How to calculate how much power your PSU needs

Hi,

I am new here and I couldn't find out how I can calculate the power which my PSU needs.

For example: I have 6 cards RX 570 OC 4 gb from asus.
On the website I bought them it said 1 card has a voltage use of 180 watts.

But I have 6 GPU: 6 * 180 = 1080 (without power for motherboard etc.)

Should I have a PSU of 1500 watt?

Sometimes people have 6 GPU and a PSU off 1000 watt, so they probably undervolt the GPU?

So if I have a PSU of 1000 watt, should I first install 1 or 2 GPU? So I can undervolt them?

Thank you