The increasing production of electricity worldwide, which is necessary to address building and transportation electrification, must be supported by the further expansion of smart grids [
1]. As a result, research on smart micro-grids established in nearly Zero Energy Buildings (nZEBs) has intensified [
2]. A nearly zero-emission building is very efficient, with its minimal energy needs usually covered by renewable sources [
3]. Micro-grids in nZEBs are increasingly designed to be self-sufficient, optimally controlling their grid transactions to safeguard their stability. The current European standards limit the net annual primary energy infusion to 20–30 kWh/m
2y [
4]. These buildings are characterized by an insulated shell and by smart features within their controls. Heating and cooling is implemented by high-efficiency heat pumps. Electricity is usually produced by photovoltaic installations. However, these sources are intermittent, opening spatial and temporal gaps between the availability of energy and its consumption by end-users. To address these issues, it is necessary to develop suitable energy storage systems for the power grid [
5]. The EPBD Directive [
6] mandates all new buildings to be nZEBs from 2021 onwards. A shift to zero-emission buildings (ZEBs) is projected to be achieved by 2030 [
7]. Energy performance optimization is attained by high shell insulation, efficient lighting and advanced heat pump technology [
8]. This concept results in a significant surplus of PV electricity around afternoon hours. A recent study on a campus building expanded the measured annual hourly load profiles with a data augmentation method [
2] and proposed an improved energy management strategy for power production and storage. The performance indices employed include PV utilization ratio, load match ratio and grid flexibility factor. The optimal sizing of the system was performed based on its impact on the grid. It resulted in a 1050 kW peak PV installation, an internal fleet of 300 electric cars and an additional fixed battery capacity of 450 kWh. Nafeh et al. presented a methodology for the optimal sizing of a proposed PV battery grid-connected system for fast charging stations for electric vehicles in Cairo [
9]. By shifting one’s attention to the big international picture, one can observe that the increasing penetration of wind farms and photovoltaic parks in modern electricity grids results in stochastic electricity production with frequent peaks, which disturb the grid’s operation and result in the dumping of large amounts of renewable energy. In an extensive study on the new power system paradigm of China, Yan et al. [
10] conducted power supply and demand production simulations based on the characteristics of new energy generation in China. They found that when the penetration of new energy sources in the system exceeds 45%, long-term energy storage becomes an essential regulation tool. By comparing the storage duration, storage scale and application scenarios of various energy storage technologies, these researchers concluded that hydrogen storage is the most preferable choice to participate in large-scale and long-term energy storage. Jung et al. compared models of optimal capacity and facility operation methods based on long-term operational changes in distributed energy resources in a building with self-consumption in Korea [
11]. The spread of energy storage systems in buildings plays a significant role, as these systems compensate for the intermittency of renewable energy and act as demand management resources through load leveling to transfer the power load [
12]. Other forms of short-term storage are combined with the above techniques. Ju et al. studied the load-shaving capabilities of a 5 m
3 thermal storage tank directly charged by the district heating supply water which was integrated into a substation of a Finnish office building [
13]. In Germany, it is estimated that achieving the full grid penetration of renewables by 2050 would require about 80 TWh of storage capacity [
14]. In that context, the economically favorable solution of water pumping in large reservoirs associated with hydro power is geographically limited. Thus, hybrid PV battery storage systems are increasingly applied in commercial buildings. Thermal storage (heating or cooling) is frequently combined with electricity storage in these buildings due to the high cost of batteries. According to a study by Chen et al., a 40% PV penetration combined with a 0.006
$/(kWh
e) energy storage investment resulted in an impressive 27.3% cost reduction in a Beijing mall, while the optimal cooling storage rate decreased from 55% to 40%. [
15]. An alternative approach with increasing research focus is the application of direct current (DC) microgrids [
16] that can improve distribution efficiency by reducing power conversion stages. Research on distributed energy resources using renewable energy on DC microgrids is gaining momentum [
17]. As battery costs decline and electricity prices fluctuate more, the battery would gradually replace cooling storage. However, there is a long way to go to reduce battery costs from the current value of 150 USD/kWh to a more economically viable 70 USD/kWh. The above restrictions have further increased attention to the use of hydrogen as an intermediate energy carrier. Hydrogen acts as a long-term storage medium for surplus renewable electricity [
18]. Its role will be critical for the electrification of energy systems and the shift to zero greenhouse gas emissions [
19]. In theory, hydrogen could be oxidized in a fuel cell to produce electricity at specific instances [
20]. Currently, about 40% of the H
2 currently consumed in industry is a by-product of large-scale chemical processes, while the remaining 60% is mainly produced as a by-product of fossil hydrocarbon extraction and processing. Since the investment in battery storage equipment is lagging due to the high costs involved, power-to-gas technology emerges as the most viable option. This involves the use of electrolyzer devices that produce hydrogen that can be stored in high-pressure tanks to fuel vehicles or be mixed with natural gas. The addition of renewable hydrogen to natural gas is a viable and promising strategy to achieve the gradual de-carbonization of the fuel mix used in electricity production and space heating. On the other hand, the use of renewable hydrogen as an automotive fuel is a valid strategy to achieve the de-carbonization of transportation, which develops in parallel with vehicle electrification [
