Inverter ACs regulate the speed of the motor to adjust temperature while fixed speed ACs start and stop the condenser. Starting and stopping the condenser to regular temperature every time draws more power, so inverter AC models are claimed to run on lesser power which translates to lesser bills. But remember, the costs of servicing and repairing an inverter AC model is way higher than the regular fixed speed models. If you are on a tight budget, buying a non-inverter AC is perfectly fine as you would want to spend less money on servicing and repairs. As far as electricity bills are concerned, if you use the AC judiciously, it makes little difference.