As long as you are in a civilized region, power is practically free. I mean, it's not free, but you'd have a hard time using enough to cost you more than a few dollars over the course of a week. Most people's electric bill hovers between $5 and $15 a month.
Batteries are extremely efficient, although they still suffer from practical limits. Electronics are usually low-power, and the MG in your head should last for a year between recharges. No, it won't run off bodyheat. Not unless you're the Human TorchTM and didn't mention it before now.
Most power comes from fusion plants. The most famous one is still orbiting somewhere in High Earth Orbit, that's Helios II. The second most famous one is Helios III, on the moon, but that one has long since been taken apart and rebuilt into the Lunar City Power Company.