This study seeks to answer a simple question: will we have enough renewable electricity to meet all of the EU's decarbonisation objectives, and, if not, what should be the priorities and how to address the remaining needs for energy towards carbon neutrality? Indeed, if not, the policy push for green hydrogen would not be covered by enough green electricity to match the “energy efficiency and electrification first” approach outlined in the system integration communication, and a prioritization of green electricity uses complemented by other solutions (import of green electricity or sustainable fuels, CCS...) would be advisable [1]. On one hand, we show that the principle “Energy efficiency and electrification first” results in an electricity demand which will be very difficult to satisfy domestically with renewable energy. On the other hand, green hydrogen and other sustainable fuels will be needed for a carbon neutral industry, for the replacement of the fuel for aviation and navigation, and as strategic green energy reserves. The detailed modelling of these interactions is challenging, given the large uncertainties on technology and infrastructure development. Therefore, we offer a “15 mins” decarbonization scenario based on general and transparent technical considerations and very straightforward “back-of-envelope” calculations. This working paper contains the calculations and assumptions in support of the accompanying policy brief with the same title, which focuses instead on the main take-aways.