The Basel city lab differed slightly from the other SONNET city labs, as it involved a collaboration between the state-owned local energy utility and the SONNET academic partner ZHAW. While the involvement of the city administration supported the initial set-up and scoping of projects with the academic partner, they were subsequently less directly involved. Nonetheless, the city lab, involving the local energy utility as the “city partner” successfully developed and tested a social innovation in energy (SIE). The SIE in focus was a smartphone app designed to encourage reducing an individual’s CO2-footprint. To conduct the outcomes evaluation and to obtain reliable data on the causal impact of the app on users’ knowledge about their personal CO2-footprint, beliefs and norms concerning climate change and their personal CO2-emissions, as well as on their environmentally relevant practices and behaviours, we conducted a field experiment, or randomized controlled trial (RCT), with around 500 volunteer participants over the course of two months from mid-October to mid-December 2020.