I agree with all of the above, it is a very common misconception of soft starters, often foisted by schlockey sleasmen who can't sell their product on other merits. I'd say it is probably over 99% of utilities that use the sliding demand window measurement method. Soft starting will not help in that case.
I once did a survey on behalf of a soft starter manufacturer of all the utilities I could find across the country, hunting for the ever elusive "instantaneous ratcheting demand meter". They do exist, usually only in small rural power coops that buy their power in bulk from other producers. In my search some 10+ years ago, I found only 11 in the entire country. Most of them that I checked on recently have stopped doing that in favor of the sliding window method, but there is still one in California that I know of, the Turlock Irrigation District. You turn on a motor, the red needle goes up and stays there until the meter reader resets it each month.
I should mention though that there are plenty of other very legitimate benefits to soft starting. Just not demand charge savings directly. But if by having a oft starter on a motor reduces the concern of and objection to restarting, users are often more inclined to turn un-used motors off. the cumulative effect of this is that power consumption is reduced, and so is the demand! I have done PLC control systems to reduce demand by cycling motors on and off without fear of motor damage because they had soft starters on them. Worked great!
