Is it possible to measure (with software, like powertop) the current power consumption on my Raspberry Pi?
No. You cannot accurately measure the power consumption of your Raspberry Pi by just software. There needs to be some hardware in place in order to do this and devices rarely have this by default.
The easiest option would be to use a wall outlet power meter or usage monitor. You can buy these for $10-$20 at a local store.
Without such hardware, the best you can get are estimations.
Multimeter in series with a bench top power supply. Or by looking at the bench power supply itself if it has an Amp meter.
Measuring the whole thing (with power supply) won't give you an accurate reading of the Raspberry pi's actual usage since you will also be measuring inefficiencies in the power supply you use.