Investment by energy firms in innovation can have substantial economic and environmental impacts and benefits. Internal R&D is the main input and driver of the innovation process, but innovation involves other activities, including capital purchases and other current expenditures. While the R&D activities of energy firms have been analysed, few studies have examined the typology of their innovation activities. Here, we analyse the impact of the main characteristics of the sector’s firms on their decisions to invest in each of three types of innovation activity: namely internal R&D; external R&D; and, the acquisition of advanced machinery, equipment or software. In conducting this analysis, we take the potential persistence of innovation activities into account. We also examine the role that different innovation objectives have on firms’ investment decisions. Given that engagement in a specific type of innovation may result from decisions that are not taken independently of each other, we analyse whether there is any complementarity between the three innovation activities. In carrying out the empirical analysis, we draw on data for private energy firms included in the Technological Innovation Panel (PITEC) for Spanish firms for the period 2004- 2013. We use panel triprobit models to examine potential complementarity.