The easy way...

Cut the +5v Supply lead, insert a DVM to measure your current consumption. Write some code to switch-ON (or work) every possible peripheral that draws current. Add a little as a 'safety margin'. Personally I never run any 78L05 over 50mA which allows for a 100% safety margin. They do get hot when you run them near their maximum, and I don't like anything getting hot in designs...