Developments in both gas and electricity are fast moving with higher risks for stranded assets in Trans-European Networks. This puts increasingly higher demands on the CBA method that is used to select priority investments. Standing still in the development of that method would be going backward. The ABC of the CBA for so-called Projects of Common Interest (PCIs) is about: A. dealing with interactions between PCIs (coordination); B. gaining trust and public acceptance (transparency); and C. deciding where the experts stop and the politics start in the valuation of PCIs (monetisation). To deal with the interactions between PCIs, we recommend additional improvements to the clustering of projects and the baseline definition in the common CBA method; and we also recognise that individual project promoters might lack the information and resources to do this, which is why we suggest that this could become a task for the ENTSOs or Regional Groups instead of the promoters. To gain trust and public acceptance, we recommend harmonised and disaggregated cost and benefit reporting, noting that we still have a long way to go, and noting that this is not even enough because the ambition should be an open source CBA model rather than a common method. To reduce the politics, we emphasise the importance of a full monetisation of the value of PCIs, and note that we could ask the Regional Groups to express their policy priorities at the start of the process via the eligibility criteria, which would also increase the transparency of the process.