We present measurements of the energy relaxation length scale $ell$ in two-dimensional electron gases (2DEGs). A temperature gradient is established in the 2DEG by means of a heating current, and then the elevated electron temperature $T_e$ is estimated by measuring the resultant thermovoltage signal across a pair of deferentially biased bar-gates. We adapt a model by Rojek and K{o}nig [Phys. Rev. B textbf{90}, 115403 (2014)] to analyse the thermovoltage signal and as a result extract $ell$, $T_e$, and the power-law exponent $alpha_i$ for inelastic scattering events in the 2DEG. We show that in high-mobility 2DEGs, $ell$ can attain macroscopic values of several hundred microns, but decreases rapidly as the carrier density $n$ is decreased. Our work demonstrates a versatile low-temperature thermometry scheme, and the results provide important insights into heat transport mechanisms in low-dimensional systems and nanostructures. These insights will be vital for practical design considerations of future nanoelectronic circuits.
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