How can I tell how many watts my PSU is using?

Power Consumption Power Power supplies
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You can estimate how much capacity your PSU needs to have for your build by looking at the power usage of the components you intend to put in the system. For the GPU and CPU you can find the TDP(Thermal Design Power) listed, this is equivalent to how many watts of electricity it consumes because it turns it all into heat.

You can assume ~10 W for a hard drive or disc drive, and ~50 W for the motherboard. Fans and memory are in single digits for power consumption so they are negligible. To find the TDP of your graphics card Wikipedia has a nice list of the manufacturer TDP ratings
nVidia GPUs
CPU data is a bit more spread out, but you can look up Intel CPU power consumption at
AMD ones your best bet is to get it out of a CPU review or from a seller's page, it usually says the TDP on the box.

Take the GPU TDP and add it to the CPU TDP, then add 50 W for the motherboard and 10 W for each drive you have, this will give you a rough estimate of the maximum possible power consumption of your system, you want to find a power supply that can provide at least this amount, preferably at least 15% more as not all the power is in the form you need.

To find out how much power your PSU is using at a given time, the easiest way is to get a power meter like a kill-a-watt power meter you can generally pick them up in your local hardware store for a reasonable price. Plug it into the wall, plug your computer into it and it will give you real time power readings, it is difficult to mathematically figure out how much power you are using so it is best to just use a tool to measure it. In the image below you can see the reading for my system on my power meter, it is a PII 955x4(125 W), 4 drives, and a 7850(130 W) with the GPU running at full load. If you were to do the math, you would need to spec for 350W at peak, but even when running near full low my power is still below the TDP math, this is because TDPs are the worst case scenario, hence why the power meter is a much easier and far more accurate way to determine your power consumption at any given moment, you are almost never drawing peak power.

Now this tells you the power that your PSU is pulling from the wall, the power it is providing to the system will be less, to figure out how much it is providing take the reading on the meter and multiply it by the power supply's efficiency. For unknown units assume 80%, for 80 Plus bronze, silver, gold, and platinum use 82%, 85%, 87%, and 90% respectively. If you have a 500 W 80 Plus gold unit and you are reading 500 W of power draw at the wall, don't be concerned, it is only actually providing 500*0.87=435 W of power, so you are not running it at full load.

This is also true when looking at reviews of CPUs and GPUs, all power measurements done in reviews are at the wall, this means they are 11-25% higher than the actual numbers being drawn from the PSU depending on efficiency which is why i suggest sticking with the TDP for the initial calculations.