How does openlane calculate power usage?

Assuming you are getting it from opensta's report_power then it is based on the tables in Liberty and activity factors (if unspecified it will propagate default ones)

thanks, so opensta does timing and power

It is a fairly basic power analysis but yes it does both

@Matt Liberty @donn which static probability and toggle rate values are used? Is there a way to specify a vcd file for more accurate power analysis?

There is no vcd capability but the set_power_activity lets you set values individually. I believe @Ethan Mahintorabi expressed interest in vcd for Google. It looks like the default is 0.1 activity on inputs which is propagated forward through the Liberty functions.