The way homes are wired, each "circuit" goes to several different plugs. The assumption is that you won't put too much load on any of them. Where high-drain appliances are expected (like the kitchen), the top plug may be on a separate circuit from the bottom plug. At least in my house, those are still shared among 2 receptacles (found by breaker trip when both microwave and electric frying pan were on at the same time: fixed by using the second circuit).

I calculated in another thread that you can only draw about 960 Watts continuously from a 120V, 15amp circuit if your power supply has a poor power factor (of 0.67 (4 of 12 amps are wasted)). The kill-a-Watt (and similar meters I assume) will tell you the power factor of the load. What is actually important is current draw. If you have an old or cheap power supply, it will have an approximately capacitive load. That causes the current draw to lead to voltage. At the zero-crossing of the wave-form; you are heating the wires with current, but supplying no power (power=current x voltage).

The good news is that the power company does not charge small users for their power factor: only wattage consumed. The bad news is that if you have very bad power factor (anything below 0.9 is bad), you can't use as much power without overloading the circuit. Power supplies with Active Power Factor Correction should give you good power factor. I have also found vacuum cleaners employ power factor correction as well: probably in the form of a capacitor (capacitive load) in parallel with the motor (inductive load).

I think just about the only way to know which plugs and light fixtures are on what circuit is to systematically turn off the  (smaller) breakers one at a time to see where the power goes out. To be honest, I have not yet done that in my own home. It will likely inconvenience all of the other power users