## 1 Introduction  
The Paris Agreement, which pledges to keep global warming well below 2 degrees Celsius above pre-industrial levels and to limit the increase to 1.5 degrees Celsius, needs a boost [1]. The highest emitting 100 cities, or so-called urban areas, account for 18% of the global carbon footprint [2, 3]. Therefore, cities are increasingly focusing on and shaping the trajectory and impacts of climate change and air quality [4, 5, 6, 7, 8, 9]. The C40 Cities Climate Leadership Group connects more than 90 of the world's largest cities, representing over 650 million people and one-quarter of the global economy [10]. C40 is focused on tackling climate change and driving urban action that reduces greenhouse gas emissions and climate risks. More than 54% of the world's population lives in urban areas (cities, towns, or suburbs) [11]; in Europe, this is almost 75% [12]. Energy consumption is growing rapidly in urban areas [7]. A smart, integrated, and combined centralized and decentralized approach is essential for creating sustainable urban energy systems [12; 13; 14; 15; 16]. By coupling energy sectors through electrification and hydrogen [17; 18; 19; 20], major problems related to the intermittent nature of many renewables, such as wind and solar, can be solved, and synergies benefiting all sectors can be created [21; 22; 23; 24; 25; 26]. Both the Hydrogen Council and the World Energy Council support and leverage the enabling role of hydrogen and fuel cell solutions around the world [27; 28]. Inspired by the concept of a \"Hydrogen Economy\" [29; 30; 31; 32; 33; 34; 35], the authors designed a 100% renewable, reliable, and cost-effective energy system for power, heat, and transportation for smart urban areas in Europe [36]. The system covers the annual energy consumption of the main energy functions in urban areas, namely road transportation and, in residential and services buildings, space heating and cooling, hot water, lighting, and electrical appliances. The heating and transportation system is all-electric in its final energy use. Heating is by means of electric powered heat pumps and transportation by hydrogen fuel cell-powered electric vehicles; no other technologies are used for these applications. Local solar and large-scale wind electricity provide all renewable energy, together with hydrogen and electricity, as intermediate energy carriers. Fuel cell electric vehicles (FCEVs) provide transportation and energy distribution and balance the intermittent solar and wind electricity production by converting renewable hydrogen into electricity. This concept of grid-connected FCEVs providing grid services when parked--also known as vehicle-to-grid (V2G)--has already been demonstrated on a small scale with one V2G-ready commercial Hyundai ix35 FCEV and an all-electric house [37; 38]. FCEVs providing power to electric appliances (also referred to as vehicle-to-load, V2L), small grids, or homes (vehicle-to-home, V2H) [39] are being developed by several FCEV manufacturers [40; 41; 42; 43], although none of them have reported connecting an FCEV to a low-voltage national AC grid. European regions have different climatic conditions [44] (including supplement of [44]), which have an impact on the energy consumption of buildings [45; 46; 47], especially for space heating and cooling [48; 49; 50; 51; 52]. In addition, the different average building and household types, sizes, and compositions in European countries also impact the energy consumption in buildings [53; 54; 55]. Vehicle ownership and the average number of kilometers driven per year determine the final road transportation energy consumption, which varies among European countries [56; 57]. The regional availability and magnitude of solar and wind energy differ significantly across Europe [58; 59; 60; 61; 62]. Wind and solar power generation across European regions exhibits hourly, diurnal, and strong seasonal behavior [63; 64], as well as intra-annual [65; 66; 67] or decadal/multi-decadal variability [68; 69; 70; 71; 72].