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I am used to looking at collocation providers where in U.S. are given as AMPs per rack (assuming 120V). I am looking into a place that rents space for servers which is listed as:

"250 Watts per usable square foot"

Using standard rack sizes with a standard hot cold isle configuration, how would this roughly translate to AMPs allowed per a rack?

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It won't. It's a marketing buzzword. – Tom O'Connor Apr 20 '12 at 16:15
Have you considered simply contacting them and asking them for a more useful number? – Zoredache Apr 20 '12 at 16:27
up vote 4 down vote accepted

The only place I've ever seen that kind of statistic used is when a datacenter is leasing space by the foot, and leave it up to the customer to figure out how to allocate power. The method for figuring out how to convert this into something actually useful for the likes of us:

  1. Take the total square-footage, multiply by 250W to get total power for the room.
  2. Take the measurements of the racks you plan on using, and get their footprint (probably in Sq/In since that's easier for us USians).
  3. Figure out the needed clearances fore and aft for servicing, add to the footprint of the rack.
  4. Convert that sq/in number to sq/ft, multiply by 250 watts to get your wattage per rack. To convert to amps, divide by voltage of choice.
  5. Look at your cage-layout and figure out what parts of the floor aren't actually in the above (door swing areas, aisles, the spot you're storing the ladder, that kind of thing) and compute the percentage of the total floor area consumed by live-system rather than these dead-spaces.
  6. Take the square footage of live space and divide the total-power number by the life-space number.
  7. Rerun the computation in step 4 to get your actual power-per-rack number.

A lot of math, and will take some diagramming to figure out, but you can get there.

It's a pain, though.

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this won;t be accurate unless you can get a definition of a "usable" square foot. (not that this doesn't cover what any reasonable person would assume is a square foot but we are apparently dealing with marketing weasels) – Jim B Apr 20 '12 at 17:25

Power measurements are usually depends on your server(s) (how many PSUs, CPUs, HDDs) and it doesn't go by square foot. You need to see what's the consumption of your server(s) and/or whatever else you're planing to hook it up in same rack to get an idea how much power you'd need.

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if you down vote provide a comment! – alexus Apr 20 '12 at 17:49
-1. You obviously didn't read the question carefully. The OP states: "I am looking into a place that rents space for servers which is listed as: "250 Watts per usable square foot". He's not asking how much power his equipment will consume, he's asking how to determine his cost based on Watts per square foot. – joeqwerty Apr 20 '12 at 17:53

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