Don’t run an absorption fridge on 12V when the engine is off. Dometic rates the 12V heating element in its RM 8551 at 130 watts, which is about 11 amps at 12 volts. The company’s own installation manual says to wire that element so it only gets power with the ignition on, “otherwise the battery would discharge within a few hours.” When you are parked, run it on propane.
The numbers, from Dometic’s own sheet
The RM 8xxx installation manual lists the RM 8551 (about 115 liters gross) at 135 W on mains and 130 W on battery. Its rated consumption over 24 hours is about 2.6 kWh on electricity or 270 grams of gas, measured at an average ambient temperature of 25 C (77 F).
Here is the math on the battery:
- 130 W divided by 12 V is 10.8 amps whenever the element is on.
- A 100 Ah battery, even if you could use every amp-hour, covers about 9 hours of that.
- Dometic’s 24-hour figure on mains is 2.6 kWh, which means the heater runs most of the day. The manual gives no 24-hour figure on 12V, but the element is only 5 W smaller.
Dometic’s automatic models agree. The manual says they select 12V “only while the engine is running in order to prevent battery discharge.” The 12V mode exists so the alternator keeps the fridge cold while you drive.
Propane is the cheap mode
A full 20-pound cylinder holds about 9,070 grams of propane, so the fridge alone would run about 33 days on one at 77 F. Summer heat, the furnace and the water heater all pull from the same tank, so you will not see 33 days in practice. But the fridge is not what empties it.
The catch: gas mode is not zero battery. The manual says the fridge’s electronics and interior light need a permanent 12V supply on a 2 A fuse. That is the control circuit, not the heater, and it is small. The furnace is a different story; see what an RV furnace pulls from the battery.
If you are thinking about switching
Compare two Dometic fridges of about the same size at the same 25 C test point. The CRX 110, a 3.8-cubic-foot (about 108-liter) compressor fridge, is rated at 0.36 kWh per 24 hours on DC. The RM 8551 is rated at 2.6 kWh on mains. The compressor fridge uses about a seventh of the energy, roughly 30 Ah a day at 12 volts. That is a load solar can carry. An absorption fridge on electric is not. (Dometic has since replaced that CRX SKU with a newer one, so check the current model’s sheet before you buy.)
Two honest limits. These are European-market Dometic models, and your fridge’s numbers will differ. Not every absorption fridge has a 12V element at all, so read the data plate or manual for a DC rating before you assume the math applies to yours.
What to do
Switch the fridge to LP (or automatic, if yours has it) when you park, and confirm it actually lit. If you are sizing a battery bank around a compressor fridge, start at 30 Ah a day and plug it into the power budget guide or the RV solar calculator. For portable compressor fridges, see what a 12V fridge costs in amp-hours.