A power supply is just a transformer or a transducer.
A power supply converts energy that comes in as alternate current with a "high" voltage (230 V and 50 Hz in Europe, 120 V and 60 Hz in North America) into the same amount of energy in a direct current mode (i.e 0 Hz) with a very low voltage (3.3 V, 5 V, 12 V) but a higher amperage. (scroll down for explanation of units)
You can compare electricity flowing through wires and cables with a liquid flowing through pipes. To make water flow through a pipe, you need to have different amounts of pressure at both ends of the pipe. When you have the same pressure ar both ends, no water will flow. This difference of pressure is the voltage in electricity.
The amount of water flowing through a pipe within a second corresponds to the amperage.
A machine that "consumes" energy (like a CPU, a light pulp or an electric engine) corresponds to a mill that is driven by flowing water. It's the product of voltage (difference of pressure before and behind the mill) and the amperage (amount of water flowing through the mill) that gives the power the machine needs to do its job.
The power supply in this picture is just a kind of turbine that uses one stream of water that has too high pressure but is flowing slowly to produce a second stream with less pressure but flowing faster. Both streams carry the same amount of energy per second, and both carry just as much as the "mills" (CPU etc.) will consume.
So, simplified: The power supply will draw only as much energy from its source as needed.
But this is only half the truth.
A power supply itself also consumes energy. This is exactly the energy that is emitted as heat from the power supply. A hypothetical power supply that really draws only that amount of energy from its source that is provided to the "real" consumers, would stay cold (at room temperature). But due to certain laws of physics this ideal power supply is impossible to be built.
The higher the maximum power is that a power supply can provide, the higher is the amount of energy it takes to heat up itself. (This strongly depends on how well crafted the power supply is.)
What does this mean?
It means: Find out how much power your system will need (take the maximum that is possible) and add some extra margin to be on the save side. This is the power your power supply should be able to provide. A more powerful power supply will not damage your system. It will just draw some extra energy to heat up itself that a less powerfull power supply wouldn't need.
But a too weak power supply can have negative effects. Your system might crash if you need to more power than your power supply can provide.
But all in all you should know, that the power supply draws more energy from the net when you need a high amount of power, and it will draw less energy when your system is idle.
Here are some physical units:
- Hz = Hertz = number of waves per second
- V = Volt = unit for voltage
- A = Ampere = unit for strength of current (aka amperage)
- W = Watt = unit for power = product of Volt and Ampere (the amount of Energy flowing through the device per second)
- J = Joule = unit for energy