After an extended power failure that caused our UPS to run down and have issues coming back up once power was restored, I'm looking into getting a generator.

What do I need to look at when specing a generator?

I know I will have to calculate how much power is actually being used. I'm not sure how to factor in the UPS units deciding to start charging their batteries back up as soon as the generator kicks in. I'm not certain how to tell what generator+UPS combinations will work properly either.

I also have to choose between diesel and natural gas - what are the pros and cons there?

I'm in Canada, using 120v single phase power, and powering about 1/2 of a 40U rack of servers (for which I will of course need to calculate the power requirements).

What other considerations are there when looking at generators? I want to ensure everything keeps working smoothly even in a long power outage.

  • Would natural gas include liquid propane?
    – Greg Askew
    Jun 25, 2013 at 1:22

2 Answers 2


First of you need to calculate Day 1 power requirements, and then future power requirements. So if your current Rack takes 30Amps at 120V now, but you might need 60Amps in the future, your Load would be around 7.2KW (this is a simplification as it isn't taking Power Factor into consideration). You will normally want to spec that to be around 80% of the Peak load of the generators.

As for Natural Gas vs Diesel. Do you current have a source of natural gas, or would you be using Tanks? What is the availability of natural gas in the case of a long outage? IS there a possibility that your city/municipality can shut off the gas feed? the pro is that if you have a feed, you don't need to fill the tank. The con is that if they do shut off the feed your are pretty much SOL (hard to go to store and get Nat Gas Tank in a pinch)..

IN the case of diesel, you will need to continue to fill the tank ever X hours (Where X is determined by the size of the tank). With Diesel, in the case of an extended outage that is going to go past your run tank, you need to make sure that your supplier is able to pump (since the pumps require power and and if your out for a long period of time, they might be as well).

Also you would potentially want to look into an ATS (Auto Transfer Switch), so that the generator will start automatically and switch over to it without you having to manually do anything. In this case you will only need say 10 seconds or so of UPS power, so charging the batter back up won't be too bad.

You haven't mentioned your cooling. If you lose power, and but continue to run the rack on generator, what about cooling the room. As teh room gets hotter the fans in the equipment will run faster, and draw more power. Doesn't help to have everything powered up if is just going to melt down in the case of an extended outage.


How much fuel does it use per hour? Where are you going to store that fuel? Who will deliver the fuel in the event of an emergency? How many of their customers are in line before you when the natural disaster of your region happens? How often will you test it's operation? Will you test it under full load? Will you be using the live load for that? How long can the generator run before it needs maintenance?

And finally: will an on site generator be cheaper than housing those servers in a quality data center?

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