## 1 Introduction  
The development and utilization of renewable energy technologies in buildings has been becoming an important way to achieve the \"dual carbon goals\" in China. Among various renewable energy utilization technologies, the solar utilization technologies have been widely concerned because of the huge potential [1, 2, 3, 4, 5]. According to the relevant statistical data in 2022, China's installed capacity of grid-connected solar power has reached about 392.61 million kilowatts, an increase of 28.1% year-on-year [6]. It is clear that the potential and prospects of solar PV technology are vast. With the development of centralized PV power plants, distributed PV systems has been becoming more attractive due to the proximity to the load side with short transmission and distribution distances and system flexibility [2, 7, 8]. According to statistics, the area of urban rooftop PV in mainland China can reach about 3.35 billion square meters and be used for the installation of PV panels [9]. In the utilization of distributed PV systems, the microgrid should be optimized with the electricity consumption characteristics of the building, such as electricity consumption and electricity consumption categories, so as to reach its full potential [10; 11; 12]. Nowadays, with the upgrading and development of power electronics technology, the application of DC electrical appliances is increasing year by year, which poses new challenges to the traditional photovoltaic grid-connected mode [13]. Therefore, the PV power generation is directly connected with the electrical appliances to form a DC-DC microgrid system, which can improve solar energy utilization more effectively than a DC-AC-DC microgrid system used to serve DC appliances [14; 15; 16; 17; 18]. With the energy conversion process simplified, the efficiency of energy conversion and the efficiency of DC appliances operating at the appropriate voltage can be increased at the same time [19]. However, different DC devices require different voltages. Therefore, it is very important to design an effective circuit system to solve the mismatch between the output of a photovoltaic system and the demand of DC equipment economically and effectively. At present, the research on the adaptation of DC microgrid can be roughly divided into two research directions, namely: power electronic buck-boost adaptation and energy management to improve power supply and distribution adaptation. In the research of power electronic step-down low-voltage DC equipment, in the literature [20; 21], a 48 V and 72 V DC power terminal is proposed to supply power to the DC load, which can save costs and realize the matching of photovoltaic output and application. However, it is still DC-DC-DC, and further step-down is still needed until all kinds of DC equipment operate normally. In Reference [22], a 12 V or 24 V DC voltage algorithm combining PI and MPPT is proposed for fixed lighting. However, DC electrical equipment is still growing, except for lighting, and the use range of single fixed voltage output is limited. Reference [23] proposed two feasible architectures to compare AC distribution in the home: one architecture is powered by a low-power DC load, while the traditional AC distribution is powered by a residual load, and the other architecture increases the level of DC distribution and load installation to the highest possible level. But multiple load distribution systems will undoubtedly increase the impact on the power grid, and from a practical point of view, large-scale circuit transformation is also difficult to implement. Reference [24] proposed the use of a supercapacitor auxiliary circuit to realize DC-DC voltage regulation. Reference [25] shows that the use of a buck-boost voltage regulator can make the extraction power and tracking efficiency have a larger working area and better performance. However, the above methods are all aimed at a single voltage output, and the adaptation range has not been improved.