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Figure 9.21 | Incremental capital expenditure on energy efficiency investment (left) and renewable heat in buildings, 2015–2021. Notes: (i) An energy efficiency investment is defined as the incremental spending on new energy-efficient equipment or the full cost of refurbishments that reduce energy use. (ii) Renewable heat for end-use include solar thermal applications (for district, space, and water heating), bioenergy and geothermal energy, as well as heat pumps. (iii) The investment in 2021 is an estimate. Source: IEA 2021b.