What Size Battery Do You Need for a 12V Fridge? (Sizing Chart)
Sizing guide · 8 minute read · Written for car camping, van life and RV owners in the US
Most people shop for a battery by comparing amp-hour numbers on a shelf. That is the wrong end of the problem. The right way is to start with what your fridge drinks in a day, decide how many days off-grid you want, and only then work back to a battery size.
Sizing forwards is guesswork. Sizing backwards is arithmetic.
Start with demand, not supply
Supply is the battery. Demand is the fridge. If you size supply before you know demand, you are guessing in the dark. So step one is always the same: get your fridge's daily energy use in watt-hours.
Running watts × hours the compressor runs per day = watt-hours per day
For the rest of this guide, assume a mid-size fridge using about 600 Wh per day. That is a labelled example, not a spec. Swap in your own number wherever you see it.
Work backwards: how many days off-grid?
Decide the number of days you want the battery to carry you without recharging. This is the step people forget, and it is the one that really sets the price.
Multiply your daily watt-hours by the number of days:
- 1 day off-grid — for weekenders who drive every day and recharge as they go.
- 2 days off-grid — the comfortable default. One day of use, one day of buffer.
- 3 days off-grid — for base-camping in one spot, where the vehicle sits still.
Those are goals, not rules. The point is that "2 days" and "1 day" produce very different batteries from the same fridge.
From days to amp-hours at 12V
Now convert energy back into the amp-hours batteries are sold in. First get usable watt-hours for your chosen number of days, then divide by the nominal battery voltage:
Usable Ah = usable Wh ÷ 12
But usable amp-hours is not what is printed on the battery. The rated figure is higher, because you cannot safely drain all of it. How much higher depends on chemistry, which the next section covers.
An example sizing chart
Every number here is an example assumption built on that 600 Wh per day fridge. Usable fractions are assumed at 80% for lithium and 50% for AGM to show the difference — check your own battery maker's published figure rather than relying on these.
| Days off-grid | Usable Wh needed | Usable Ah at 12V | Lithium rated Ah (~80% usable) | AGM rated Ah (~50% usable) |
|---|---|---|---|---|
| 1 day | 600 Wh | 50 Ah | ~65 Ah | ~100 Ah |
| 2 days | 1,200 Wh | 100 Ah | ~125 Ah | ~200 Ah |
| 3 days | 1,800 Wh | 150 Ah | ~190 Ah | ~300 Ah |
Read it in one direction and the trade-off jumps out: to get the same runtime from AGM, you buy a physically bigger, heavier battery than you would with lithium. Read it in the other direction and you see why "what size battery" and "what chemistry" are really the same question.
Lithium vs AGM: how much you can actually use
The chart above bakes in the single biggest difference between the two chemistries: how much of the rated capacity you are allowed to draw.
Lithium batteries are commonly rated for a deeper discharge, so a large share of their amp-hours is usable. That is why a smaller lithium battery often replaces a much larger AGM one.
AGM batteries are usually kept to a shallower discharge to protect their life, so you use a smaller share of the rated number. More of what you pay for stays locked away.
Neither is wrong — the point is that rated amp-hours are not comparable across chemistries. Two batteries with the same label can deliver very different usable energy. Always check your battery maker's published depth-of-discharge or usable-capacity figure before you compare.
Want the numbers done for you?
The free 3-page Portable Fridge & Power Buying Checklist walks through this same formula, then the 7 fridge checks, the 8 station checks and the 24-hour test — in order.
Charging is half the decision
A battery is only as good as what refills it. If you size the battery and forget the charging, you have planned half a system. Ask what puts energy back in, and how fast:
- Driving. Your vehicle's charging system tops the battery up while the engine runs. Excellent on travel days, useless on a base-camp day where the vehicle does not move.
- Solar. Refills while you sit still, but output depends on sun, angle and season. A cloudy day gives you a fraction of a clear one.
- Shore power. Reliable and fast where a hookup exists, no help at all off-grid.
This is why two campers with identical fridges end up with different batteries. The one who drives every day can get away with less capacity than the one who parks for three days. Charging changes the size you actually need — so decide your charging plan before you decide your amp-hours.
The mistake: sizing on one mild night
The most expensive error is sizing on a pleasant trip. On a mild night the compressor barely runs, the fridge holds temperature, and almost any battery looks fine. Then you take the same setup into a hot weekend, the fridge runs far more hours, and the battery goes flat by morning.
Always size on the worst realistic day, not the best one. Think about your hottest planned trip: a full fridge opened all afternoon, and a run of cloudy days with no driving.
Mild-weather numbers are a trap that feels like proof. Design for the hot day and the mild trip takes care of itself.
Quick answers
What size battery do I need for a 12V fridge?
Start with your fridge's daily watt-hours, multiply by the days you want off-grid, divide by 12 to get usable amp-hours, then account for your battery's usable fraction. For a 600 Wh-per-day example fridge, two days off-grid is about 100 usable Ah.
Is a lithium or AGM battery better for a 12V fridge?
Lithium usually lets you use a larger share of its rated capacity, so you can buy a smaller, lighter battery for the same runtime. AGM is typically kept to a shallower discharge. Check your battery maker's published depth-of-discharge figure to compare fairly.
Can I run a 12V fridge on one battery?
Often yes, for one to two days, depending on the fridge and the battery's usable capacity. Beyond that you will want charging — from driving, solar or shore power — to keep the battery topped up rather than relying on capacity alone.
Keep reading
- How long will a 12V fridge run on a 100Ah battery?
- How many amps does a 12V fridge draw?
- What size solar panel runs a 12V fridge?
- Will a 12V fridge drain my car battery?
This guide may contain affiliate links. If you buy through them, we may earn a commission at no extra cost to you. We only link to products that fit the numbers above. Frostamp is not an electrician or an electrical engineer — this is general information, not professional advice. Always follow the manufacturer's instructions for your exact model.