If you want to put on the floor i would suggest to use some kind of base. A shelf and some industrial wheels would be perfect. The problem is that every little dust particle on your carpet would eventually go into the power supply. Assuming that your power supply is on the bottom of your case (with the fan grill facing the floor).
If you really have to put it on the carpet, turn the psu inside the case upside down (fan grill facing the top of the case). This is not ideal for the psu, as it will suck the hot air from the case. Especially if you have a top flow card instead of reference.