Hi Liam,
The 'bible' that I generally refer back to called 'Solar that Really Works' and is written by Collyn Rivers and he says the following:
"A correctly installed electric compressor fridge of 30-80litres is likely to draw about 0.7 Ah/day for each litre of its internal capacity, but more in warmer places." This is based on fridge makers specs of 4*C thermostat temp and an external ambient temp of 25*C. For each degree celsius colder that the thermostat is set to, the energy draw increases by about 5%.
Your fridge will either be on or off as it cycles during the day. So if for example your fridge draws 2.5 amps for 12hrs each day (on average) then it adds up to 30Ah/day right?
From your avatar you are in Perth, and as it is end of December/early January, the Peak Sun Hour contour tables say that you should be getting around 7.5 peak sun hours each day in good sunny weather, and this is 'chasing the sun' (or regularly moving your panels to face the sun) as mentioned by TD.
So, you then multiply the peak sun hours (7.5) by the real output of your solar regulator and divide that by 12 V, you will get the number of Ah/day you are going to likely put back into the battery to manage your fridge. As long as you are getting 30Ah/day or more, then you will be fine for as long as you have beer to keep cold.
Your regulators output is a big factor in how cold your beer will stay, and in this regard you get what you pay for, and there is plenty written elsewhere on this topic.
If it is cloudy or raining, then expect to only get around half of the real output of your panels, or about 3 to 3.5 peak sun hours.
The golden rule, according to Collyn, is to have at least 120-150W of solar for every 100Ah or battery capacity if you want to be sustainable. Now given that you only want to be running the fridge for 3-4 days, you can probably just do as advised by TD and ET and use minimal solar capacity to keep putting something back, as the fridge slowly runs the battery down.
If you are also running a few camping lights then it might run down slightly quicker, depending on the type of lights.
Worst case - start your engine and run it for an hour or so on your second and third days!
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