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Re: [datacenter] Understanding power billing

Hmmm, I don't seem to be getting all the messages from the list, so hopefully this doesn't repeat what someone has already said (apologies if so)...

Kilowatt-hours are not so mysterious. A 100-watt light bulb left on for one hour will use 100 watt-hours of electricity. Divide that by 1000 for kilowatt hours.

We figure power using the following cocktail-napkin approach, which works for us:

1) Measure current at the running device. We use what we lovingly refer to as a "Croda Cord" for this purpose: it's a regular 6-foot power cord with about 6 inches of the vinyl jacket removed to expose the three conductors within (THEIR insulation is not removed, however!). You can easily put a clamp meter on the hot conductor this way. Let's call the server measurement 2 amperes, for example.

2) Multiply amps by 120 (volts, assuming a typical single-phase circuit) and divide by 1000, which gives you KVA: 2 x 120 / 1000 = .24KVA.

3) Convert KVA to kilowatts by applying your "power factor". This is typically a multiplier from 0.8 to 1.0, depending on effeciency. Let's just use 1.0 to keep it simple. .24 x 1 = .24 kilowatts.

4) So now, that .24 kilowatt server left on for one hour will use .24 kilowatt-hours of energy. There are 720 hours in a 30-day month, so it will use 720 x .24 = 172.8 kilowatt-hours in that month.

5) Multiply this by your average cost per kilowatt hour. You can find the average, of course, by just dividing the amount of your bill by the number of kilowatt hours reported used. Let's say it averages $0.15/KWh. Then, $0.15 x 172.8 = $25.92, your cost to power that server 24x7 for a month.

You must also apply a "delivery factor", which accounts for the cost of removing the heat that the server generates as well as losses and other overhead in the system. I've heard from reliable sources of factors of 5 or higher in some unfortunate facilities, but numbers in the 3 range come up a lot (ours happens to be lower still). Factoring this in, it really costs about $75 per month to operate that server, cool it, give it stable power, and have lights around it so you can work, etc.


Charles Sprickman wrote:
On Tue, 18 Oct 2005, Eric Fullar wrote:

It depends on your utility and how much power you use.  Our electric bill
here in Eugene has three or four components:

Similar here, basic charge, delivery charge, "demand" charge.

But I'd like to back up a bit. We're not charging for power, I'm just looking to make sure that we are figuring out our pricing correctly and not left holding the bag.

Backing up even more, I really don't understand the whole "kilowatt hour" metering scheme. If anyone has a link to a good explanation of that, I'd appreciate it.

I've found that the most reliable way to get electrical usage measurements is to have an electrician put an ammeter on each circuit at the panel. To
figure out the actual power used, you'll need to measure power factor as
well.  If you have a lot of the same kind of equipment, you can measure a
typical configuration and do the arithmetic. Seasonic makes a nifty little
meter you can use yourself to do this (
http://www.pcstats.com/articleview.cfm?articleID=1704 ) .

We're just a little shop, so I just bring in a tiny little tool, the "Kill-A-Watt": http://www.the-gadgeteer.com/review/kill_a_watt_electric_usage_monitor_review

It reads watts, VA, amps, power factor, voltage, and the mysterious Kw/H.



Eric Fullar
Univ. of  Oregon Telecom Services

-----Original Message-----
From: Charles Sprickman [mailto:[email protected]]
Sent: Tuesday, October 18, 2005 12:53 PM
Subject: [datacenter] Understanding power billing

Hi all,

Hope this list is still active...

I'm working on a VERY small in-house "ghetto co-lo" setup.  So far, so
good, but one thing I'm stuck on is figuring out our actual power costs so
that I can give management some numbers along the lines of "this server
consumes 120 Watts, which costs us $X/month".

Any pointers to how kilowatt/hour billing is calculated? I was looking at
our power bill and it's pretty complicated.  We get billed a certain
amount at $X/kw/h, and then after a certain amount of usage $Y/kw/h.  I
don't mind just averaging that, as all I really need is an approximate

Any input, STFW, etc. appreciated. :)


Charles Sprickman
Bway.net - New York's Best Internet - www.bway.net
[email protected] - 212.655.9344

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