I have not read all the comments but here is my take based only on personal experience.
i run my gears ( s9s , L3s, D3s, Z9 minis ) on very unstable voltage, it moves to as high as 235v ( very seldom) and drops to as low as (190v), the average is 220v but it's rarely stable.
never really had an issue with PSU , so far only 1 failed.
the funny part is this > i do have most of the miners run on a voltage regulator , the psu that failed was running on it, so i would only assume that psu would die either ways.
i use a regulator that looks pretty much like this but different brand >
https://www.alibaba.com/product-detail/Automatic-voltage-stabilizer-5KVA-20KVA-single_62000979590.html?s=pmy set up is 3 phase, so i have 1 on each phase, they are supposed to do 10kw but since i don't trust these cheap manufacturers i only use like 7-8kw , and they regulate at range of 180v to 240v , they always try to keep it flat at 220v.
but since i have more miners than what my 3 regulators can handle, i run some on unregulated voltage that goes from 190v to 235v as mentioned above.
never really had an issue, since all these psu are apws that suppose to run on 100-264V as stated by bitmain.
now the S15 however is a whole different story, if you look here >
https://shop.bitmain.com/product/detail?pid=00020190222170435964U2e2qb490657you can see the min voltage is 200v while the max is 240v, so anything out of this range will probably damage the s15 psu,
they also state this on the s15 specs page.
4. Please kindly note that the required input voltage for S15 is 220V.
also this in red
(1-3) Caution: Wrong input voltage may probably cause miner damaged
this means that they have probably tried those PSUs out of that range and burned them, they never really state such an exact voltage on any of the other miners, so i would be very careful using these S15s on anything that is not flat 220v.
* UPS is a big no > the cost is super high, why by expensive batteries that you never really going to use ??
* the voltage regulators like that one i bought cost me about 500$ each , i can't recall exactly those are bought long time ago.
there are also 20kw and 30kw that will surely cost cheaper as in price per regulated kw, but not sure about the shipping and stuff, you can't ship those using airfreight they are heavy as f, so you would want to use sea freight or try to buy them locally.
* keep in mind that there is a good amount of loss in power when using these regulators , that can vary. so DYOR
* also there are many scammers on alibaba and aliexpress so be extra careful.