How much electricity does an external USB hard drive use when it's not being used?

How much electricity does an external USB hard drive use when it's not being used?

My external USB hard drive requires its own power source. I noticed that if I don't use it for a while, and then try to access it later, it takes a while to startup, meaning that it was in some kind of sleep mode. How much electricity is it using when it's powered on but not being used?