But actually you raise a great question.

First, your PUE assumption is absolutely correct – our PUE today hovers around 2.0 and if our UPS usage was at maximum, yes, we would need a generator solution of 1300 kVA.

However, there are a couple of assumptions that you didn’t take into account.

First, counting the total load on our redundant UPS systems and the building mechanical load, we are below 500 kVA in the total power to run the facility and our present data room floor is approaching 100% occupancy. At today’s load capacity, one of our two 750 kVA generators will easily maintain power to the entire facility. So, with the expected power requirements to support our customers, we are actually more like 2N+ in available back up power.

Second, when we built the facility, we designed in scalable components that would allow us to maintain the N+1 generator approach. So we already have the paralleling switch gear sized to accommodate four total generators with all the panel boards, breakers, and conduit infrastructure in place.

This gear is a little unusual in a commercial facility, but it allows us to run multiple generators as a single utility feed to the facility and the Kohler systems provides us with a very clean interface to manage their operation.

And we’ve already prepped the pads and conduit in the generator yard to accommodate the additional generators we’ll be adding to the facility.

And as an extra added bonus, the Kohler switch gear controls our on-site load bank which allows us to test these generators (no matter how many we have) with varying load without having to bring in expensive portable load banks or (horrors!) put the data room on generator just to test their ability to handle load (yes, we did hear that someone was doing that).

So, as we look at going forward, one of our main planning issues is not determining if, but when we need to bring in the additional generators to support increasing load in the facility. If growth dictates, we’ll end up with 3000 kW of generator capacity which would support a UPS load of about 1300-1500 kVA, depending upon the PUE and other factors as you rightly point out. But suffice it to say, regardless of how our customer power needs grow, we’ll never get ourselves in any less than an N+1 situation.

If you have a 750KW generator with a 650KVA UPS? In the middle of the summer, you need to allow for an operating PUE of 2.0 or higher to handle the loss in the UPSs & HVAC load – which means that you need 1300 KVA to operate the data center. Sounds like you need both generators to run the data center at load. That’s N, not N+1 on the generators.