Some help understanding how PSUs work?

been looking to finally getting around to building my own pc and the only real thing that Im lost at is how to buy a PSU. I googled a while, but I didnt really understand most articles on PSUs. so, can some of you help with my questions?

1. does a higher wattage PSU mean that it uses more electricity than a lower wattage one, or is power comsumption based more on the components of a pc? for example, will an 650W psu get me a bigger electricity bill than a 500W psu powering the same components?

2. is the power usage of each component summed up for a total power consumption grade?

3. I was looking at an HD6950 and it says in the requirements that it needs a 500W psu with 2 75W connectors. does this mean it uses 150W from the pins + the power from the pcie slot? why is 500W psu a requirement?

4. there might be more to consider when buying a psu(depending on how the previous questions are answered), but what would be a reliable/common psu to have powering an ivy bridge/sandy bridge i5 cpu and an hd6950?

I read a few articles here on THG forums, "How to determine how much power you need", "The PSU Efficiency Myth", and "Computer Power Supplies - A Guide". my brain kind of hurts after reading those. I have a year of college electronics in my head and Im still learning. I believe I might understand a complex answer if its what you have.

thanks for your time.
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More about some understanding psus work
  1. Best answer
    1) If you don't take efficiency into account, then a PSU only draws as much power as it needs. If your components only require 200watt out of the wall, then no matter what PSU you use (200watt or 100000watt), it will only draw 200watt.

    2) It varies widely between ratings/standards/companies. Be more specific. I dont think I understand the question.

    3) These requirements take into account an average computer. For a better requirement rating, you must look at the current consumption and rail at which the card operates. Their estimates are just a ball park assuming you use a certain computer.

    4) Stick to good brands: Seasonic, Corsair, Antec, XFX...etc.

    Since you say you know a thing or two about electronics, here is my advice. Don't worry about wattage. Worry about the individual voltage rails components use (most use 12v) and the current that each consumes. Then look at a power supplies current ability at those specific rails. Wattage ratings can be messed with by manufacturers. For example a 650watt corsair power supply is rated at 650 watts on the 12V rail alone, on top of 100-200watt on the 3.3 and 5 rails. Other manufacturers may advertise the rating by adding the total wattage of all rails.
  2. thank you for the awesome answer dude. very helpful.
  3. Best answer selected by bustapr.
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