To size a van battery bank, you add up the daily watt-hours of everything that runs off it, convert that total into amp-hours at the battery’s voltage, then multiply by a safety factor for days without charging and divide by the depth of discharge your battery chemistry can handle. It really is three operations, and they only give a reliable answer when you run them with the list of your own appliances in front of you.
1. Daily use in Wh: each appliance’s wattage times the hours it runs on a typical day.
2. Convert to Ah: divide that total by the nominal voltage, which is 12.8 V for a LiFePO4 lithium battery.
3. Capacity you need once you size a van battery bank: daily Ah times a factor between 1.5 and 2, divided by 0.8 for lithium or 0.5 for AGM.
Not in the mood for math? The “Where your van battery size should land” table at the end of this article gives you the ballpark for three travel styles, based on our example.
List what each appliance draws before you size a van battery bank
Every number that follows comes out of this list, the real starting point when you size a van battery bank. Go through each device that will run on the house battery, write down its wattage from the label, the manual or the spec sheet, and give an honest estimate of how many hours it runs on a normal day. Wattage multiplied by hours gives you watt-hours per day.
Spread occasional use across the week instead of counting it as daily: a 60 W laptop used for two hours every other day averages out to one hour a day, so it adds 60 Wh to your daily total.
Two items need special handling. A compressor fridge cycles on and off, so its rated wattage tells you little about what it really consumes; use the 24-hour figure the manufacturer publishes, which climbs as the cabin gets warmer. Anything run through a 12 V to 230 V inverter also loses some energy, shown in the inverter’s efficiency rating.
| Appliance (example) | Power | Hours per day | Daily use |
|---|---|---|---|
| Compressor fridge | manufacturer’s 24 h figure | 360 Wh | |
| Roof fan | 30 W | 3 h | 90 Wh |
| Laptop | 60 W | 1 h | 60 Wh |
| LED lighting (4 puck lights) | 4 × 3 W | 4 h | 48 Wh |
| Charging two phones | 2 × 15 Wh | one charge | 30 Wh |
| Water pump | 60 W | 15 min | 15 Wh |
| Total | 603 Wh | ||
The wattages and hours are example values picked only to show the math: swap in your own figures before you size a van battery bank for real.
Amp-hours, days of autonomy and depth of discharge
Batteries are sold in amp-hours while your list is in watt-hours: Ah = Wh ÷ volts, the conversion you need before you size a van battery bank. A so-called 12 V LiFePO4 battery is really 12.8 V nominal: the Victron Energy datasheet for its Lithium Smart batteries explains why: a single cell sits at 3.2 V, four in series give 12.8 V. For a lead-acid AGM battery, stick with 12 V. Running the example through that step, 603 Wh becomes 47.1 Ah per day on lithium and 50.25 Ah on AGM.
A bank that holds exactly one day of power leaves you in the dark the first grey sky, so you need more than a single day’s use, adjusted for how much of the battery you can safely drain. FarOutRide, a widely used build guide for van electrics, sets out the formula in its guide to sizing a van electrical system: daily use × factor ÷ depth of discharge = battery size in Ah. Its factor is 1.5 for sun seekers, 1.75 for regular users and 2 for snow chasers, with the most economic depth of discharge at 80% for lithium and 50% for AGM.
| Travel style | Factor | LiFePO4 lithium (÷ 0.8) | AGM (÷ 0.5) |
|---|---|---|---|
| Sunny, fair-weather trips | 1.5 | 88.3 Ah | 150.8 Ah |
| Regular year-round use | 1.75 | 103.1 Ah | 175.9 Ah |
| Winter and snow | 2 | 117.8 Ah | 201 Ah |
For the same daily use, AGM needs far more rated capacity, since you only allow yourself half of it: that is why the math leans towards lithium once you size a van battery bank for more than a few dozen amp-hours a day, before weight even enters the conversation.
Cold weather changes how you size a van battery bank
The winter factor covers sunless days, but temperature also cuts the capacity you can actually draw. In the Victron datasheet, the 100 Ah battery drops to 80 Ah at 0 °C and to 50 Ah at −20 °C: to keep 117.8 Ah usable at freezing, aim for roughly 147 Ah of rated capacity. Victron also gives a charging range of +5 °C to +50 °C: a LiFePO4 bank in an uninsulated underbody box may refuse to charge on a frosty morning, which argues for keeping it inside the heated living space.
Depth of discharge also decides how long the battery lasts. Victron rates its lithium batteries at 2,500 cycles at 80% depth of discharge, 3,000 at 70% and 5,000 at 50%: going slightly bigger than the formula means each day takes a shallower bite out of the bank, which lasts longer as a result.
Mistakes that skew the result before you buy
Most bad battery purchases come from an incomplete list rather than a wrong formula. Writing down the fridge’s rated wattage instead of its 24-hour consumption badly overstates what it really draws, while something that stays on all the time, like an inverter left switched on, sometimes never makes it onto the list at all. Dividing your watt-hours by 12 V when you size a van battery bank for lithium also inflates the need.
Finally, plenty of people stop at the exact figure the formula spits out. A 103 Ah battery may not exist in any catalog, and rounding up to the next capacity actually on sale is the only way to keep the margin the factor was meant to give you. Rerun the numbers for every season you travel in before you size a van battery bank for good.
Where your van battery size should land
| Travel style | What to aim for |
|---|---|
| Weekends and summer trips | Factor 1.5: 88 Ah of lithium on the example, so a 100 Ah battery leaves some headroom |
| Regular travel in mild seasons | Factor 1.75: 103 Ah of lithium on the example, meaning the next size up from 100 Ah |
| Winter use or full-time living | Factor 2 plus the cold correction: around 147 Ah of rated lithium capacity on the example |
These three profiles show how to size a van battery bank by season as much as by daily use. Run the numbers with your own appliances before you open a single product page, and always round up to the next capacity actually on sale: the battery’s size then sets how much solar and how big a charger you need to refill it.