Not exactly... the alternator provides "up to" a certain amount of electricity, but as more of that electricity is actually used for things it gets harder for the engine to spin the alternator and so you end up using (slightly) more gas.
The alternator (or really any electric generator) has a magnetic field, and a lot of coiled up wire. Moving the wire thru the field generates electricity, the faster the wire moves the higher the voltage is. But the more current is moving thru the wire (more load, more things using the electricity), the more it pushes back against the magnetic field (because that current in the wire generates its own field) and so the harder it is to make it move.
The alternator (or really any electric generator) has a magnetic field, and a lot of coiled up wire. Moving the wire thru the field generates electricity, the faster the wire moves the higher the voltage is. But the more current is moving thru the wire (more load, more things using the electricity), the more it pushes back against the magnetic field (because that current in the wire generates its own field) and so the harder it is to make it move.