12V Fridge Power Consumption: Why the Wattage Isn’t the Number That Matters

Almost every article on this answers the wrong question.

They tell you a 12 V compressor fridge draws 30 to 60 watts. That’s true, and it’s nearly useless on its own — because that figure barely changes. A fridge in a cold spring and the same fridge in a hot August both draw about 45 watts while the compressor is running.

What changes, enormously, is how long it runs.

That’s the number that decides whether your battery lasts one night or three. The same fridge can use 22 amp-hours a day or 76, and the difference has almost nothing to do with the fridge.

This page is about that: what actually sets the duty cycle, how to measure your own, and why the cheap 12 V cooler you were considering is the worst possible choice on both counts.

The number everyone quotes, and why it misleads

A 12 V compressor fridge draws 40 to 50 watts while the compressor runs. At 12 V that’s roughly 3.5 to 4 amps.

This figure is remarkably stable. It’s set by the compressor — most van fridges use a Danfoss/Secop BD35 or similar — and the compressor either runs or it doesn’t. It doesn’t run harder because it’s hot outside.

So where does the variation come from?

From the thermostat. The fridge cools to its set temperature, the compressor stops, the contents warm up, and it starts again. The proportion of each hour it spends running is the duty cycle, and that’s what heat, insulation and your habits actually change.

Same fridge, same 45 watts, two different days:

• Cold day: 25 % duty cycle, 6 hours running, 22 Ah
• Hot day: 85 % duty cycle, 20.4 hours running, 76 Ah

Three and a half times the energy, from an identical appliance.

That’s why quoting the running wattage tells you so little, and why a single measured figure from someone else’s van is a starting point rather than an answer.

For a typical 40-50 litre compressor fridge drawing 45 W while running, set to around 4 °C. Ambient means the temperature inside the van, not outside — and inside is usually warmer.

A sanity check against a real measurement. Build A Green RV measured a Norcold NB 751 with a Danfoss BD35 compressor over two days at 80 °F (27 °C), using an inline watt-meter. Result: 42 Ah per day, 3.6 A while running, roughly 50 % duty cycle. That sits exactly where this table predicts, which is a reasonable reason to trust the rest of it.

The three types, and what each one costs you

Three different technologies get sold as “12V fridges”, and the difference in what they take from your battery is enormous.

The absorption fridge deserves a warning. On gas it’s efficient and it’s why they exist. On its 12 V electric element it’s a resistive heater — over 2,000 Wh a day, which is more than most van battery banks hold in total. Some RVs will happily let you select electric mode off-grid, and it will empty the battery in a few hours. Use gas when parked, electric only on shore power or while driving.

And the 230V/120V domestic fridge question, because people ask. Yes, you can run one through an inverter. No, it isn’t a good idea. You pay the conversion losses, you pay the inverter’s own draw — 4 to 18 W continuously, whether the fridge is running or not — and the inverter has to stay switched on twenty-four hours a day to catch each compressor start. A 12 V compressor fridge connected directly to the battery avoids all three. More on what an inverter costs you here.

The 12V cooler that isn’t a fridge

Before compressor fridges came down in price, I bought a thermoelectric cooler — one of the cheap ones with a small fan in the lid. It cost me about €20.

It barely cooled anything. It could keep something already cold from warming up for a while, but it never worked as a fridge. For a trip of several days it was useless, and in practice I ended up putting a bag of ice in it to do the job it was supposed to do itself. Eventually I took it out of the van.

That’s the part everyone who’s owned one recognizes. Here’s the part almost nobody knows.

It uses more energy than a real fridge. Considerably more.

A thermoelectric cooler uses the Peltier effect: current through a junction moves heat from one side to the other. There’s no compressor, no cycling, and no thermostat worth the name in the cheap ones. It runs continuously the whole time it’s switched on.

Why it can’t reach a safe temperature. A Peltier element only moves heat across a fixed differential — typically 15 to 20 °C below ambient. On a 30 °C day inside the van, the best it manages is 10 to 15 °C. Food safety needs below 5 °C. It isn’t refrigeration, it’s “slightly less warm”, and that’s why mine only worked with a bag of ice in it.

