Measuring Electric Usage for Specific Circuits?

[Jesse Smith]

I'm setting up a single-family residence for multiple living units. I'd prefer not to install more breaker boxes and meters if I don't have to. 

I know it's possible to monitor usage for specific circuits using a few different device options.  The most popular seem to be ones that attach around the wires on the circuit breaker, allowing me to isolate the usage to circuits within each rental unit.  If I can determine precisely how much energy each living unit consumes, then I could pay the power bill and bill each tenant accordingly. 

Do any of you use this method?  If so, what specific device do you use to monitor the consumption?  An internet-connected device would be ideal, so I could monitor remotely.  Thanks!