How Much UPS Do You Need?

On September 24, 2010, in Computing, Hacks, Hardware, by Anthony Lake

For all of you who find yourself wondering, here’s a basic by-the-seat-of-your-pants rule of thumb for picking an uninterruptible power supply.

1. First, figure out the power consumption of your computers. The best way to do this would be to use a watt-meter. In general, a reasonable rule of thumb is to add up the wattage rating of all the computer’s power supplies and divide it in half.

Odds are, if you’re using only a low end video card and only one hard drive, the required power would probably be around 90 watts… or with a high end card or multiple drives, figure 200 or more watts continuous.

2. You will want to purchase a power supply whose RMS rating is equal to or greater than the number you calculated in step one.  On most power supply brands, multiply the VA rating *.6, or just use the watts rating if it’s actually written on the sales material. For instance, the APC1250LCD says “1250 VA” on the box. Multiplying this number by .6 gives us 750, so we can safely keep 750 watts of servers running – using the rule we applied above.

Here’s an example:

I have three servers, each with a 380 watt power supply. That’s a total of 1140 watts. We’re going to divide that number in half, as per rule (1) above, thus, we’ll need a UPS that can deliver 570 watts RMS, plus we’ve got a monitor, a modem, and a switch that I think I’ll throw in the mix… together, they draw about 100 watts total, so, for a nice safe number we’ll need a UPS that can supply 670 Watts continuous. The APC1250LCD mentioned above will do nicely to power these servers at least long enough to get them a graceful shutdown.

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