Saturday, November 11, 2017
The main problem I'm trying to solve is providing power for the (hopefully short) period of time between the time we lose main power and the time my backup generator kicks in. Generally that's around 30 seconds. A secondary problem I'd like to solve is to mitigate the effects of “dirty power” (brownouts, momentary outages, surges) on the computer equipment in my office. We had an example of a momentary outage last week, and it took me a couple of hours afterwards to get everything back up and running. It was that experience that prompted me to pull the trigger on ordering a UPS.
The one I bought is an “online” type, meaning that it has an inverter that continuously outputs 60 Hz sine wave 120 VAC. That inverter is powered from a battery, and the battery is continuously charged when there's 120 VAC available at its input (in other words, when the power is up). Most UPSs designed for home or small office use are a different type: “standby”. These UPSs only produce power when the main power fails, and there's always a brief “glitch” when the power switches over. Furthermore, standby UPSs aren't very good at dealing with brownouts or intermittent power outages. The datacenters I managed all used online UPSs, as hard experience with the problems caused by standby UPSs long ago convinced datacenter operators to stay away from them.
Another thing I learned while managing datacenters is that you really don't want to run an online UPS anywhere near its rated capacity. Doing so will reduce reliability (not exactly a feature in a UPS!) and, perhaps surprisingly, cause them to make much more noise. The latter is because in order to cool the power electronics (for the inverter, primarily) at maximum capacity, the fans in the unit generally run full blast – but when running well below their capacity, they can (and usually do) run much more slowly.
So I selected a UPS with roughly twice the capacity I actually need (I need 1,500 VA, so I bought one rated at 3,000 VA). Everything I read about it validated the thought that this would reduce noise, and I know enough about electronics to know that it will also increase its lifetime. The particular model I selected is 120 VAC in and 120 VAC out, matching the power I had available at the place I wanted to mount it, and the power I need for my computers.