A founding assumption and aim of the sociotechnical approach to sustainability transitions was the need to develop frameworks to understand major systemic changes that would be required across the entire chain of production, distribution and consumption. However, most studies have so far focused on partial aspects of the entire chain, often a single, radical technology innovation. Therefore, since the founding ambition remains largely unrealized, the paper aims to contribute to transition scholarship by developing an approach for ‘whole-system’ analysis. As a second contribution, we argue that this broader unit of analysis calls for greater attention to the architecture of the system in terms of how constituent elements are linked to one another. To elaborate this point, we develop a reconfiguration approach, based on conceptual extensions to the multi-level perspective, analysing both techno-economic developments and socio-institutional developments. This approach draws attention to the multiplicity and interdependencies of change processes that constitute transitions, including incremental change, component substitution, symbiotic add-ons, knock-on effects and changes to the system architecture. A third contribution is to make an empirical whole-system analysis of the low-carbon reconfiguration of the UK electricity system between 1990 and 2016. This is important and timely, because it allows socio-technical transition approaches to ‘speak’ at the same empirical whole-system level that dominates current long-term, low-carbon (modelling) analysis and associated political and public debate. This consequently enables a demonstration of the added value of the whole-system reconfiguration approach. Our findings show that early reconfiguration of the UK electricity system was dominated by modular changes within the generation and consumption subsystems; and more recently, how these earlier changes have triggered a new focus on the whole system architecture, anticipating deeper changes to the linkages between the generation, network and consumption subsystems.