# Determining UPS power rating to use for a SME data centre

I'm doing this the first time.

I'm trying to understand what power rating of a UPS to use, the scenarios is that I will have 4 Dell PowerEdge R440 rack servers, each server has a Dual Hot Plug Redundant Power Supply (1+1) 550W. Furthermore, I need to add UPS with redundancy to this.

1. Am I correct in understanding that the 550W refers to the power the device uses at any given time and NOT per hour ?

I have been seeing UPS from various manufacturers, I have used their calculators as well.

1. One UPS site mentioned Full Load Runtime of UPS (5 min, 1920W) what does this mean, is this the total Watt power it can provide at any given time, if yes, So in my case each server is 550W, meaning that 550 x 4 (+2 for future growth) servers = 6600W in total, so am I understanding correct that I need to look for a UPS that has a power rating higher than 6600W per UPS (2 for redundancy), that will have a 10 minutes of backup time to allow for equipment shutdown time ?.

And also where does a PSU come into the picture, is it needed, I know that PSU converts AC to regulated DC, but is it needed for my scenario ?

• for 1. Yes. A watt is an amount of power used at a point in time. A watt hour is the average power drawn over an hour. – davidgo Jan 19 at 7:57
• For 2 - theoretically, but generally dual power supplies are there for redundancy purpisrs, and will not draw anuthing close to double the power, ad computers only draw the power they need. To power 4 x 550 watt units you need something which has a kwh rating greater then 550x6= 3300kwh or better. (make sure you look at kwh and not va ratings). You should buy 2 of these units for redundancy, and feed one PSU from each pc on each UPS. – davidgo Jan 19 at 8:03
• The full runtime rating is seperate from thr kwh rating, and depends on the batteries. It is often possible to get additional battery banks. – davidgo Jan 19 at 8:06
• A couple of other considerations - generally Online UPS's are more expensive to buy and run thrn line interactive ones but offer better protection and componentry. In a mission critical environment this is what you want. Oversizing the Kwh rating is also a good idea if you can afford it as larger rated units need heavier duty components and are more robust. They will often also have larger battery packs which will give you more runtime for your stated load. – davidgo Jan 19 at 8:10
• IMHO this question is not asking "what to buy" but is instead asking "how to determine what to buy" (I'm trying to understand what power rating of a UPS to use). I'm voting to reopen. – Twisty Impersonator Jan 19 at 10:48