Running a 12V camp fridge around the clock plus a CPAP machine overnight draws roughly 700Wh to 1,000Wh per day, depending on how you run the CPAP. In practice, that means a 100Ah LiFePO4 battery covers a single night in eco mode, while a 200Ah caravan battery is the safer choice for weekend trips or anyone who sleeps with a heated humidifier switched on.
The stakes here are higher than keeping the milk cold. A CPAP machine is a medical device, and waking at 3 am to a silent mask in a national park campsite is the scenario every camper with sleep apnoea wants to avoid. The good news is that sizing a battery for this combined load is straightforward arithmetic, and this guide from Outbax walks through every step of the runtime calculation using real figures for Australian conditions.
Here is what you need to know about the overnight load, the hidden losses, and the battery capacity that actually gets you through.
How Much Power Does a Camp Fridge Use?
Compressor Draw and Duty Cycle Explained
A standard 40L to 50L single-zone camp fridge draws about 40W to 60W while the compressor is actively running. The compressor does not run constantly, though. It cycles on and off to hold temperature, typically running 30% to 50% of the time. That on-and-off pattern is known as the duty cycle, and it is the reason a fridge rated at 50W does not consume 50W every hour.
Real-World Daily Consumption
Once the duty cycle is factored in, real-world consumption averages around 20W to 30W per hour. Take a midpoint of 25W and run it over a full day, because a camp fridge never sleeps: 25W multiplied by 24 hours equals 600Wh per day. That single figure is the foundation of the whole calculation.
Why Australian Summers Push It Higher
Ambient temperature drives the duty cycle. On a 35-degree January afternoon in the bush, the compressor works far harder than it does parked in a shaded caravan in autumn. Budget towards the top of the range in summer, park the fridge out of direct sun, and keep it well stocked, since a full fridge holds its temperature better than a half-empty one.
How Much Battery Does a CPAP Machine Use While Camping?
The Heated Humidifier Is the Real Load
CPAP power draw varies enormously depending on settings. With the heated humidifier and heated tubing switched on, a standard machine such as a ResMed AirSense 10 or AirSense 11 pulls around 40W to 60W, because heating elements are hungry. Over an eight-hour sleep, that is 50W multiplied by 8 hours, or roughly 400Wh per night. The humidifier, not the blower, is doing most of the draining.
Eco Mode Changes Everything
Turn off the heated humidifier and heated tube, and the same machine sips just 10W to 15W. The nightly figure drops to around 96Wh, less than a quarter of the heated setup. Many campers find a dry setup perfectly comfortable in humid coastal conditions, and it is the single biggest battery-saving option available to a CPAP user off-grid.
Adding It All Together
Combine the two appliances and the daily demand lands in one of two bands. The maximum comfort setup, fridge, plus heated CPAP, needs about 1,000Wh per day. The eco setup, fridge, and unheated CPAP need about 696 Wh per day. Because this is a medical device, the sensible approach is to size for the worst case rather than the best.
Inverter Losses and Safety Buffers: Why Rated Capacity Lies
The 15% to 20% Inverter Tax
You cannot use 100% of a battery’s rated capacity in the real world. Running appliances through a 240V AC socket forces the inverter to stay on, and DC-to-AC conversion plus idle consumption wastes 15% to 20% of your stored power as heat. Inverter efficiency matters most overnight, when the inverter idles for hours to feed a low-draw device.
The 20% Safety Margin
On top of conversion losses, add a 20% buffer. It protects battery health, covers a hotter-than-expected day, and guarantees the CPAP does not cut out before dawn. Apply both adjustments, and the eco setup needs about 850Wh of real capacity per night, while the heated setup needs about 1,200Wh. Outbax publishes usable capacity and continuous output figures on its product pages, which makes this comparison easier than squinting at marketing numbers.
What Size Caravan Battery Do You Actually Need?
Battery capacity is usually listed in amp-hours. Convert amp-hours to watt-hours by multiplying by the nominal voltage (12.8V) for LiFePO4 chemistry. Here is how the common sizes map to the demand figures above.
One Eco Night: 100Ah
A VoltX 12V 100Ah LiFePO4 Battery holds about 1,280Wh, comfortably above the 850Wh eco requirement. It suits the solo camper who runs the CPAP dry and heads home the next day. It is also roughly 60% lighter than a lead-acid deep-cycle battery of similar usable capacity, which matters when it lives in a ute canopy.
Here’s what one of our customers said about this battery:
“Well packed. Unit is sealed well with good lift handles. 13+ volts on delivery. Accepts a top up charge very well. 12 months later and going still good.”
Weekends and Heated Comfort: 200Ah
The VoltX 12V 200Ah Pro LiFePO4 Battery delivers about 2,560Wh, enough for two heated nights or three eco nights without a recharge. At $799.20 on current Outbax pricing, it costs less per watt-hour than most 1,000Wh sealed power stations while carrying more than double their capacity. For a couple running a camp fridge and a CPAP together, this is the sweet spot.
Here’s what one of our customers said about this battery:
“Very happy customer so far. was quick to deliver good communication and tracking. The battery fits in the same space as the old 100amp battery with only minor adjustments. only time will tell as to how long it will last however with a 5-year warranty, I'm pretty confident it will last the distance. definitely recommend Outbax.”
Longer Stays Off-Grid: 300Ah or Solar
For three nights or more without mains power, step up to the VoltX 12V 300Ah Pro LiFePO4 Battery at around 3,840Wh, or the value-priced Gentrax 12V 300Ah LiFePO4 Battery. Alternatively, pair a 200Ah battery, such as the VoltX 12V 200Ah Slim LiFePO4 Battery, with a VoltX 12V 130W Fixed Solar Panel, and let daytime sun replace what the fridge consumed overnight. In most Australian conditions, 130W to 200W of solar will keep a fridge and CPAP loop sustainable indefinitely.
Ports, Protection, and Recharging: Getting the Setup Right
Run the Fridge on DC and the CPAP on Pure Sine Wave AC
Connect the 12V fridge to a regulated DC output rather than through the inverter, which sidesteps the conversion loss entirely. A regulated port holds a steady voltage as the battery drains, preventing the low-voltage error that shuts many fridges down mid-trip. For the CPAP, if you must use its standard 240V plug, insist on a pure sine wave inverter. Modified sine wave units can overheat and permanently damage sensitive medical equipment. Better still, use the manufacturer’s DC converter cable and skip the inverter altogether.
BMS Protection and Charging Between Trips
Every quality lithium battery includes a battery management system, or BMS, which guards against over-discharge, overcurrent, and temperature extremes. That protection matters twice over here, once for the battery and once for the medical device plugged into it. Between trips, recharge with a dedicated lithium battery charger rather than an old automotive charger, since LiFePO4 chemistry needs a specific charging profile to reach full capacity and preserve cycle life. Outbax stocks matched chargers alongside its battery range for exactly this reason.
Size Once, Sleep Soundly
The method is simple enough to do on the back of a camp chair box. Audit each appliance; allow 600Wh per day for the fridge and 100Wh to 400Wh per night for the CPAP; add inverter losses and a 20% buffer; then match the total to the real capacity. For most Australian campers running both appliances, a 200Ah lithium caravan battery is the answer, with 300Ah or a solar panel for longer stays. Browse the Outbax caravan battery range to compare capacities, usable output and current pricing, and get the sizing right before the next trip rather than at 3 am during it.
