Daily Blender Tip 337: How to use IES lights

Jan van den Hemel writes:
IES lights are an essential component of making your architectural visualisation renders look realistic. Luckily, they are easy to set up in Blender! This quick tip shows you how.

Do you know what to do with the power and size values to make the light intensity more accurate to real life? Since the IES file also contains these values.