If I buy a power supply @ 600W but actually my media center (HTPC) doesn't need something more than 400W, is any the problem and is more energy used ? (The price for 400 W and 600 W is the same. My media center has a AMD E-350 APU so it doesn't need too much power.)
400W will do just fine. If the price is the same.. and you want to sleep peacefully at night knowing you have more than enough power for your htpc, then feel free to go for the 600w. Just make sure they are from reputable brands. You can check reviews on sites like johnnyguru/overclock/tomshardware/whatever you can pull out.
yes, both are reputable brands, my only concern is if the 600W one is using more energy by doing the same thing.
Just because it's plugged into your wall, doesn't mean it will run at 100% the whole time. You should check which one has better efficiency. That may save you a bit on the monthly fee.. otherwise.. the 600w won't use more than it has to. You don't need to worry about a thing. I hope this solves everything.