Hm. Air has a heat capacity of approximately 1 kJ/(kg K). Its density is 1.2 kg/m^3. I don't know how big the fridge is, but someone elsethread said 600 litre

Hm. Air has a heat capacity of approximately 1 kJ/(kg K). Its density is 1.2 kg/m^3. I don't know how big the fridge is, but someone elsethread said 600 litres, so I'll use that. (And assume that it's empty.) That's 0.6 m^3, and so has a mass of ~700 grams. Assuming ambient air temperature of 20°C and a fridge temperature of 4°C, that's a temperature delta of 16 K. Therefore the energy required to chill that air is 0.6 * 16 = ~10 kJ. 10 kJ is 0.003 kWh. Electricity costs about 20 US cents per kWh. Assuming a factor of ten fudge factor for cooling efficiency, then we can be reasonably confident that opening and closing the fridge would cost less than 0.03 * 0.20 = 0.006 dollars = 0.6 cents. That's not a lot. Does anyone have any figures on how good fridge insulation is, for comparison?