Ever wonder how much power a PDU draws?

Hi again!  Soon I will be moving a rack of about 8-10 servers, two 48 port switches, an MD1000 full of disks, and some other odds an ends to a location equipped with a 240v 30A circuit.  Naturally, I need something to plug into the L6-30R receptacle and because I will be remote of this setup I’d like to be able to flip a server or switch off entirely in case of issue.  There are times where an iDRAC just goes brain-dead and the server needs to be unplugged to reboot it, but if you don’t have some hands at the location…  So, combine the need for  at least 20 power connections and the remote switching, I figured I’d pick up a PDU (Power Distribution Unit).

I ordered an APC AP7941 off of eBay for about $85 shipped.  The key about this strip is that it has an acceptable input voltage range of 200 – 240v.  So, a 240v receptacle off ordinary household mains will suffice.  Be careful if you go to buy a PDU on eBay!  I would say approximately 90% of them are for 3-phase power, and rated for 208v input which you will not be able to utilize without an expensive phase converter.  Ordinarily, if you see them for really cheap, check the model.  You don’t a 3-phase model for home (or some businesses!).  You’ll also find that the 120v models tend to be pretty expensive – this is because they do cost more money new sometimes, but in the used market 99% of people buying a used PDU are at home or small office and have only 120v.  Ordinarily you’ll plug the PDU into a UPS but for my purposes it will be direct into a receptacle.

That said, I received my Zero-U AP7941 except it was in a box about 12″ x 12″ x 20″.  Oh boy – a Zero-U should be long and skinny.  Turns out, it was an APC AP7900 – a 120v  1-U PDU… not what I wanted at all!  I checked the listing  and sure enough I did place a bid and buy the AP7941 so the company had just sent the wrong device.  No big deal after sending them an email they realized the error, send me the right unit out and a shipping label for the wrong one (they were great to deal with).  While I had the wrong one I figured I would reset the username/password of the device  since it was not reset when I received it (it was an asset  from a cloud hosting service in Houston, TX).  Note to businesses – you should really zero out your configurations before selling off parts.  This PDU had the community string for read/write SNMP settings, hostnames, domain, etc. for assets that I could ping externally!  Rather than pull the hashes and such to try and crack some passwords, I reset it using the proper RJ12-Serial cable which the next person will not likely have and won’t be able to log into the unit.  I reset it to the standard user of apc and password of apc.

Today my AP7941 showed up in a 6′ long box but before I got to messing with it I wondered – how much power does a PDU use itself?

APC AP7900

APC AP7900

APC AP7900 power draw

APC AP7900 power draw

While the unit booted up it only used 3w which was nice and I thought for a second that it would only use that at idle.  But, once it finished booting up, even with the outlets turned off, it consumed between 8.2 – 8.6w of power.  Not a  huge deal at the end of the day but considering there are pfSense appliances drawing only 16w this is a little high.  But, with the beauty of remote managed outlets versus not, it is what it is.

Mind you these two units are old and deprecated – the current offerings may consume less power.

I couldn’t test the power consumption of the AP7941 since it’s got an L6-30P connector but I assume it is similar if not higher considering it has  24 ports instead of 8.

Here’s the AP7941 powered up just for kicks:

APC AP7941 powered up

APC AP7941 powered up

APC AP7941

APC AP7941

Both units are in fantastic used shape though I suppose that’s to be expected as they obviously came from a real data center based on the configuration on the units.  The AP7941 was from a different hosting center but it also had the read/write SNMP criteria set along with all host names, outlet labels, domain names, etc.

One thing worth noting – I flashed the PDU firmware and application firmware on the AP7941 since I am keeping it.  I didn’t want to risk bricking the AP7900 though it’s a really easy process unlikely to fail.  Unfortunately the only L6-30R receptacle I have is in my garage for my electric heater so I can’t really use this in full, but it’s meant for another location anyway.  Just though some people might get a kick out of seeing how much power is consumed just by the PDU itself!  Thanks for reading!

 

Author: Jon

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8 Comments

  1. Can you clarify what you meant by “Be careful if you go to buy a PDU on eBay! I would say approximately 90% of them are for 3-phase power, and rated for 208v input which you will not be able to utilize without an expensive phase converter. ”

    This is true, but AP7941 is always supports 200-240V input voltage. Are you somehow finding AP7941 that are for 3-phase?

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    • No the AP7941 is fine, but there are other models on eBay usually for $50 – $75 which seems like a good deal until you realize they’re for 208V.

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      • Thanks, I’m assuming the AP7941 I received free from a friend is not going to be usable for my home lab. It’s mark as 200-208VAC, 24A, 50/60Hz.

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        • That sounds like its a 3-phase unit. You can make it work but you need to get a transformer which is usually about $100 – $150 which may not make it worthwhile. It’s a drag because there are so many 3-phase units on eBay for cheap and it’s easy to pick up the wrong one!

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  2. Nice article! I prefer the Eaton ePDU’s though, using a eMAA10 has a usage of about 5 watts if I compare my PDU usage to my PSU output. 🙂

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    • I like Eaton superchargers but never used their PDUs. I checked that model on eBay not many come up. That’s why I went APC, they’re all over eBay. What are some of the features of the Eaton vs APC?

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      • Jon, depending on the options EAton provides you with several advantages. for example redundancy capabilities, monitoring of power delivered by the ePDU (eatons PDU strip like shown in the picture) switching capabilities as well.

        I suggest also that when looking for Eaton distribution not to use PDU because those are extremly programable and they are also for high capacity power distribution. ePDU is what you will be looking for and Ebay will not have them as they are customizable for different requirements and designs.

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