China Mobile identified energy efficiency as a major problem for 5G. Massive MIMO - 4G or 5G - is a major energy saver. The MMIMO advances are not quite enough to keep up with expanding usage but important.
China Mobile estimates each upgraded cell requires 50% more power. When you are installing 2 million cells, that adds up. (China plans 600,000-800,000 5G cells in 2020.) I've seen higher estimates.
Of course, each cell carries three to ten times as much data so energy and cost per bit are much lower. The 5G standard aims for a 90% reduction of energy per bit, although I don't believe that has been achieved outside the laboratory.
With an infinite number of antennas and other requirements rarely met in the field, interference can be reduced to very little.