No hard fast rule. Mikey was called to dinner and ate food as normal. Or rather simulated it. If it is required it was never established but I suspect she can have more then one kind of food source now and chemical energy is still useful to have. Direct electrical feed is possible but I don't think they worked out how to do that yet. They can store energy and know how to direct it to tasks the most efficiently so can probably go ages between feeding.
I suspect every mechanical changed is different though.
Digital fairies eat icons. They are yummy. (at least to effie)
