Power supplies: what am I doing wrong?

[CTho9305]

Alright, lets take a theoretical system and figure out how big a PSU it needs (or will benefit from)

watts +5 +12

proccessor 72
video* 35
NIC (some intel 10/100 NIC) 1.25 .250A 0
sound* 15
HD (maxtor diamondmax plus 60) 41.2 .550A 2.950A
burner (some random burner) 11
mobo* 30

total: 205w assuming 100% efficiency.
total assuming 80% efficiency VRM: 223W

Now... the video is a wild guess. that is unreasonably high based on the cooling... a 30 watt CPU needs much more than video cards hav.

The sound card also can't be drawing 15 watts. That is double a pentium 200... and sound cards dont have heatsink/fans like p200s do.

The motherboard probably doesn't use 30 watts either, based on the miniscule heatsink, and (occasionally) a fan.

An asterisk above means I guessed.

Now what people tend to say is that power supplies can only supply so much current on each rail, so a 235W psu probably won't be able to handle it. Unfortunately, I couldn't find detailed enough info to be more accurate. Does anyone have this information?

 

[veryape]

Why bother with all the research and just get a 350 or 400 watt power supply, since they're really not much more than their smaller counterparts and will definitely do the job without any hassle.

Forgive me if you are doing this research for something important, I was just thinking that if I need to think that much about how much power my system needs, I likely need something big, and don't want to have just enough to power my system. I want to have enough and extra for when I want to upgrade.