A compressor doesn’t work that way. It reaches its set temperature and holds it whether the van is at 15 °C or 40 °C — it just runs for longer.

Where a thermoelectric cooler does make sense: a day trip, plugged into the vehicle while driving, keeping drinks cool. That’s it. As the fridge in a camper it’s the most expensive option per amp-hour you can buy.

And for scale at the other end: if a fridge at 4 to 5 amps already looks demanding, air conditioning asks for 98.

What actually changes your duty cycle

If the duty cycle is what decides your consumption, these are the things that move it. Most of them are free.

Ventilation behind the fridge. The compressor and condenser have to dump heat somewhere, and if that heat sits in a sealed cabinet the fridge is fighting itself. A fridge built into a tight box with no airflow can run half again as long as the same unit with a vent at the bottom and another at the top. This is the single biggest thing most people get wrong at build stage, and it’s almost impossible to fix afterwards.

Where the van is parked. Direct sun on the side the fridge sits against raises the ambient around it well above the cabin temperature. Shade is worth more than any setting.

How cold you set it. Every degree colder costs run time. If you’re storing drinks and vegetables rather than raw meat, 6 °C does the job that 2 °C is doing.

What you put in it, and at what temperature. Loading warm food or drinks means the compressor runs for hours pulling that thermal mass down. Chill things on shore power before you leave if you can — it’s a one-off cost paid at the campsite rather than a repeated one paid by your battery.

How often the door opens. Cold air falls out every time, and the compressor has to replace it. In hot weather, frequent opening can add 20 to 40 % to the daily figure. Grouping what you need into one trip genuinely helps.

Insulation of the fridge itself. This is the difference between two units with identical compressors and identical labels. It’s also the thing that separates a cheap 12 V fridge from an expensive one, and it doesn’t appear on the spec sheet as a number.

Voltage arriving at the fridge. A fridge receiving 11.6 V instead of 12.6 V because of undersized cable works harder and runs longer for the same cooling. It’s an easy thing to check and a common one to find. Symptoms and test here.

The setting on your fridge that protects the battery

Almost every 12 V compressor fridge uses a Danfoss/Secop BD-series compressor, and that compressor has a battery protection setting most owners never touch. It decides at what voltage the fridge shuts itself off to avoid flattening your battery.

On lead-acid, set it to “high”. The fridge stops earlier, you lose some cooling on a bad night, and the battery survives to do it again next week. On lithium, “medium” is usually fine, because the BMS handles the real cut-off.

How to change it: on most units it’s a jumper or a small switch on the compressor’s control module, behind the fridge. Some newer models expose it in the digital menu. Look in the manual for “battery protection” or “voltage cut-out”.

It takes five minutes, costs nothing, and it’s the single most useful thing you can do to stop the fridge slowly killing the battery. Check the values against your own manual — they vary slightly between compressor models.

How to measure yours over 24 hours

Everything above is a model. Your fridge, in your van, in your climate, is a number you can measure — and it’s the only one that really matters for sizing a system.

What you need: a battery monitor with a shunt, or an inline DC watt-meter. The cheap watt-meters cost around $20 and are adequate for a test, though not something to leave permanently installed.

How to do it:

1. Start with the battery charged, and note the reading.
2. Isolate the fridge if you can — measure on its own circuit rather than the whole system. If you can’t, run the test with everything else switched off.
3. Use the van normally for 24 hours. Open the door as you would. Put food in. Don’t baby it, because you’re measuring real use, not best case.
4. Note the ambient temperature, ideally the maximum and minimum inside the van. Without that, the number means nothing next season.
5. After 24 hours, read the accumulated watt-hours or amp-hours.

Converting the result:

If the meter reads 420 Wh over 24 hours:

420 Wh ÷ 12 V = 35 Ah per day

That’s your figure — for that temperature. Repeat it once in summer and once in winter and you have the range your system actually has to cover.

