There are several articles in this blog comparing MFDC vs AC.
COMPARE MFDC VS AC IN RESISTANCE WELDING?
IS DC OR MFDC BETTER THAN AC IN SPOT WELDING?
They explore the advantages and disadvantages of both systems. AC being the long term tried and true with some applications and MFDC being the new comer that has become the main product used on automated lines.
The referenced articles describe the various advantages and disadvantages of both systems. The question raised here is what is the electrical cost advantage. MFDC heats faster than AC since there is no zero cross over. This faster heat up of course will vary by part and final weld schedule. There is no hard value that can be given but the breaker can generally be reduced 20% for MFDC installations vs AC..
Other savings in quality and good part production per shift will generate a much higher value in savings.
ADVANTAGES & DISADVANTAGES OF MFDC AND AC
Reference: RWMA – Resistance Welding Manual 4th Edition
AWS – Welding Journal, Q & A July 2019