Can you chain two UPSes together? No
Everybody probably has this thought at some point: I have two UPSes rated for 20 minutes of battery time; can I plug them into each other and get 40 minutes of power? The short answer is ‘no’. It’s just not a good idea, and not just because it voids your warranty. Here’s why.
The power that a UPS draws is significant. This is because in addition to getting power to your devices, it has to charge the battery. Hence, you’ll actually be drawing significantly more power through the UPS than if you were just running the devices. This means that you’re going to kill the power rating of the first UPS. Would this be more than just the first UPS? Possibly, if the first UPS could handle that power drain, but it also ignores some other things.
Consider, for example, the electricity output. Electricity curves coming from the power company are a nice smooth sine wave. This is what your devices expect. UPSes do not — their curve approximates a sine wave using steps. So instead of rolling your computer down the handicapped ramp, you’re pushing it down the stairs. This is okay for the short term, but it’s considered “dirty” power, in the sense that it’s not reliable and smooth — there’s a lot of harsh fluctuation.
Enter surge protectors and UPSes. Being that they’re designed to cover dirty power issues such as brownouts, how would they handle actually receiving power from the first UPS? The answer is probably not very well, because it’s not clear how they would respond to the dirty power. They might not respond to the power, or they might be damaged by the stairstepping curve. (Incidentally, they do make UPSes with smooth sine wave curves, but they’re pretty rare and expensive.)
So, it’s a bad idea, don’t do it. If you want to use two UPSes, then split equipment between them — run the monitor on one, and the computer on the other, and don’t power protect items such as scanners and printers. Just don’t expect to add the UPSes together for double the time — it isn’t going to happen.
