How Many Amp Hours Do I Need? Sizing a Van Battery Bank

Fifteen different ways of asking the same question turn up in van forums every week. How many amp hours for a van. Is 100Ah enough. How many panels for a 200Ah battery. What size battery for a fridge. They all have one answer, and it is arithmetic rather than opinion.

This is a community-evidence guide, per How We Test. VanTested has not put a battery on the bench. Every number below is a published manufacturer figure, a third-party metered result, or an owner report, and it is labeled as one of those. The method is the useful part; the numbers are illustrations you should replace with your own.

If you would rather do the arithmetic than read about it, the battery calculator runs these steps for you, and the solar calculator handles the recharge half.

Step 1: list the loads, honestly

Every sizing exercise fails in the same place, which is this step. People list the fridge and the fan, forget the laptop, forget that the water pump runs, forget the diesel heater’s glow plug draw at startup, and then wonder why the bank underperforms its spreadsheet.

Write down every load, its draw in watts (or amps, if that is how it is specified), and the number of hours a day it actually runs. Not its maximum rating. Its real duty.

The one that catches everyone is the fridge, because a compressor fridge does not run continuously. It cycles. Its instantaneous draw and its daily consumption are very different numbers, and only one of them belongs in your spreadsheet.

Load	Typical published or reported figure	What to put in the spreadsheet
12V compressor fridge, 35 to 60L	Roughly 4 to 11A while the compressor runs, depending on model	Daily total, commonly reported around 25 to 40 Ah/day in moderate conditions; third-party metered tests of the Dometic CFX3 range put it around 21 to 31 Ah/day
Roof fan	MaxxFan Deluxe dealer figures cite roughly 0.2A at low speed and up to about 4A at maximum; one owner’s ten-speed test ran 0.1A to 2.8A	Hours per day at the speed you actually use, which is usually not maximum
LED lights	Commonly 0.2 to 0.5A per fixture	Fixtures times hours, and it is a smaller number than people expect
Water pump	Commonly 3 to 6A while running	Minutes per day, not hours
Laptop	45 to 100W depending on machine and charger	Hours of actual charging, plus inverter losses if it is on AC
Phones and small devices	10 to 20W each while charging	An hour or two each, and it adds up in a group
Diesel heater	Commonly around 10A during the startup glow cycle, far less while running	Startups per day matter more than run hours

Those figures are claims and reports, not our measurements. Use the spec plate on your own gear where you have it, because model-to-model variation inside a category is larger than the category range suggests.

Two loads deserve their own treatment rather than a row in this table. Air conditioning is one: a 12V mini split is a different order of consumption and cannot be fed from a power station’s DC port at all, which we covered in do 12V air conditioners need a separate power source. Induction cooking is the other, and the honest answer there is that it is a short, enormous draw that sizes your inverter and your cable, not your battery.

Step 2: convert to amp hours

Multiply each load’s watts by its daily hours to get watt hours per day. Add them up. That total is the number that matters, and it is worth writing down before you convert anything, because watt hours are the only unit that compares systems fairly.

Then divide by your system voltage:

• 12V system: watt hours per day divided by 12 equals amp hours per day
• 24V system: divide by 24
• 48V system: divide by 48

This is why an amp hour rating on its own tells you almost nothing. A 100Ah battery at 12V stores about 1,280 watt hours nominal. A 100Ah battery at 48V stores four times that. If you are weighing higher-voltage architectures for a bigger build, the trade-offs are in 12V vs 48V and the always-on inverter.

Work an example. Fridge at 30 Ah/day, fan at four hours on a middling speed, a few lights, phones, and a laptop for three hours:

Load	Daily figure	Watt hours/day at 12V
Fridge	30 Ah/day	360
Roof fan	~0.9A for 4h	~43
Lights	3 fixtures, 4h, ~0.3A each	~43
Water pump	~4A for 10 minutes	~8
Phones	2 devices, ~15W, 2h	60
Laptop, via inverter	60W for 3h, plus ~15% conversion loss	~207
Total		~720 Wh/day, which is ~60 Ah/day at 12V

Sixty amp hours a day is a realistic number for a working van build without air conditioning. Your number will differ, and the point of doing it yourself is finding out by how much.

Step 3: decide how many days you need

Multiply your daily amp hours by the number of days you expect to go without a meaningful recharge. This is where honest self-assessment beats aspiration.

If you drive most days, that number is close to one, because the alternator is refilling as you go. If you park in one spot for a long weekend in poor weather, it is three or four. If you are a full-timer who works from the van and moves twice a week, size for the gap between moves rather than the average.

