Pete's correct it all come out of the petrol tank. A little known fact is the electric fan uses more energy, when in use, than a mechanical fan. This is because the process converts mechanical energy into electrical energy, generator, then back again to mechanical energy, electric fan. The mechanical fan run straight from the engines energy.
The main advantage with electric fan is it only comes on when require. Shorter warm up time, No over cooling in the winter. Better cooling when stuck in traffic.
Disadvantages - if you drive your car hard in the summer chances are the electric fan will kick in and use more power than a mechanical type. When the electric fan stops working you could have over heating problems in the summer.
Dave