The water and wastewater sectors of England and Wales (E&W) are energy-intensive. Although E&W’s water sector is of international interest, in particular due to the early experience with privatisation, for the time being, few published data on energy usage exist. We analysed telemetry energy-use data from Thames Water Utilities Ltd. (TWUL), the largest water and wastewater company in the UK, which serves one of the largest mega-cities in the world, London. In our analysis, we: (1) break down energy use into their components; (2) present a statistical approach to handling seasonal and random cycles in data; and (3) derive energy-intensity (kWh m⁻³) metrics and compare them with other regions in the world. We show that electricity use in the sector grew by around 10.8 ± 0.4% year⁻¹ as the utility coped with growing demands and stormwater flooding. The energy-intensity of water services in each of the utility’s service zone was measured in the range 0.46–0.92 kWh m⁻³. Plans to improve the efficiency of the system could yield benefits in lower energy-intensity, but the overall energy saving would be temporary as external pressures from population and climate change are driving up water and energy use.