14]. In this respect, despite the high incentivization of plug-in hybrids in Germany and the EU, spark-ignition (SI) engine cars continue to dominate new registrations. If one additionally considers hybrids, which are mostly equipped with SI engines, the total amounts to two-thirds of new car registrations in 2023. On the other hand, using battery-powered electric trucks is not a viable option due to their covering of long distances at continuous near full load operation, which requires a large battery size and weight, as well as a long battery charging time. Under these circumstances, hydrogen, having the highest energy density of all fuels, with a lower heating value of 120 MJ, remains an ideal candidate for CO
2-neutral, long-range, heavy-duty vehicles, which cause the battery capacity to be stressed to the limit. The above trends and perspectives lead to the development of advanced hydrogen-fueled, spark-ignition engines and high-pressure hydrogen storage tanks with advanced technology. The market growth for H
2 storage tanks for mobile and on-site storage is expected to exceed a value of USD 7 billion by 2030 [
21]. Large-scale synergies may be attained through the exploitation of the above technologies in the building sector, where a significant number of research works examine optimal grid integration in commercial and public nZEBs. These studies profit from on-site measurements and the modeling of a building’s and grid’s operation, taking into account the existing constraints and mutual information exchange [
22]. The use of the storage capacity of employees’ electric cars is now a workable option to provide short-term electricity storage in commercial and public buildings. This option may be profitably combined with the usual electric vehicle charging infrastructure [
23]. This includes both wired charging and wireless charging. Interoperability is crucial in the charging process and is the subject of intensive research [
24]. New legislation already seeks the optimal exploitation of alternative charging sources by means of price regulation [
25]. Engel et al. studied the effects of the increase in the share of electric vehicles to local peak loads and the slope of the evening ramp in a residential feeder circuit [
26]. Thomas et al. [
27] studied a smart grid in a university campus building, which comprised renewables, a fixed storage system and a fleet of electric cars. They employed two-year actual smart metering data to create discrete probabilities for PV production scenarios and found that the uncertainty in PV production could lead to a threefold increase in the daily system cost estimation. Such effects must be relaxed by adopting smart charging and exploiting the effect of more households being involved in one feeder circuit. Fachrizal [
28] optimized a workplace with PV-powered chargers using load matching. Office nZEBs comprise a popular field of study in this context [
29,
30]. The optimized in-house charging system should be capable of reducing the cost for electric car owners by simultaneously protecting the grid [
31]. The optimal sizing of a hybrid solar photovoltaic (PV) and battery energy storage (BES) system for grid-connected commercial buildings was studied in [
32]. The authors optimized the energy system of a grid-connected university building in Malaysia. They attained a 12.3% reduction in electricity cost, a 22.6% reduction in annual energy consumption, and a 15.85% reduction in peak demand. Although battery storage is an established option for short-term photovoltaic electricity storage, long-term, seasonal energy storage is mandatory because of the significant seasonal variations in the heat pump’s electricity consumption. A convenient solution is to produce hydrogen from electrolyzer units powered by solar electricity [
20]. Go et al. studied a H
2 storage system comprising an electrolyzer, H
2 tank and a fuel cell for seasonal storage [
33]. They found that the H
2 storage system compensated for a self-discharge loss of the battery for seasonal storage. Hai et al. modeled an autonomous residential building with four occupants for Kuwait’s capital, which was powered by solar panels and stored excess energy in a hydrogen tank [
34]. Zahra et al. comparatively studied the effects of battery versus hydrogen storage in a typical residential building in Iran [
35]. The prospect of using fuel cells to transform stored hydrogen to electricity is a subject of intense discussion due to the higher efficiency in energy conversion [
36]. However, these applications are at a continuous experimental stage [
37]. Moreover, extensive comparative lifecycle and reliability assessments of these alternatives would be necessary prior to their widespread application [
38,
39]. On the other hand, another electric power source is already available as standard equipment in commercial and public buildings in the form of diesel-powered engine generator sets for emergency power supply. A study of the effect of fueling a building’s genset using natural gas [
40] pointed to its optimal sizing as an additional power source, resulting in limited electricity imports from the grid. Now, the problem with this type of configuration is caused by the lack of long-term electricity storage. The export of large quantities of electricity to the grid during hours with high insolation is the result in these cases. A comparison of the time slots with significant export to the grid during weekends with the load curves of the Greek electricity grid indicated that these quantities are not favorable for grid stability. The current work proposes a significant improvement to the micro-grid’s design by adding the in situ production of green hydrogen with a novel power system layout and timing. Green hydrogen is only produced during the weekends by means of commercially available alkaline electrolyzer units. The hydrogen produced is pressurized and stored in a high-pressure vessel. It can be employed to fuel an advanced state-of-the-art technology, direct-injection, spark-ignition engine that powers the generator set. This genset can be optimally dispatched by the smart grid during weekdays to minimize external grid interaction. The optimal sizing of several components of this micro-grid is critical to the success of this concept. An energy analysis of the optimized system indicates that the interaction between short-term (battery) and long-term (hydrogen) storage is beneficial to the overall system’s performance and may profitably match with the hydrogen infrastructure already in progress. All of this leads to a novel concept with a high reliability and degree of technological maturity for immediate application.