Power Supply energy use

I am confused by this. We all know that power supplies are mostly categorized by wattage. But how does that equate to how much power it actually uses? Is a 400 watt power supply using 400 watts constantly? Or is that number per hour, etc..?? When i receive my electric bill it is measured in KWh, which is kilowatt hours, or energy transfered at a rate of 1,000 watts an hour. However much a power supply uses in watts, how often is it using that? How is that measured? Sure it’s 400 watts, but constantly? Thanks!!