Abstract
<title>Abstract</title> <p>Photovoltaics can exhibit favourable installation-level energy return on investment (EROI), yet electricity must also be delivered when solar and wind generation are insufficient. This study evaluates how the energy balance changes when the boundary is expanded to reliable electricity supply. A fleet-equivalent benchmark model includes component turnover, grids, storage, renewable overbuild, dispatchable backup, and fuel supply. It is not a simulation, optimization, or real-system prediction. Photovoltaics provide the accounting reference in an illustrative Central European mix of 80% wind and 20% photovoltaics, while the firming penalties arise more generally from weather-dependent generation. Expanding the boundary is decisive. The unfirmed renewable fleet retains an EROI of 10.7-16.0, but does not provide continuous electricity. Battery load-balancing references decline from 7.1-12.5 for 1h to 0.7-2.3 for 24h. Seasonal hydrogen yields 3.4-7.7; adding a 1h battery reduces the combined range to 2.9-6.8. Pipeline-gas and LNG backup yield only 1.2-2.6 and 1.0-2.2. The lifecycle-carbon advantage narrows accordingly. Under present industrial supply chains, GWP100 intensities rise from 38-75 gCO2eq/kWh without firming to 80-289 gCO2eq/kWh for hydrogen and battery-hydrogen firming, and 121-327 gCO2eq/kWh for gas backup. These favourable benchmarks set orientation, site, curtailment, and delivery factors to unity; real deployment lowers EROI and raises emissions. The results identify firming as a critical energetic constraint: reliable decarbonization requires firm low-carbon generation with sufficient net-energy surplus to sustain a resilient industrial society.</p>