Powering a 12V fridge off-grid: how to work out the battery, solar and wiring
Fridge makers publish their power use in different units. Here's how to turn those numbers into a daily figure, size a battery around it and wire it so the fridge doesn't cut out on a full battery.
On a camping trip without mains power, the compressor fridge is usually the biggest thing running off your second battery. Getting the system right is mostly arithmetic, but the arithmetic is harder than it should be, because fridge makers don't publish power use in the same way. Our 12V fridge comparison lines up five portable compressor fridges using their makers' own figures. This piece explains how to turn those figures into a battery and charging plan.
Four ways makers state power use
Look at the published power figures for the five fridges in the comparison and you'll find four different kinds of number.
Dometic gives an average consumption for the CFX5 45: 0.76 Ah per hour at 12 V, measured with 32 °C outside and 4 °C inside. That's the most useful kind of figure for planning, because it already averages out the compressor switching on and off.
SnoMaster gives a rated current: 3.75 A at 12 V for the SMDZ-TR42S and 5.9 A for the SMLS-50. Dometic also lists a rated input of 9.4 A for the CFX5 45. A rated current is roughly what the fridge draws while the compressor is running. Because the compressor cycles on and off, the daily total is lower than that figure multiplied by 24.
National Luna gives a wattage range: 40 to 75 W running for the Legacy Smart range, from its manual. To get amps at 12 V, divide watts by 12, so 60 W is about 5 A while the compressor is running.
Engel's Australian distributor gives a current range for the MT-V45F of 0.5 to 3.0 A. That shows the low and peak draw, but not how long the fridge spends at each, which is why you can only compare two fridges directly when their makers use the same kind of figure.
Turning it into a daily number
If the maker gives an Ah-per-hour figure, multiply by 24. Dometic's 0.76 Ah/h works out to about 18 Ah a day (0.76 × 24 = 18.24), at Dometic's test conditions of 32 °C outside and 4 °C inside.
That figure will move with the conditions. Real use depends on how hot it is outside, how cold you set the fridge, how full it is, and how often the lid opens. A fridge in a closed canopy in the Northern Cape in December is working much harder than one in the shade in spring. Dometic lists an operating ambient range of 16 °C to 43 °C for the CFX5 45, which is the outside air temperature it is designed for.
If your fridge's maker gives only watts or a rated current, ask them for an average daily figure at a stated outside temperature. Working it out from the peak current gives a number that is too high; working it out from the low end leaves you short.
Allow extra on top of the daily figure for hot days, for cooling down a fridge you've just loaded with warm drinks, and for anything else on the same battery, such as lights or a phone charger.
Sizing the battery
The National Luna manual suggests at least a 100 Ah battery, with 100 to 150 W of solar where 6 to 8 hours of direct sun is available, for remote use where solar is the main charging source. Use that as a starting point, not a rule.
How much of that 100 Ah you can actually use depends on the battery type. Lead-acid batteries, flooded or AGM, last longer if you don't drain them deeply, so you can plan on using only part of their rated capacity. Lithium iron phosphate (LiFePO4) batteries tolerate deeper discharge. Check your battery maker's recommended depth of discharge, then work out how many days the usable capacity covers at your daily fridge figure.
If you drive every day, the vehicle's charging system will top the battery up. If you plan to stand still for several days, solar or a generator has to replace what the fridge uses, and cloudy days make that harder.
Wiring it so the fridge doesn't cut out
A fridge that keeps cutting out on a battery you know is charged is often suffering from voltage drop: the cable is too thin or too long, so the voltage at the fridge falls below its cut-out point while the battery itself is fine.
National Luna's manual specifies at least 4 mm² cable between fridge and battery and a 15 A fuse close to the battery on the positive cable. Follow your own fridge maker's cable size and fuse rating, keep the run as short as you can, and use a dedicated circuit rather than a shared cigarette-lighter socket where possible.
Set the low-voltage cut-out
Most compressor fridges have a low-voltage cut-out that switches them off before the battery goes flat. SnoMaster's SMLS-50 lets you choose 10 V, 10.7 V or 11.8 V. If the fridge runs off your starting battery, use the highest setting so you can still start the engine. On a separate auxiliary battery, follow the battery maker's guidance, because lithium and lead-acid batteries have different safe minimum voltages.
Airflow and mounting
A fridge sheds heat through its vents, and if that heat can't escape, the compressor runs longer and uses more power. SnoMaster asks for 150 mm clearance around the SMDZ-TR42S. Keep bags and dust away from the vents.
The fridge also needs to be tied down to rated points or a fridge slide. The NL 52 Legacy Smart weighs 24.4 kg empty, before any food or drinks go in.
Who can fit it
Dual-battery systems, DC-DC chargers and solar regulators are best fitted by a qualified auto electrician or an experienced fitment centre. Several in our directory list battery and 12V work, including GearUp 4x4 in Noordhoek, Kamelback 4x4 in Worcester, Xplorer 4x4 in Pretoria and Tzaneen 4x4.
Before you book the work, have your daily power figure, the fridge's cable and fuse requirements, and your battery type written down. A fitter can then size the system to the fridge you actually own. The full spec table is in the 12V fridge comparison.