Similar to the Matrix movie where humans were used as a source of electricity but different by using electric eels. Farmed like tuna what would be the electricity exchange to the food they eat?
I don't have the math at hand for the actually energy efficiency, but let's just start from the storage capacity.
Basically, an electric eel wouldn't make a very good battery. They could be a decent, albeit fairly inefficient capacitor, but they actually store little energy. Using data from Wikipedia on Electric Eels it works out to something less than 2 joules (less than 1kilowatt over 2 ms), whereas an AA battery is around 12000 joules.
You're going to need a very, very big tank to store any appreciable amount of energy, before you even worry about how much food it will take you, let alone the animal welfare concerns with forcefully discharging a tank of eels at your whim.