When the crew (Sanji and Ussop) saw the Arc Maxim powered by Enel's Lightning they were surprised by how the machine worked autonomously without a power source. But then how does the crew power the fridge?
Presumably, this Water-Drawing Machine is used to power the refrigerator. As pointed out in the comments, all you need is a rotary motion source to use electricity at all times.
Although, the crew stores most of their food in the Storage Room, as indicated by this image:
You may also note that there are 3 orange trees, taken from Bellamere's plantation, which are used as a source of emergency food: