The answer is energy efficiency.
Energy efficiency was the largest energy source for the world between 1974 and 2010, according to the first ever Energy Efficiency Market Report, published by the International Energy Agency.
The Energy Efficiency Market Report, gives as an example, energy efficiency contributed 63 exajoules (EJ) of avoided energy use in 2010, which is larger than the supply of oil (43 EJ), electricity (22 EJ), or natural gas (22 EJ).
This information was featured in a recent edition of the High Performing Buildings Newsletter. G/BA founder Dave Grumman is on the editorial board for High Performing Buildings Magazine. Dave shared the energy efficiency statistic with G/BA staff members saying, “What a coincidence!” Pointing out that the period of time included in this statistic corresponds with G/BA’s 40 years of providing energy efficiency consulting and engineering. The photo featured here is Dave Grumman opening Enercon, Ltd. (original name of G/BA) in 1973.