This paper presents a comparison of the performance of an online inertia estimation method for three different C37.118 compliant PMUs in a laboratory. Inertia is a critical part of ensuring stable system frequency and improving operator awareness of inertia will help preserve high quality frequency control in power systems with high levels of wind generation and converter interfaced generation. This study is necessary as inertia estimation is based on active power and rate of change of frequency measurements recorded immediately after a disturbance. Therefore, the transition time and inherent difficulty of estimating derivatives may mean that not all PMUs can support inertia estimation. Indeed, the results presented show that not all PMUs are capable of supporting inertia estimation. Therefore, it must be acknowledged that the installation of a PMU at a bus does not necessarily imply that it will be possible to perform inertia estimation at that bus.