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The PSU I have reads 400W on the label. Its a cheap local one and has only 15A on its 12V rails, so I'm sure that its lower than indicated. Is there any way I can measure the real PSU wattage by any software, instrument, or any means. Please do not suggest kill-a-watt meter as its not available in my country.

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Do you want actual consumption, or rated maximum? – pjc50 Oct 29 '12 at 16:12
What is on the other parts of the label? 15A at 12V is 180Watt. If there are rails supplying 220 watt via +3.3 or +5 volt then the label might be right. Having written that: 1) Outputting max 45% of the PSU's max rated power via +12v seems rather low. 2) To make thigs more complicates the output numbers added are often bigger that the maximum rated power. Meaning you can not draw the maximum from all rails simultaneously. – Hennes Oct 29 '12 at 16:58

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@Jook is right about the difficulties of testing the capacity of a power supply outside of a lab. It's expensive and potentially dangerous.

For real-time power consumption, if you can't invest in a Kill-A-Watt, you can try building a separated power cord that you can use with an inductive amp meter. These kinds of meters only work when you can measure the hot wire directly. If you put the hot and neutral wires both through the meter, they'll cancel out. (Explicitly measuring the hot line is basically what the K-a-W does.)

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The Kill-A-Watt is great for this, but his question specifically said that was not an option. – techturtle Oct 29 '12 at 17:03
@techturtle Ah! I didn't see that. I've expanded my answer. – dafydd Oct 29 '12 at 17:10
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If you want to know, what is the max my PSU could provide - I would say NO, there is no such thing, because you would have to actually use this amount of power until your PSU fails.

This would not be a good idea.

There are surely devices to test your PSU, outside your PC.

And you might be able to determine an avagerage powerconsumption, by taking in all power-consumers of your setup. There might be software for this already - at least I saw some website, which basically takes average ratings for typicall consumers and adds them up according to your configuration.

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I suspect you could increase the drawn current (i.e load) until the measured output voltages drop below some usable level (or until the PSU trips out safely) – RedGrittyBrick Oct 29 '12 at 16:37
True. That assumes the PSU has safeties though. I am not sure if this holds for what is described as a 'cheap local PSU'. – Hennes Oct 29 '12 at 16:55
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the watt rating is meant to be how much is can supply, not how much it will use under no load.

you could use a clamp meter to see the AC current consumption.

to measure how much it can supply, you need a calibrated variable "load". I have used 12V lightbulbs to do this in the past...

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+1 for clamp meter. – RedGrittyBrick Oct 29 '12 at 17:12

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