If you have a Powercab you can enable the built in meter to see what you power usage is really like: press prog/esc 6 times, look for "set cab params" press enter, on the "show track current" screen press 1 to enable. The Powercab has a 2 amp capacity. If you are upgrading from the Powercab the first booster MUST BE the SB5 Smartbooster!
If you have a PH-Pro 5 amp system, the only way to get an accurate reading of your power usage is with a meter specifically designed for DCC such as the RRAMPmeter. A fantastic item, it can be panel mounted or used to spot check “in the field” http://tonystrains.com/rrampmeter-by-dcc-specialties/ If you find out you need more capacity, buy a booster such as the PB5 and daisy chain the control bus. Take a look at these diagrams: https://ncedcc.zendesk.com/hc/en-us/articles/200590789-Booster-Diagrams