Using PC's PSU to power monitor

[Giraffe]

19" monitor, 35W, fed by external PSU rated 12V 3.3A (near enough 40W.

The external PSU sits there getting warm and taking up space and gathering dust. I wondered if it would be reasonable to take a 12V line out of the PC to replace the external PSU.
The PC's PSU is rated at 330W and is taking 45W input so, if there are about 4A spare on the 12V rail (I'll need to check the load and rating for the 12V) is there any reason not to do this?

The PSU is aPFC and reaches its best efficiency between 20% and 80%; atm it's running at 36W output (assuming 80% efficiency) which is 11%. The monitor would make it about twice that which would be within the above figures.

[watercooled]

You might save a bit because of improved efficiency but just how much depends on how efficient the original adapter is - does it get very hot or just a bit warm? If you have a power meter see how much it's actually drawing now for comparison. There's no reason it shouldn't work but remember doing this could void warranties - especially that of your monitor because you're not using the supplied adapter but it the MFR would ever know is a different matter (according to some/most PSU warranties you shouldn't even use a different kettle lead). Also remember that the monitor will only work when the PC is powered up which could be a problem if you use a KVM or something.

[Georgy291]

im powering quite a few non pc stuff from my power supply.

firstly i soldered my own schematics for fans and fan controllers

seconds the free view card i got doesnt get any signal. so i bought a ebay signal amplifier and it required a external powersupy (12v) so rather than buying a separete powersuply i just soldered it to a old HDD plug,

thirdly im powering is the speakers. they are really really old, but they are very high quility...i havent heard anything better yet thats cheap. they originaly were powered by my old monitor but then that died so the powersuply went to. again i pluged in to my 12v rail on the powersuply and it all works nicely.

i dont even have to remember to unplu that stuff when pc is off to save power...its just isnt powered anyway

[Giraffe]

Thanks for the replies.

@watercooled

Warranties aren't a problem as I built the PC and the monior is >3yo.
I don't use a KVM (never seen the point in any from of networking for 1 PC!) and if I need to do a swap I have 2 more monitors.
I'd connect it to a Molex rather than solder anything ( the 12V lead from the PSU is hard-wired).
I'll check the power draw, but that's only part of the reason.

@Georgy291

Looks as if it's possible to wire in more thing - speakers I like. I don't know what they draw; the wallwart is cool and I can't see any difference in overall draw from the strip with speakers off/on/playing music.

Part of the reason is to get the PC's PSU in to it's optimal range - no big deal, just a whim!