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That's still not saying much, unfortunately — you are going with "authoritative" vs "informative" argumentation ("trust me" instead of "this is why"). Explaini

That's still not saying much, unfortunately — you are going with "authoritative" vs "informative" argumentation ("trust me" instead of "this is why"). Explaining what are the conditions that need to be met for one UPS not to cause issues to another daisy chained off of it would help the reader understand why that's a bad idea.

zootboy 3y ago
The main technical reason is that the vast majority of UPSes cannot take what they give. If you have a 1000 VA UPS that is loaded at or near 1000 VA, it will draw significantly more than 1000 VA during periods of recharging, or if you make use of the non-protected outlets on certain models, or if the model is a double-conversion type. Thus, daisy-chaining two identically-sized UPSes will never work; they always need more input power capacity than they can output. Then there's the issue of output waveform. Cheaper UPS models put out a hideous square wave or modified square wave waveform. This is adequate for being fed into a PC's PSU to be turned right back into DC, but most UPS line status detectors will not be happy being fed in such a bad waveform and will consider the grid faulted. Thus, as soon as you have an outage, both UPSes will go into battery mode, and you will not be able to use the "upstream" UPS's capacity. I suspect that even nicer UPSes may have issues accepting the power generated by another UPS, as their voltage and frequency regulation may not be good enough to satisfy the grid stability tests. But the larger part of my reasoning for why you shouldn't daisy chain UPSes is that it's attacking the problem of reliability from the wrong angle. By daisy chaining UPSes, you have added an additional single-point-of-failure to your system. If instead you used a redundant PSU, you would have decreased the number of single-points-of-failure. In my personal experience, I have seen far more power supplies die than I've seen UPSes, so the first thing I would do if I wanted a more reliable power setup is go for the redundant power supply. Only then would I consider attaching multiple UPSes (but I probably wouldn't under normal circumstances).
- Dylan16807 3y ago
  > The main technical reason is that the vast majority of UPSes cannot take what they give. If you have a 1000 VA UPS that is loaded at or near 1000 VA, it will draw significantly more than 1000 VA during periods of recharging, or if you make use of the non-protected outlets on certain models, or if the model is a double-conversion type. Thus, daisy-chaining two identically-sized UPSes will never work; they always need more input power capacity than they can output. It really depends on the model. The UPS I have takes uses barely any watts to recharge over a period of multiple hours.

zootboy 3y ago

The main technical reason is that the vast majority of UPSes cannot take what they give. If you have a 1000 VA UPS that is loaded at or near 1000 VA, it will draw significantly more than 1000 VA during periods of recharging, or if you make use of the non-protected outlets on certain models, or if the model is a double-conversion type. Thus, daisy-chaining two identically-sized UPSes will never work; they always need more input power capacity than they can output. Then there's the issue of output waveform. Cheaper UPS models put out a hideous square wave or modified square wave waveform. This is adequate for being fed into a PC's PSU to be turned right back into DC, but most UPS line status detectors will not be happy being fed in such a bad waveform and will consider the grid faulted. Thus, as soon as you have an outage, both UPSes will go into battery mode, and you will not be able to use the "upstream" UPS's capacity. I suspect that even nicer UPSes may have issues accepting the power generated by another UPS, as their voltage and frequency regulation may not be good enough to satisfy the grid stability tests. But the larger part of my reasoning for why you shouldn't daisy chain UPSes is that it's attacking the problem of reliability from the wrong angle. By daisy chaining UPSes, you have added an additional single-point-of-failure to your system. If instead you used a redundant PSU, you would have decreased the number of single-points-of-failure. In my personal experience, I have seen far more power supplies die than I've seen UPSes, so the first thing I would do if I wanted a more reliable power setup is go for the redundant power supply. Only then would I consider attaching multiple UPSes (but I probably wouldn't under normal circumstances).

Dylan16807 3y ago
> The main technical reason is that the vast majority of UPSes cannot take what they give. If you have a 1000 VA UPS that is loaded at or near 1000 VA, it will draw significantly more than 1000 VA during periods of recharging, or if you make use of the non-protected outlets on certain models, or if the model is a double-conversion type. Thus, daisy-chaining two identically-sized UPSes will never work; they always need more input power capacity than they can output. It really depends on the model. The UPS I have takes uses barely any watts to recharge over a period of multiple hours.