In the coming renewables-based energy transition, global electricity consumption is expected to double by 2050, entailing widespread end-use electrification, with significant impacts on energy efficiency. We develop a long-run, worldwide societal exergy analysis focused on electricity. Our 1900–2017 electricity world database contains the energy carriers used in electricity production, final end-uses, and efficiencies. We find world primary-to-final exergy (i.e. conversion) efficiency increased rapidly from 1900 (6%) to 1980 (39%), slowing to 43% in 2017 as power station generation technology matured. Next, despite technological evolution, final-to-useful end-use efficiency was surprisingly constant (∼48%), due to “efficiency dilution”, wherein individual end-use efficiency gains are offset by increasing uptake of less efficient end uses. Future electricity efficiency therefore depends on the shares of high efficiency (e.g. electrified transport) and low efficiency (e.g. cooling and low temperature heating) end uses. Our results reveal past conversion efficiency increases (carbon intensity of electricity production reduced from 5.23 kgCO2/kWh in 1900 to 0.49 kgCO2/kWh in 2017) did little to decrease global electricity-based CO2 emissions, which rose 380-fold. The historical slow-pace of transition in generation mix and the need to electrify end-uses suggest that strong incentives are needed to meet climate goals.