Power supply sizing – Help me with my homework

Greetings,

Have never been a hardware guy, and am taking PC Maintenance to finish up my BA in Networking.

I am not looking for the answer directly, just hoping to get pointed in the right direction to get the most out of this.
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The question is:

Powersupply, 450 watts. When and why would you need it.

I want to take it a step further as to the difference between rated or maxed.
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What I am looking for are web sites that explain this. Where do YOU go when your looking for answers to a question like this?

Also, tools. What do YOU use to determine your power consumption so you know how “big” of a power supply you will require?

Thanks! 😀