@ op I have 1 stabilizer was not cheap it was rated to run 2 s15's
it does keep volts at 220 with 228 to 246 in it is really close to 220 out.
it was poorly wired and will never run 2 s15's
it runs 1 s15 fine but cost was about 180 Usd maybe 162 with a coupon
a link
https://www.ebay.com/itm/Norstar-5000-Watt-Step-UP-and-Down-Voltatge-Regulator-Transformer-Converter/332955089461?since the unit works for 1 s15 well I found this one price was 121 with an offer
it claims to do 4000 watts which would be 2 s15's
it has not arrived.
https://www.ebay.com/itm/Norstar-4000-Watt-Step-Up-and-Down-Voltage-Transformer-with-Voltage-Regulator/332607784072?if it is better built and does 2 s15's correctly it would only be about 61 or 62 an s15
that would be ballpark okay if you have power doing 243 to 252 a lot.
unit comes tomorrow.
Now only 2 or 3 psu's on s15's have been reported to die on this website.
assuming they are true reports the input volts were 243-246 for sure maybe higher.
this is a very small data sample.
I am far more certain that the power protection unit with the lcd screen won't do 2 s15's
since 14 gauge wire is not good enough to do over 3000 watts
I am less then certain that s15 psu's blowup and die in the 243-245 volt range.
I will post info on the new protection unit here tomorrow .
but dust humidity and of course room temps are more critical then power exactly 220 volts
in a world of perfection.
the mine would be solar and wind powered at 1 cent a kwatt
dust filter
fan cooled to 25c or 77f
humidity at 35%
stable power to psu at 220 volts
psu supplying a constant correct dc value
psu under a 50-60% load
ac under a constant 75% load
all wires one gauge better than needed.
all of the above at cheap cost.
but world is not perfect.
so all of the above needs to be selected to get best results at best price.
paying 60 dollars to protect one s15 psu is at best an okay choice
I rather pay 60 dollars to keep 5 s15's at a good temperature range
or pay 60 dollars to manage the dust for 10 s15's
or pay 60 dollars to manage the humidity for 3 s15's