We tested the influence of a private commitment strategy, in which people pledge to change their behaviour, on energy saving behaviour. We found that the private commitment only influenced energy saving behaviour when the behaviour was perceived to be relatively effortful. When people found it easy to engage in the behaviour, the private commitment did not promote energy saving behaviour. Importantly, we tested the underlying mechanism why private commitments may influence energy saving in households. Our results show that when behaviours are perceived to be relatively effortful, the private commitment strengthened people's personal norm to engage in the behaviour. That is, after making a private commitment they felt more morally obliged to engage in the behaviour they committed to. In turn, a stronger personal norm was positively related to energy saving behaviour. People's injunctive norms and environmental self-identity did not explain why making a private commitment changed energy saving behaviour when this behaviour is perceived to be relatively effortful. Our findings contribute to the literature by providing more insight into why and under which circumstances private commitments may influence behaviour. Our results suggest that only when people find the behaviour somewhat effortful a private commitment may increase their personal norm to engage in the behaviour, thereby making it more likely that they actually do so.