hi, I.m alf, but grandad alf, no wizards here. anyway, 1Kwh is equal to about 4amps. I take it that you have 250v fridge working via an inverter from a 12v.dc circuit. the ratings you give mean nothing that you can calculate, only that in normal use with the fridge almost full of food etc. it would run for a total in a week of about 3 hours. therefore, on a normal 250v household circuit it would cost 3x your kwh rate, say about 12p per unit, so 36pence.on an inverter which is probably running from your diesel engine, it will, at a guess, draw about 400watts,= 1 amp approx. I have no doubt some ohms law expert will quote lenz law or parameters of magnetism in conversion of d.c. to a.c., but I think as a layman thats as close as you will get. I do think the annual consumption is a bit optimistic. I must add one more thing,have a good look at the back of the fridge, there must be an information plate somewhere that says something like: 230v 350w. etc etc.
alf ( the non wizard).