How high is the wattage of a MacBook 13?

I want to have my MacBook online several days.
I wonder how high the current drain of a MacBook Pro could be?
Someone said about 15W if this is correct then:

15W * 24 hours = 360Wh
1kWh / 360Wh = 0,20€ per kWh / x
x = 0,074€ a day

Can it be really this cheap?

Thanks a lot.