January 13th, 2008, 01:04 PM #1
Calculating Power Consumption & Green Savings
Any EEE's (or equally sharp) people out there please check my figures and let me know of errors or omissions. Earth and I thank you!
...a 500W PSU running at 60% load (300W system demand) under 75% efficiency will result in a net consumption of 400W. A 90% efficient PSU under the same conditions will draw 333W. Thatís an annual usage difference of 578kwh. At $0.08/kwh thatís an annual energy cost savings of $46.
500W * 60% load = 300W system load demand
300W / .75 = 400W total consumption (or 100W psu overhead)
300W /.9 = 333W (or 33W psu overhead)
(33W * 24hours * 365days * .08 / 1000 ) - (100W * 24hours * 365day *.08 / 1000) = cost difference at $0.08/kwh
I'm assuming that load is net and does not include the psu overhead, else my equation should look like this:
300W = X / .75
X = 225, which would be the system load demand on a 60% load with a 500W PSU running at 75% efficiency, and thus an overhead of 75W.
I'm aiming to calculate power saved by moving green, both in analyzing the upgrades of current PSU's as well as the money saved by incorporating green into new system purchases. Greatly appreciated all.
Last edited by SiliconJon; January 13th, 2008 at 01:06 PM.
January 16th, 2008, 09:34 PM #2
You're math seems right, BUT, you're making the assumption that the computers are running at full power 100% of the time, 24/7/365. A more normal usage pattern of... say 8 hours a day... will result in only 1/3 of the savings you've calculated.
It's also worth doing a return on investment analysis. Let's say you save $20 a year with the new PSU, and the PSU costs $80. It will take four years for the savings to offset the extra cost. That's not a bad payback rate, but there are better ways to spend your "energy efficiency" budget, such as upgrading to fluorescent lighting if you haven't already done so.
Let's also not forget that by throwing out an older PSU that is still functioning, you are introducing lead and possibly other heavy metals into the landfill. Personally I would wait till your current PSU dies before upgrading to the new high efficiency PSUs, which hopefully are ROHS/WEEE compliant (aka lead free).
January 16th, 2008, 09:42 PM #3
- Join Date
- Oct 2001
- San Diego, CA
wow... I wish my electric rate was $0.08 kwh... Last I checked it was almost $0.14, and that was in 2005"Opinions not based on knowledge are ugly things"
January 16th, 2008, 09:47 PM #4
I think it may actually be .10 kwh, and that's due to the large consumption rate. Our electric rates are tiered - the more you use, the less it costs.
That's a good point, DanU - I am over calculating the savings a bit at the given electric rate, though the higher rate will offset that error a bit.
January 17th, 2008, 01:42 AM #5
have you thought of purchaseing an item called the KillAWatt?
This might give you a better handle for idle and max load usages to make a more accurate figures you can calculate.
January 17th, 2008, 01:01 PM #6
Already have one - and it does come in handy. The first thing I did with it was calculate how soon a new refrigerator would complete pay for itself versus keeping the older less efficient one around. I hadn't proposed using it on the servers as I would prefer seeing the power signal tested to ensure the KillAWatt does not lessen the quality of the electrical current before I plug it into expensive machinery.
January 17th, 2008, 02:22 PM #7
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