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Sizing a generator to power your UPS backup power

Sometimes, it doesn't make sense to migrate to someone else's datacenter/cloud platform/solution, and instead it makes more sense to "roll your own" solution. If you are going to be using your own datacenter instead of outsourcing it to someone else, you need to make sure your power system is rock solid. You need to make sure your emergency power grid can handle the load. Sizing a generator for your UPS capacity is harder than it looks. You can't just buy a 50kw Generator and 50kw UPS, they have to be sized accordingly. In this post I'll try to help you plan some rough numbers as a starting point. For accurate numbers you should hire a professional and not just use this guide.

Before we start

Before going down this route, I would like to say that going this route is often more expensive than colocating or hosting in a cloud environment. Professional datacenters and cloud providers have economy of scale, so rolling your own solution like this typically isn't cost effective. However if you are in a remote environment or have legal requirements it may mean you have to do this.

How large of a power draw do you expect to have?

First lets look at how much power you think you're going to draw. Most datacenters operate on the 80% rule, meaning that they don't let out more than 80% of the total capacity. This is usually due to equipment drawing more power as it starts up, or to cover spikes in power draw. If you are going to have redundant power from 2 grids, then this number is cut in half, so 40% of the maximum capacity. This is because if one "side" fails it will start to pull from the other. Many devices do not pull power evenly when they have multiple power supplies, sometimes they will draw power only from one side and not the other, it all sort of depends on the device and is more detail than I wish to go into here.

So, if you add up all your devices and it comes to 16kW of power usage, you want to plan for 20kW of total power (80% rule, 40kW if you want N+1). So how does this compare to the size UPS you need?

How large of a UPS?

Unless you aren't going to have any more room for future capacity, I would plan for expansion later. There are several types of UPS's that allow you to add more batteries, add more units, etc. Make sure you have more than just your 20kW (in this example) of power!

How large of a generator?

This is arguably the hardest part. Unless you are in the MW range (megawatt), your generator needs to be on average twice the capacity of your max load. The reason is that the sine wave output needs to be as close to even as possible, and with a big voltage/power drop when the UPS kicks over or your automatic transfer switch drops capacity onto the generator you don't want it to "shudder" and produce a poor waveform, thus triggering your UPS to stay on battery power, and then flipping back and forth. For this example, you should get at least a 40-45kw generator (about equivalent to a 3.5L chevy engine in size with a big alternator on it).

Here's an example of what a sine wave looks like from the power company versus a generator (you need an oscilloscope to see for yourself):

Almost all UPS's come with load/power conditioning these days, but just in case you should confirm that your UPS does, it will help smooth out the sine wave output as shown above.

Sizing chart example:

Load UPS size Generator
Normal - 80% max 16kW 20kW 40kW
N+1 Redundancy - 40% max 8kW 20kW 40kW

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