Municipalities play a key role in transitioning to a climate neutral and 100% renewable energy system. Although alternative energy system models, feasibility analyses and strategic energy plans are being developed, municipalities lack the know-how, funding, authority or political demand needed to implement the plans. Collaborative design and implementation of energy plans through cross-sectoral networks could help overcome some of these barriers. However, there is little empirical evidence of how collaboration could be sustained during implementation and how it could be institutionalised in a permanent governance network, resulting in a perception that collaboration is 'all talk and no action'. This paper presents a comparative case study of two municipal energy governance networks in Denmark with two different institutional designs: the municipality-led Energi2020 network in Ringkøbing-Skjern and the public–private network administrative organisation ProjectZero in Sønderborg, in which the municipality participates as a partner. It was found that institutional design influences the ability to mobilise private funding, the access to municipal resources and the degree of political control. In addition, it has important consequences for the design and evaluation of energy plans, their outputs and outcomes and the accountability for their implementation. As a result, Ringkøbing-Skjern has been more successful in renewable energy installation, while Sønderborg has been more successful in improving industrial energy efficiency and sector coupling. It is concluded that ultimately, the two approaches need to be combined to develop a smart energy system that relies on 100% renewable energy, energy efficiency and sector coupling. [Display omitted] • Frontrunner municipalities implement energy plans through institutionalised collaboration in cross-sectoral networks. • Collaboration can be institutionalized in a municipality-led network or a network administrative organization. • Municipality-led networks can build on municipal resources but are also politically controlled. • Network administrative organisations can mobilise more private funding and are less vulnerable to political change. • Institutional differences affect the design and implementation of energy plans and, eventually, the energy transition. [ABSTRACT FROM AUTHOR]