how to know the power that a working Arduino require

MarkT:
Multimeter on amps range connected in series with the power supply?

Thank you for your reply.
Yeah I can do this, but I can not do this all the time. This project is like a outdoor project, I can not holding multimeter all the time. Also, I can not read data from Arduino itself.
Since I have Wifi, I want to read it from web. I can do interface things.