A typical pairing draws 45 to 70 amp-hours across a cold night. Here is the arithmetic, and what it means for the size of your battery bank.
Run a 12V compressor fridge and a diesel heater together on a winter night, and you can expect them to pull somewhere between 45 and 70 amp-hours from your caravan battery over 24 hours. That is roughly half to two-thirds of a 100Ah battery, and it is why so many caravanners wake up in the Victorian High Country or out past Broken Hill to a flat bank and a warming fridge.
The good news is that neither appliance is a mystery. Both draw current in predictable patterns, and once you understand duty cycle, amperage draw, and the heater’s glow-plug surge, you can budget your daily load with confidence. At Outbax, we would rather show you the working than quote a spec sheet, so here is how the numbers stack up for Australian winter touring.
How Much Power Does a 12V Caravan Fridge Actually Draw?
A 12V compressor fridge typically draws 4 to 5 amps while the compressor runs, but it does not run continuously. Real caravan fridge power consumption depends on how often the compressor cycles on, which is where most overnight budgets go wrong.
Compressor Amp Draw Versus Rated Wattage
A common 60L compressor fridge is rated at around 45 to 60 watts. Divide by 12 volts, and you get a running draw of roughly 4 to 5 amps. That figure only applies while the compressor is working. The rest of the time, the fridge sips a fraction of an amp to power its thermostat and display.
Why Duty Cycle Decides Your Real Daily Figure
Duty cycle is the percentage of time the compressor actually runs. In mild winter conditions, a well-insulated fridge might cycle at 30 per cent. Park it in the sun, pack it with warm groceries or open the door often, and that climbs towards 50 per cent. Cold ambient air works in your favour here, which is one small mercy of winter caravanning.
A Worked Example Over 24 Hours
Take a 60L fridge drawing 5 amps at a 35 per cent duty cycle. That is 5A × 0.35 × 24 hours, or 42 amp-hours per day. In watt-hour terms, about 504Wh. Call it 30 to 45Ah daily depending on conditions, and you have the first line of your daily load budget.
Diesel Heater Power Consumption: Surge, Settle, and Overnight Totals
Diesel heaters burn fuel for warmth, so their electrical demand is modest. Diesel heater power consumption follows a distinctive three-stage pattern, and only one stage is heavy.
The Glow Plug Surge
At startup, the glow plug ignites the fuel and briefly pulls 8 to 12 amps for the first few minutes. This surge is short, so it barely registers in your daily total, but it matters for a different reason: a nearly flat battery bank can dip below its low-voltage cutoff under that load, and the battery management system will disconnect to protect the cells. If your heater refuses to fire on a cold morning, this is usually why.
Running Draw Once the Burner Stabilises
After ignition, the heater settles to running the fuel pump and combustion fan. On low, that is often 0.5-1 amp. On high, expect 1.5-2 amps. Most heaters spend the night modulating between the two.
What Eight Hours of Heating Adds to Your Daily Load
Assume an average of 1 to 1.5 amps across an eight-hour night, plus the ignition surge and a shutdown purge cycle. You land at roughly 8-15 amp-hours. It is a light load compared with the fridge, but it arrives entirely overnight, precisely when nothing is recharging the bank.
Adding It Up: Your Combined Daily Load Calculation
Put the two appliances together, and the daily load calculation looks like this:
- 12V compressor fridge: 30 to 45Ah per day
- Diesel heater, eight hours overnight: 8 to 15Ah
- Combined total: 45 to 70Ah, or roughly 540 to 840 watt-hours
The Loads People Forget
LED lights, the water pump, phone and tablet charging, and a TV can quietly add another 10 to 20Ah. A realistic winter daily load for a couple in a caravan sits between 60 and 90Ah once everything is counted.
How Much of a 100Ah Battery Does That Leave You With?
Against a 100Ah LiFePO4 battery like the VoltX 12V 100Ah Lithium LiFePO4 Battery, a 60 to 90Ah daily load leaves very little margin, and against an AGM battery, which should only be discharged to around 50 per cent, it does not fit at all. This is the honest answer to how long a 100Ah battery lasts with both appliances running: about one night, with nothing in reserve for a cloudy day.
Sizing a Lithium Battery Bank for Fridge and Heater Together
Once you know your daily load, sizing becomes straightforward. The goal is at least one full day of capacity, plus a margin for weather that can keep your solar panels idle.
Why LiFePO4 Usable Capacity Changes the Maths
A lithium battery for a caravan, such as the VoltX 12V 200Ah Pro LiFePO4 Battery, gives you close to its full rated capacity, because LiFePO4 cells tolerate deep discharge without damage. A 100Ah lithium battery delivers roughly double the usable capacity of a 100Ah AGM, at less than half the weight. That is why lithium has become the default for serious winter touring.
Here’s what one of our customers said about this battery:
“I’ve had these batteries connected in parallel for about a year feeding a 2.5KW inverter, and they have worked faultlessly over this time. Have been off grid for 4 days running aircon for about 3 hrs a day, as well as air fryer, coffee machine and everything else that runs off the 12v side, with 650 watts of solar and by the end of the week we still had 100% on batteries. Excellent buy and excellent quality.”
Matching Capacity to Trip Style
For weekend trips with decent solar input, the VoltX 12V 100Ah LiFePO4 Battery comfortably covers a single night, provided you top it up each day. For most couples running a fridge and heater together, the VoltX 12V 200Ah Pro LiFePO4 Battery is the comfortable choice, giving two to three nights of autonomy. For extended off-grid stays or bigger loads, the VoltX 12V 300Ah Pro LiFePO4 Battery or the value-priced Gentrax 12V 300Ah LiFePO4 Battery in the Outbax caravan battery range provide genuine multi-day security.
Recovering the Daily Load
Whatever you draw overnight must go back in. A lithium battery charger, like the SRNE 12V 30A DC-DC MPPT LiFePO4 Battery Charger, matched to LiFePO4 charging profiles, restores the bank quickly on mains or generator power. Meanwhile, a VoltX 12V 130W Fixed Solar Panel, or two, will replace a typical winter daily load when the sun cooperates. Budget your solar for winter sun hours, not the summer brochure figure.
Cold Nights, Low Voltage Cutoffs, and What Your BMS Is Doing
Battery Capacity in Cold Weather
Lithium cells deliver slightly less usable capacity below about 5 degrees, and most reputable batteries will refuse to accept charge below zero to prevent cell damage. On frosty mornings, expect your solar to sit idle until the battery warms. Plan your capacity margin with that delay in mind.
Low Voltage Cut Off Versus the Glow Plug Surge
When a depleted bank meets a 10-amp glow plug surge, voltage sag can trip the low voltage cut-off. The BMS protection built into the batteries Outbax stocks does its job when this happens: it disconnects before the cells are damaged. The fix is capacity headroom, not a workaround. If heat degradation is your summer concern instead, the same BMS logic protects the pack at the other end of the thermometer.
Budget the Load First, Then Buy the Battery
The method is simple. Work out your fridge’s duty cycle, add the heater’s overnight draw, count the small loads, then size a battery bank with at least a full day in reserve. For most winter caravanners, that arithmetic points to 200Ah of lithium, such as the VoltX 12V 200Ah Slim LiFePO4 Battery. If you would like a second opinion on your numbers, the Outbax team is available on 02 888 10 333, or you can browse the full caravan battery range online with free shipping Australia-wide.
