of the rated capacity without the same cycle-life penalty. Its own LiFePO4 datasheet lists 100% depth of discharge.
That is why the calculator asks for depth of discharge (DoD). A 100 Ah lead-acid battery used to 50% gives you about the same usable energy as a 50 Ah lithium battery used fully. Always check the datasheet for the exact battery you are considering. Our battery types guide explains the chemistries.
Step 4: Convert to the size on the label
Batteries are sold in amp-hours (Ah) at a voltage, or in watt-hours. The conversion is simple: Wh = Ah x volts, so Ah = Wh ÷ volts. The same energy at 24 V needs half the amp-hours it would at 12 V.
The full formula the calculator uses:
Battery Wh = daily Wh x days x (1 + margin) ÷ depth of discharge
Battery Ah = battery Wh ÷ system voltage
The margin covers inverter losses, cold weather and devices you forgot. Inverters turn battery DC power into household AC power and use some energy doing it. Lithium batteries also cannot be charged below freezing without help, and cold reduces what lead-acid can deliver; see winter battery care.
House battery calculator
Example rows are made-up round numbers to show the method, not measurements. Replace them with the watts on your own device labels. Runs in your browser; nothing is sent anywhere. The days, margin and depth-of-discharge settings are example choices, not recommendations for your setup.
| Device | Watts | Hours per day | How many | Wh per day |
|---|
A worked example
Example only. Using the calculator's made-up starting rows: a 60 W fridge running 5 hours (300 Wh), 20 W of lights for 4 hours (80 Wh), a 50 W pump for half an hour (25 Wh) and 60 W of charging for 2 hours (120 Wh) add up to 525 Wh a day. Two days with a 20% margin is 525 x 2 x 1.2 = 1,260 Wh usable. At 80% depth of discharge that is 1,260 ÷ 0.8 = 1,575 Wh, or about 131 Ah at 12 V. At 50% depth of discharge (lead-acid) it becomes 2,520 Wh, or 210 Ah at 12 V.
How you recharge matters as much as size
A battery only lasts as many days as you planned if it gets refilled. Most van and RV batteries charge three ways:
- Solar: size the panels with our van solar calculator.
- Driving: the alternator can charge the house battery through a DC-DC charger. See alternator charging explained.
- Shore power: a charger running from a campground outlet.
If you recharge every day, you may need fewer days of autonomy. If you park in the shade for a week in winter, plan for more.
Built-in battery or power station?
The same math works for a portable power station. Its capacity is printed in Wh, so compare your result with that number, then check how much of it the maker says is usable. Our power station vs built-in system guide compares the two. For the whole picture, see camper van electrical systems, explained.
Common questions
Should I use 12 V, 24 V or 48 V? Most small vans use 12 V because vehicle parts and accessories are 12 V. Higher voltages carry the same power with less current, which allows thinner cables. Victron's free Wiring Unlimited guide explains the trade-off. If you are not sure, stay with 12 V and ask a qualified installer.
Can I mix old and new batteries? Follow your battery maker's manual. It will say whether, and how, batteries can be combined in one bank.
Does a bigger battery always help? Only if you can recharge it. A large battery you can never fill just adds weight and cost.
Estimates only. Confirm against your battery's datasheet and, for permanent wiring, a qualified installer.