All of the electricity is generated by the vehicle alternator.
A small part goes to making sparks, the rest to operating all the various systems, stereo, etc.
The more electricity used, the more load on the alternator and the more difficult it turns.
So, the system increases fuel flow to compensate for the additional load on the engine.
Our Code 3 lightbar, with two 3-bulb 360s on each side = 12 bulbs, with all the other lights, caused the engine rpm to drop when turned on.
Immediately the engine was given more fuel to maintain minimum rpm.
But it was an obvious indication of how much load the alternator imposed on the engine.
There is NO FREE LIGHT!
The battery mainly stores energy, (electricity converted to chemical energy which is converted back to electricity) to start the engine.
The battery also discharges when sudden electrical demands occur, and helps even them out.
The more electrical use, the more fuel expended.
Wind resistance, which rises exponentially with speed, causes the greatest
fuel consumption. To gain better fuel mileage, I rarely drive our Expedition at posted speed . Emergent transports use a lot of fuel, especially if 100 mph on Interstates. That is when I wish we did not have the exterior Streethawk light bar.