Let's say 200W idle power, which is good ballpark figure for mid-high-end PCs these days and is easily verifiable on many review sites.
at 80% efficiency, actual power draw will be 250W
at 75% efficiency, 266.66W
so 16.66W saved
16.66W * 24HRS = 399.84 Watt-Hours / day
* 30 days = 11.995 kWH
* P7.90 / kWH = 94 pesos per month
If he is consuming 400W average, then that's close to the P200/mo figure estimated.
Also, if the efficiency gap between the current PSU is bigger, say 70% old vs 90% new then that's P600/mo easily.
It really depends on how crappy his current PSU is, and how much use the computer gets throughout the day but overall I'd say those figures are quite realistic.
What is not obvious in the above calculations is while the savings may be 94pesos/month, the electricity bill is 250W*24*30*7.90 or 1422 pesos/month. My recent Meralco bill puts the cost per KW-Hr at over 10pesos/KWHr
I bought an Energy Logger from Alexan. It can read power consumed and store consumption patterns for several days. I also have a couple of FSP epsilon 500W 80plus PSUs. I assume it is similar in design to the Supersonic although it is more expensive at over 3K.
Here are my observations.
The idle power consumption of my I5-750 rig with one hard drive is about 85W. I has an 9600GT non-green. Everything is stock. I don't play games but I estimate that the consumption will shoot up to 200W when gaming. So this is half the estimated average above.
I also measured it before changing the mobo from a C2D E7500. The old rig with the same video card had three HDDs and idled at 110W. The difference may be in the reduced hard drives but I think the speedstep on the I5-750 is superior to the the one on the E7500.
Yes, I have been buying only 80plus rated PSU to save on power. But you can save as much power by switching to newer more efficient CPUs. I have an older E6400 rig and the processor seems to run hotter.
That old E6400 rig also used an FSP epsilon 500W PSU (used slightly longer than 1 year). One day it won't power up and the fan LEDs that normally stay constantly lit when powered up, were flashing indicating an unstable voltage. I replaced the FSP with an HEC cougar 550W and the motherboard would now boot. But I had also to replace the mobo because some parts of it like the PS2 keyboard would no longer work. After that experience, I somewhat lost faith in the FSP brand.
Last weekend, I had the PSU checked out by a PSU repair expert and after testing and replacing a couple of suspected caps, he can't find anything wrong with it. I tried it once more and it would now boot the E6400 rig.