An increasing number of Danish local governments use green web applications, which are websites intended to convince users to reduce energy consumption in order to mitigate climate change. However, to date, no empirical evidence has been published that demonstrates whether green web applications have this intended effect. This paper, based on a survey of 329 Danish small and medium enterprises (SMEs), examines the perceived effects of a green web application named klimabevidst.dk (which can be translated as climateawareness.dk) and explores the motivation of SMEs to use the application. As 29–56 % of SMEs declare that the application has had a medium or large energy reduction effect on their businesses, the paper concludes that klimebevidst.dk can be a relevant component in local government climate change mitigation campaigns aimed at small- and medium-sized enterprises (SMEs). This paper also shows that the perceived effects of using the web application correlate strongly with the extent to which the web application is used (i.e. with the number of the application’s energy reduction guides read). Moreover, the perceived effect is larger in smaller SMEs, in SMEs that prioritise climate change and the health of their employees, and in SMEs that do not prioritise economic benefits. The present study is, however, unable to quantify the reduction; further study is needed to produce a more refined picture of the role played by the web application in increasing energy efficiency.