Reverse cycle air‐source heat‐pumps are an increasingly significant load in New Zealand and in many other countries. This has raised concern over the impact wide‐spread use of heat‐pumps may have on the grid. The characteristics of the loads connected to the power system are changing because of heat‐pumps. Their performance during under‐voltage events such as voltage dips has the potential to compound the event and possibly cause voltage collapse. In this study, results from testing six heat‐pumps are presented to assess their performance at various voltages and hence their impact on voltage stability.