It takes more horsepower to run the fan so you are using more fuel. That is the primary purpose of the fan clutch, to reduce fuel consumption when the fan isn't needed.
I have 2004 volvo d12 engine and I don't have power coming to fan clutch solenoid. Replaced temperature sensor and AC high switch sensor, still nothing has changed, my fan is constantly on. Anyone know anything about this. Help is appreciated. Thanks