But then Emily showed me the spreadsheet where she has tracked their energy use, measured in kilowatt hours (kWh). In a recent year before the heat pump installation, the house consumed a total of 28,000 kWh, which would mostly have been gas heating. In the latest year, they used just 10,000, despite having had some adult children move home in that time. In other words, they’ve cut their energy use by almost two-thirds.