**approach. This problem is especially apparent within the literature using expenditure-based approaches.** Often, even if researchers use the same metric, approaches on how to construct this metric deviate; this problem is especially apparent within the literature relying on expenditure data focusing on the affordability aspect of energy poverty. We identify the following inconsistencies in the literature:

- **On scaling energy expenditure and measuring energy costs:** While some scholars divide energy expenditure by adult equivalents, others scale it by the number of household members (Schulte and Heidl, 2016) or the OECD equivalence scale (Hills, 2012). A third possibility is to use absolute values altogether (Thema and Vondung, 2020). The rationale behind this debate is that residential energy use is subject to important economies of scale (Schulte and Heidl, 2016). There is no consensus in the literature on how to address this point. Moreover, different researchers consider different components of energy costs. For example, one could include transport-related energy costs or abstract from them. Finally, there is debate on whether theoretical energy needs for a household with certain characteristics at current prices or actual energy expenditures reported by households should be used.

- **On using household income or expenditure:** In addition, there is variation in the literature on defining the denominator of the energy spending share. Several papers relate energy expenditure to household income (e.g., Thema and Vondung (2020); Hills (2012); Saghir (2005)) while others relate it to household expenditure (e.g., Freund and Wallich (1995); Schulte and Heindl (2012); Laderchi et al. (2013)). At the same time, several measures abstract from scaling household energy spending altogether and use absolute energy costs (Thema and Vondung, 2020). When using income as the denominator, there is again an inconsistency with respect to whether to consider disposable income before or after housing costs (Herrero, 2017).<sup>24</sup>

- **The threshold used to define energy poverty.** Lastly, there is no consensus in the literature to date on the threshold one should use to define those who are energy poor. While energy poverty is multidimensional, in practice, most definitions of energy poverty specify a resource threshold in terms of a maximum acceptable proportion of household income devoted to energy consumption. The threshold can be based on an absolute or relative measure. In an absolute measure, the threshold is a fixed percentage of the income/expenditure spent on energy, whereas a relative threshold is based on a median or average energy burden. The most used thresholds are the 10-percent measure, the twice median (M2) threshold, the half median (M/2) threshold, and the Low-Income High Costs (LIHC) measure (Thomson et al., 2022). In some cases, researchers and policymakers also use thresholds for subgroups of the population, such as households with a similar composition or the same number of rooms, or might apply several thresholds simultaneously, such as a threshold for energy expenditure and income (King Baudouin Foundation, 2018).

- **National vs. subnational threshold.** Using a consistent nationwide threshold may also be problematic, as it overlooks the variations in energy requirements between households living in different regions. Using the same threshold can lead to lower energy poverty rates in warmer zones and higher rates in colder zones. The rationale behind this pattern lies in the higher energy needs for domestic heating services prevalent in colder regions, thus leading to increased domestic energy usage. Conversely, individuals residing in warmer regions exhibit lower heating energy needs, resulting in decreased domestic energy usage. Consequently, applying a uniform national threshold indicates a higher likelihood of surpassing the threshold

> 24 For example, the Energy Poverty Barometer developed in the case of Belgium relates energy expenditure to disposable income after deduction of housing costs (King Baudouin Foundation, 2018).

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