The error in using amp hours as an indicator of power usage is not large where the voltage the system is running at is relatively constant as is the case when using nominal 12v batteries and recharging systems in a boat. However this would not be the case with say a large installation
like a multi kilowatt solar power installation
back feeding to the grid.
If you put an amp meter on the power supply to your fridge you will notice that when you first load the fridge and start the process of pulling down the temperature the compressor will pull high ampage, then as it cools the ampage will reduce. Where it settles eventually will depend upon the temperature setting of the thermostat.
How often the compressor cycles will depend upon how often you introduce warm material and how effective the insulation
is. Opening the box regularly allows warm air into it and poor insulation
allows lots of heat to bleed in so the compressor will cycle often.
The load that the compressor works against depends upon the temperature in the condenser. If you have a poor condenser which is not pulling the heat out of the compressor discharge vapor the compressor will have to work harder than it would with an efficient condenser and consequently will draw more amps.
My BD-50 compressor pulls about 8.5 - 10 amps with a warm box and drops back to about 4.5 - 5.5 once the temperature pulls down and the compressor starts cycling. I am at present changing the system from an air cooled condenser to a water
cooled one in an attempt to reduce the head
pressure and amps draw of the compressor. I mostly cruise
in the tropics where air cooled condensers are not very effective.