As Hans said, manual defrost fridges are cheaper to run. But some equivalent manual-defrost and frost-free fridges share most of their parts including their compressor. The extra energy needed is for the blower motor(s), the defrost timer, the defrost heaters (which are on for short periods) and what accounts for the biggest difference is the extra work that's required from the refrigerating system that has to run constantly for about two hours to balance the temperature after the defrost periods.
However, newer frost-free fridges beat both older types as they have much more efficient components. But many seem to lack the long term durability of older units.
