RV Fridge Basics -updated October 2019

An 85 litre compressor fridge
An 85 litre compressor fridge

Fridge Types

Most fridges are essentially pumps. They move heat from where it is not wanted to where it does not matter. All work like this, but differ in how they do it - and also how well they do it. They also differ in their ability to cool in hot weather. The major differences in how well they work, however, are largely due to well they are installed. 

  • Absorption (Three Way)

These (in RV use) are intended to run from the alternator whilst driving, and from the RV's auxiliary battery for short roadside stops. They are meant to run on grid power or LP gas at all other times.

 None can realistically be run from solar or battery power for long: one of only 120 litre draws over 10 amps (120 watts/hour at 12 volts). Larger ones draw 25 amps or more.

These fridges slash electricity usage, and save on solar modules, battery storage, and/or generator fuel. When run on LP gas an 8.5 litres cylinder typically lasts three weeks.

Three-way fridges must be installed correctly. Far from all are, resulting in poor performance. As a result they gained an undeservedly poor reputation from caravan owners.

If installed properly they work well for extended stays away from grid power. They cannot, however, freeze a large mass of fresh fish etc. in reasonable time, and thus not suited for serious fishers.

They work via liquid that has a very low boiling point that is heated until it vaporises (boils). It then flows into an evaporator inside the fridge, where it absorbs unwanted heat that is then dissipated externally.

  • Compressor Fridges

These are now by far the most commonly used in RVs.

They work by mechanically compressing a gaseous refrigerant to a liquid, generating heat as they do so. The heated liquid is then cooled thus releasing the pressure and allowing the liquid to vaporise and absorb heat from the refrigerator’s contents. That heat is then released, via cooling fins, to where it does not matter. 

Those intended for mobile use 12/24 volt motor driven compressors but almost all run optionally on 230 volts ac. They are thermally and electrically efficient - particularly the top of the range units made from 2018 - that have variable-speed compressors. All  run well from solar.

Fridge Energy Usage

Until 2014 or so, most compressor fridges ran until just below their preset temperature. A thermostat then cut the power, restoring it when the internal temperature rose a degree or two above the set level. In ambient temperatures of 25 degrees C or so they typically ran in a 40:60 on/off ratio. This ratio varied as the contents cooled and that may be all the time in very hot areas.

Fridge vendors usually quote their product's steady-state energy draw, but daily draw substantially relates to how long the fridge cycles on - as opposed to off. A fridge that draws 1.25 amps but cycles on for a total of 16 hours a day uses 20 Ah/day.  Another that draws 1.5 amps but cycles on for 12 hours a day, uses only 18 Ah/day. So consider only their daily draw.

At 27.5 degrees C ambient, and set to 4 degrees C, most efficient compressor fridges of 40-80 litre draw about 0.7 Ah/day per litre. This progressively falls – to about 0.5 Ah/litre/day for fridges over 150 litres or so.

Those post-2014 compressor fridges that have variable speed motors (they run constantly adjusting speed as needed) use about 25% less energy. 

Fridge-freezers, with the freezer at -14 degrees C  to -18 degrees C, draw only marginally more once their content is frozen - because their insulation is thicker.  All fridges increase their draw by about 5% per every 10 degrees C higher in ambient temperature and by the same amount if set colder.

Energy usage varies slightly from brand to brand - with the variable speed units generally more efficient) but installation, ambient and set temperatures and usage all affect consumption. If freezing is not required, energy is saved by using a chest freezer, providing it can be set to +4 degrees C. Not all can.

Many RV users prefer an electric fridge if travelling extensively and spending only a day or two at powered sites. Given space for solar modules, an energy-efficient 12 volt fridge can be used successfully for extended periods away from grid power. However, a fuel cell or a generator is advisable for energy back-up during periods of little sun.

Or hook up your RV to the tow vehicle and go for a long drive.

Top- versus Front-Opening Fridges

Top-opening refrigerators are marginally more efficient than door-opening units. This is because cold air is retained when opened. That lost from door opening units can be minimised by using plastic drawers to block that cold air flow.

One minor drawback of top opening fridges is that water vapour condenses in the bottom of the chest and needs removing every week or so. Another is that the most needed items somehow migrate to the least accessible area.

Portable Fridges

Many RV owners would like to have a second fridge inside their tow vehicle. This is worth considering as one is handy when shopping (although an esky filled with ice works just as well for short distances).  A way that works well is a 40 or 60 litre compressor fridge in the tow vehicle, powered by about 240 watts or so of solar on its roof plus a 100 amp hour (ideally AGM or LiFEPO4 battery). This enables you to run the fridge permanently from solar alone. This is not feasible if your roof carries a boat up there – but that is never a good idea anyway.

If tow vehicle rooftop solar is not used, the tow vehicle fridge will need to be run from a second battery in the tow vehicle anway. So-called ‘dual battery systems’ are common in 4WDs that are used extensively in the bush, but having two fridges run from only one power source is pushing one’s luck. It is far more reliable to duplicate the power source. Or have one fridge that is electric and the other fridge LP gas.

Pic: ARB

Pic: ARB

RV Fridge Standards

Unlike domestic fridges, there are no local performance standards for RV fridges. European-designed three-way fridges, however, must meet EU Standards that include ‘Climate Classes.’ These define the ambient temperature at which the fridge must deliver its claimed performance.

Climate Class SN (sub normal), is from 14 degrees C to 32 degrees C, N (normal) from 18 degrees  C to 32 degrees C, ST, (sub tropical) from 18 degrees C to 36 degrees C - and tropical) to 43 degrees C.

Regardless of brand, Climate Class ‘T’ fridges can only be positively identified by the letter T following the ‘Climate Class’ box on their compliance/rating plate

 RV Books believes that only the ST (but preferably the more costly T) units are the only three-way fridges suitable for travelling in the hotter parts of Australia and the northern island of New Zealand.

RV Fridge Summary

An RV fridge of 80-100 litres is likely to be ample for most RV users, not least if one realises that freeze-dried food is equally edible, but can be stored at ambient temperature.

Fridge energy draw depends mostly on how well it is installed and used. A correctly installed RV fridge may draw half the current of an incorrectly installed one and cool far better in hot climates as well.

Do not overfill RV fridges (whilst making sure nothing can break in transit). Avoid putting in hot items. Set the internal temperature no colder than the recommended +4º C (fridge) and -14º C  to -18º C (freezer).

Putting items into your domestic fridge before a trip and then transferring them quickly to the RV fridge before departure is a good way to reduce RV fridge power consumption. If you have a fridge-freezer, always conserve energy by de-thawing overnight frozen food needed the following day.

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