Then account for what you can actually use out of the rating. Lead acid is conventionally planned at about half its rated capacity to avoid shortening its life. LiFePO4 packs are commonly specified for far deeper discharge, so much more of the rating is planning capacity. Manufacturers differ here and the specification for your pack is the authority, not a general rule.

And add margin, because the spreadsheet is always optimistic. Around 20 percent covers inverter efficiency losses of roughly 10 to 15 percent, voltage drop in the wiring, capacity loss in cold weather, and the loads you forgot. Nobody has ever regretted this margin.

So: 60 Ah/day, two days of autonomy, 20 percent margin, and a chemistry that lets you use most of the rating puts you in the 150 to 200Ah region at 12V. Change any one of those inputs and the answer moves a long way, which is precisely why copying somebody else’s bank size is a bad plan.

Step 4: the step that actually decides whether the build works

Now check that you can put the energy back. This is the part that sinks builds, and it is barely marketed.

A bank sized for three days that recharges at a rate covering one day of consumption is not a three-day bank. It is a bank that gets a little emptier every day until something stops working. Capacity is the buffer; recharge rate is the budget.

There are three inputs and they are not equally reliable. Solar is weather and season, and it is the one people over-trust. Shore power is a campground, which is to say it is not a plan. That leaves the alternator, which is why an alternator-to-bank DC-DC charger such as the BLUETTI Charger 2 is frequently the most consequential component in a van electrical system. We worked out what that energy costs in what a kWh from your alternator actually costs, and it is cheap enough that drive time is the input most vans can genuinely count on.

The check is arithmetic again. Take your daily amp hours, then look at what your charge sources deliver in a normal day rather than a perfect one. If the total is smaller than your daily draw, buy the faster input before you buy the bigger battery. More capacity does not fix a deficit; it just delays it.

Panels are sized against daily consumption for the same reason. A widely used planning figure in the van community is that a panel returns roughly three to five times its rated wattage in watt hours on a good day, less in winter and far less in shade, but latitude and season move that enough that a rule of thumb is a starting point rather than an answer. Run it properly in the solar calculator.

The wiring and fusing question, which is not the same question

Sizing the bank tells you nothing about what cable to run. Battery capacity is about energy over a day; wiring and fuses are about current in a moment, and the loads that decide them are the brief, brutal ones: the inverter at full tilt, the compressor at startup, the induction hob.

Two rules worth carrying:

Size the cable for the current and the distance, then check the voltage drop. Long runs at 12V lose meaningful voltage in the wire, and the fix is thicker cable, not a bigger battery. The wire gauge calculator does this properly.

Fuse to protect the cable, not the appliance. A fuse exists so a fault cannot turn your wiring into a heating element. It is sized to what the cable can carry, and it goes as close to the power source as the run allows. Every connection to the battery gets one.

If you are still deciding on the architecture that this bank lives inside, power station or DIY battery bank covers that call, and the 12V output ceiling every van build forgets covers the limit that catches people who chose a station.

Three worked builds

Numbers to sanity-check yourself against. All three assume 12V, LiFePO4, no air conditioning, and the margin already added.

Build	Loads	Daily draw	Autonomy	Bank that fits
Weekender, drives most days	Fridge, fan, lights, phones	~40 Ah/day	1 to 2 days	~100Ah, if alternator charging is in the build
Remote worker, moves twice a week	The above plus laptop, monitor, more fan hours	~60 to 80 Ah/day	3 days	~200 to 300Ah, with solar and alternator both
Full-timer, parks for a week	The above plus water pump, heater startups, winter loads	~80 to 100 Ah/day	4 to 5 days	400Ah and up, and at this size the higher-voltage question is worth asking

The pattern to notice: the bank sizes climb with autonomy far faster than they climb with load. Two extra days of sitting still costs more capacity than adding a laptop to the list. If your bank estimate is coming out very large, check that assumption before you buy batteries, because a day of driving is cheaper than 200 amp hours.

What would settle this properly

Everything above is arithmetic over published claims and owner reports, which is what it is labeled as. What would improve it is measurement: a logged season of real consumption from a working van against the spreadsheet that sized it, so the gap between planned and actual has a number rather than a 20 percent hand wave. That is a long-run logging job rather than a bench test, and it is on our list.

Until then, the most valuable thing in this guide is the discipline of Step 1. Almost every undersized bank we read about in forums traces back to a load list that was missing something, not to bad arithmetic.