One honest note about cheap meters. Build A Green RV ran exactly this test and reported that the meter reset its accumulated total partway through, for no apparent reason, and that its internal wiring looked light for its claimed rating. Treat a $20 meter as a measuring instrument for a two-day test, not as permanent installed equipment.

Sizing the battery for it

The fridge is the largest continuous load in most vans, and it runs whether you’re awake or not. So it’s the load the system should be designed around, not the one added at the end. It’s usually not the only continuous-ish draw either — if you’ve added a 12V hot water system, that one has its own power profile worth accounting for separately, covered here.

Start with the number, not the fridge. Take your daily amp-hours from the table or from your own measurement — and take the hot-weather figure, not the average. Call it 50 Ah a day for a typical fridge in summer.

Then apply what your chemistry actually gives you:

• AGM 100 Ah → about 50 Ah usable → one day, and that’s the whole battery gone, with nothing left for lights, pump or charging a laptop
• AGM 200 Ah → about 100 Ah usable → two days, more realistically a day and a half with other loads
• Lithium 100 Ah → about 90 Ah usable → just under two days
• Lithium 200 Ah → about 180 Ah usable → three days plus

This is where the AGM-versus-lithium argument stops being theoretical. A 100 Ah AGM sized “for the fridge” is a battery that reaches 50 % every single day, and lead-acid batteries die from being emptied. That’s how you get a battery that fails inside two years. The full comparison, including what a degraded AGM looks like.

And solar has to cover the same number. If the fridge takes 50 Ah a day, your panels have to put 50 Ah back in — before anything else. In good summer sun a 200 W panel might deliver 50 to 60 Ah, so a single 100 W panel does not cover a fridge in July, whatever the packaging says.

The practical rule: work out the fridge first, in hot weather, then size the battery and the panels around it, then add everything else. Doing it the other way round — battery first, fridge added later — is how most under-sized systems get built.

And the fridge is only one line on the list. There are five errors in the calculation itself, and every one of them makes your real consumption higher than your figure.

The overnight number is the one that matters

Daily consumption is useful for sizing, but it hides the question people actually care about: will the fridge still be running in the morning?

The day and the night are not the same.

At night the van is cooler, so the duty cycle drops — maybe 25 % instead of 45 %. Over twelve hours that’s about three hours of running, so roughly 135 Wh, or 11 Ah.

That sounds comfortable. But consider what else is true at night:

• Solar contributes nothing. Every amp-hour comes out of the battery.
• The battery is at its lowest point of the cycle, because it’s been discharging since sunset.
• Nobody is watching. If the fridge cuts out at 3 a.m., you find out at breakfast, along with the food.

So the sizing question isn’t only “does the battery cover the day”. It’s “does the battery still have usable capacity at 6 a.m., after twelve hours with no charging” — and that’s where a bank sized to the daily average comes up short.

A rough guide: if your battery is at 50 % when the sun goes down, an AGM has nothing left to give. It should be near full at sunset, not half.

Why the fridge is what kills batteries

There’s a reason the fridge deserves this much attention, and it isn’t the total energy.

It’s that the fridge runs every night, all night, all year. No other load does that. Lights go off. The laptop gets charged and unplugged. The pump runs for seconds at a time. The fridge keeps cycling while you sleep, in December and in August.

That means the fridge is what determines how deep your battery goes every single night — and lead-acid batteries don’t die from being used. They die from being emptied, repeatedly, and put back only partially.

I know this because it’s what happened to mine. My 150 Ah AGM is under a year old and already degraded: normal voltage at rest, collapse under any load, and charge acceptance that has halved. The causes were a heater running through winter nights and a fan running through summer ones — the same pattern a fridge produces, just seasonally. What that degradation looks like in detail.

The practical conclusion: size the bank so the fridge takes it to 70 or 80 % overnight, not to 50 %. That means roughly 20 % more capacity than the calculation says you need.

That margin costs money once. Replacing a battery every eighteen months costs more, and it costs it repeatedly.

Frequently asked questions