How much hardware can you put onto a 500w power supply before "IT" starts to hit the fan.

To work out how many watts you system uses you need to know a few things.

The first is how much wattage each component connected to the motherboard uses, the second is from what voltage supply it runs from the Psu you have as Power supplies provide multiple voltage ranges called power rails.

And the amount of amps a bit of hardware also requires from that power rail of the PSU you have.

With most things for example like a graphics card that fits to the Pci-e slot of your motherboard it will quote the power specifications in watt consumption and the amp draw at that maximum wattage that the device requires.

For example look at the bottom of the list for power consumption for a 750 Ti Nvidia graphics card fitted in a system at the end of the specification information sheet bellow.

http://www.geforce.co.uk/hardware/desktop-gpus/geforce-gtx-750-ti/specifications

The peak or stated output of a Psu is always with the combined wattage each of the different power rails of the psu can output maximum in wattage The 12v rail, 5v rail, and the 3.3v power rail.

Each power rail of the Psu details how many watts, and amps it can provide for a device that uses one of it`s power rail feeds to run in your system.

You simply total the devices on what wattage they require in your system at what voltage, and the amps to drive each of the devices and add them together for the overall maximum power draw or feed required from the PSU you have.

It is important to note what the stated power consumption is of each hardware device connected to your motherboard.
when it is fully utilized or used, that value should always be taken into account it`s highest power consumption based on it state in the system.

For example a graphics card draws it`s peak power in watts, and amps of the 12v power rail of the fitted power supply unit in your system.
When the Gpu is fully utilized in gaming on the system.

There are power supply calculators on the internet if you do a search.
You just need to know how many of each listed hardware devices you have in your system of what bran or specification to calculate what power supply and it`s wattage you need for safe running.