Energy Consumed During Flash Read / Write

The datasheet lists the current during erase or write operations, and the duration (on older MSPs, it depends also on the provided oscillator).
You should be able to calculate a maximum value from this in Ws or Joule.

However, the real energy usage depends on more factors, e.g. data written. I'm pretty sure, I'll require more energy to write teh whole flash to 0x00 than for writing 0xFF to it. Or any combination in-between.