Don is right. Heat is wasted electricity. As for efficiency it all depends on the I**R (current squared X Resistance) = Watts or heat. You can play games by cutting the wire size for 240 vs 120 wire runs and this can negate any I**R savings but the one-time cost of the wire run will be less. However using a lower resistance run for a 240 application will reduce the recurring cost of wasted heat. Another reason for using 240 on residential high current devices is to balance the load on the legs from the pole transformer.
Al