The cooling ability of RV fridges relates to how well they are installed, not major differences between products. Their installation manuals show how to do this. Most buyers and many ‘professional’ installers, however, appear to ignore them. Because of this few RV fridges work as well as they should. This is particularly so with ‘three-way’ fridges that run from your choice of (LP-gas or /110/230/12 volts. These must be installed as specified. They rarely are and have an undeserved reputation for not performing.
It is usually possible to improve cooling and reduce energy draw of such fridges. It is harder to do this if they are inside a packed towing vehicle. Even then, a fair bit can be done to help. How to do this is shown in detail in rvbooks.com the Camper Trailer Book.
Engel 80-litre fridge. Pic: Engel.
Cooling ability of RV fridges
Fridges do not make ‘cold’. They pump heat from where it is not wanted, to where where it does not matter. The more common compressor fridges do this via intermediate compression/expansion. The sketches below show the two most common types of compressor.
Most fridge makers specify energy draw at an internal temperature of 4℃. A well installed RV compressor fridge so used is likely to draw about 0.7 amp hours/day per litre of its capacity. Each 1℃ colder thermostat setting increases energy draw by about 5%. Each 1℃ increase in ambient temperature does likewise. A fridge that draws 50 amp hours/day on a cool garage floor is likely to draw close to twice that in a warm 4WD.
Once the fridge’s contents are cooled, the energy needed to maintain that relates to its insulation and door seal losses. Only minor energy is lost when the door is briefly opened. This is because air does not store or readily transfer heat. A door-opening freezer, however, wastes energy if frequently opened (as children may). This loss can be reduced by installing high-fronted plastic drawers.
The simple and effective Engel swing-motor.
The Danfoss equivalent. Both work well and reliably. Drawings: RV Books.
Many fridge makers quote energy use in terms of current drawn. This can mislead. Most electric fridges cycle from fully on to fully off every few minutes. A fridge that draws 2.0 amps for 14 hours a day thus uses 28 amp hours/day. Another that draws 2.5 amps for 10 hours a day uses 25 amp hours a day. It is the total daily current draw that matters.
Many new fridge compressors, however, run constantly. They maintain temperature by varying their speed. This saves energy because electric motors draw about twice their running current for a second or two each time they start.
There are currently no specified performance standards for compressor-driven RV fridges. But given competent installation and usage the better brands are generally similar. They are also increasingly efficient.
Most campers trailers have electric-only fridges (12/24 volt DC and grid power). They run from the tow vehicle’s alternator, often with solar and/or generator back-up, or even solar alone. These fridges may cool well if the installation is less than perfect, but they will draw excess power. They may not, however, cool properly in very hot places.
These are intended to run from the alternator whilst driving, and the battery only for lunch stops etc. They run on LP-gas or grid power at all other times. They draw far too much energy for solar or long term battery operation.
The EU has standards for these fridges. The ratings are SN, N, ST and T. The ST rating (36℃) is fine for warm climates. A T-rated unit (43℃) is needed for tropical climates.
Fridges must be installed properly to work as intended. To cool properly, a fridge’s heat must be able to escape. Air intake and outlet vents must be unobstructed.
Fishing camps etc.
Camper trailer owners who fish seriously need a large fridge or fridge/freezer (and often a second in the tow vehicle).
A typical efficient chest fridge/freezer draws approximately 0.7 amp hour/day per litre of its capacity. A 200 litre unit thus draws about 140 amp hours/day in normal use. It will draw up to twice that, however, to freeze newly caught fish. It will draw and even more in tropical areas (not least as these may also stay hot all night).
One or two such fridge/freezers will run directly from a 1 kW generator, but use a great deal of fuel. It is better (and quieter) to use a 2 kW or 3 kW generator driving a high-current battery charger to charge a lithium battery bank of about 300 amp-hour. A lithium battery bank of that size accepts massive current. Very-high output battery chargers are available. The system will then need running only for an hour or so each morning and evening.