The plant air compressor is a high volume, 4 cylinder, 60 gallon Quincy with a 3 phase 208 volt motor. The compressor itself is a "V" with two cylinders on each bank. Both banks feed a common tube going to the tank. It pumps up to 125PSI. The HP is not listed on the motor. It does not trigger a demand reading on the meter.
Indeed...generally it's for a period of 15 min or even 30; not inrush of a single (small--yes, for commercial loads 10 hp is a _very_ small load, iggy, even though for those who are just used to household and hand tools it does seem sizable :) ).
Here's a good description of light commercial general billing practices in general; iggy needs to call and find out the details of his particular utility billing but as said earlier it's virtually certain he doesn't have enough loads available to come even close to triggering the demand billing penalty (even if he tried his darnedest to do so! :) )
He's worrying over a non-problem at this point almost certainly.
Most demand metering is based on something like a 15 minute demand period. That is: The real power consumption is integrated over a 15 minute period. Verify the details with your local utility, of course.
Efficiency aside, it will take about the same real power to accelerate a motor with either a line start or soft start system.
You can reduce the inrush current peak (which is a combination of real and reactive power), but you'll end up extending the duration. So with a 15 minute integrating period, it'll just be a wash anyway.
Correction: That should be 'same energy'. Power integrated over time. So, unless the motor takes appreciably longer than 15 minutes (or whatever the demand interval for billing is) to accelerate, the improvement will be small.