Consumption?

[PcGk]

I'm building a new system, nothing too powerful (dual-core pentium E4500). I am having trouble choosing a power supply. I would like to build an efficient system to keep my energy bill as low as possible. I will be leaving my PC on 24/7 as I run a PVR/HTPC. If I were to buy a PSU that is rated at a much higher wattage than I actually need, would this increase energy consumption, or does it not matter? I.e- would a 600+ Watt PSU use more energy than a 400ish Watt PSU?

 

[ceewi1]

No, a system will only draw as much power as is needed. A higher wattage power supply will not result in higher energy consumption.

 

[PcGk]

Thank you.
Also, would an 80 PLUS efficiency PSU save me much over a >70% efficiency PSU if I plan on leaving my PC on 24/7? How much could I really save on my energy bill?

 

[ceewi1]

Depends a lot on the system specs and how the system is used (e.g. CPU usage, etc...). Doing some calculations, though:

If we assume an average of 150W DC draw:
At 70% efficiency (that's very low for any modern PSU), this will result in a 214.3W AC draw.
At 85% efficiency (this is realistic for many 80+ PSUs), this will result in a 176.5W AC draw.

That's a difference of 37.8W.

When used 24/7, that's a difference of 37.8 * 24 * 31 / 1000 = 28.12kWh per month.
If you're paying, for example, 8c per kWh that's a difference of 28.12 * 0.08 = $2.25 per month. With those sort of differences, the higher efficiency PSU would recover the additional purchase cost fairly quickly, but PSUs with 70% efficiency aren't so common anymore.

Of course, aside from the saving in terms of your power bill, the excess power used in the PSU with lower efficiency will be converted to heat, and the PSU with higher efficiency will generally be quieter, if that's a consideration. The unit with higher efficiency may also be a newer design as well.

 

[PcGk]

Depends a lot on the system specs and how the system is used (e.g. CPU usage, etc...). Doing some calculations, though:

If we assume an average of 150W DC draw:
At 70% efficiency (that's very low for any modern PSU), this will result in a 214.3W AC draw.
At 85% efficiency (this is realistic for many 80+ PSUs), this will result in a 176.5W AC draw.

That's a difference of 37.8W.

When used 24/7, that's a difference of 37.8 * 24 * 31 / 1000 = 28.12kWh per month.
If you're paying, for example, 8c per kWh that's a difference of 28.12 * 0.08 = $2.25 per month. With those sort of differences, the higher efficiency PSU would recover the additional purchase cost fairly quickly, but PSUs with 70% efficiency aren't so common anymore.

Of course, aside from the saving in terms of your power bill, the excess power used in the PSU with lower efficiency will be converted to heat, and the PSU with higher efficiency will generally be quieter, if that's a consideration. The unit with higher efficiency may also be a newer design as well.

Thanks very much for the calculations. Would it be correct to assume a charge of roughly $20-30/monthly for leaving the PC running 24/7?

 

[Yeti]

Thanks very much for the calculations. Would it be correct to assume a charge of roughly $20-30/monthly for leaving the PC running 24/7?

That's a bit high. I have my HTPC running 24/7 plus a couple other computers running intermittently, and my elctric bill is in the mid 20's for ~300 kW-hr.