Average photoelectric heating efficiency ϵ, equated as cooling efficiency [C II]/FIR, in M42, M43, and NGC 1977, divided into the previously defined subregions, as function of the ionization parameter γ = G₀ T^1∕2∕n_e. OMC1 includes [O I] cooling, but excludes the BN/KL region and Orion S. Parameters for Orion A are given in Table 7. The photoelectric heating efficiency in PDRs and diffuse sight lines from the literature is plotted as open blue squares (cf. Fig. 9 in Paper I). Data for the diffuse ISM are taken from Gry et al. (1992) and van Dishoeck & Black (1986), data of NGC 2023 are from Hollenbach & Tielens (1999), data for the Horsehead and L1630 are from Pabst et al. (2017), data for IC 59 and IC 63 are from Andrews et al. (2018), data for the Orion Bar, S6, M43 west, and OMC2 are from Herrmann et al. (1997), and data for the Orion Veil are from Salas et al. (2019). The red curve is the theoretical prediction of Eq. (43) by Bakes & Tielens (1994).