Let me put it this way .... when I converted an office building from T-12's with conventional ballasts, to T'8's with electronic ballasts .... the math gave me a very conservative number! IIRC, I was expecting savings of about $250/ month .... while the actual saving was nearly $400/mo.
Actual amp readings, before and after, showed about a 35% reduction in current drawn. This is not considering the lesser load on the air conditioning, due to the lower operating temperature.
I was changing things 'bulb for bulb.' If you are replacing 4 lamp fixtures with 3 lamp fixtures, expect further savings.