Scholars [51] conclude that energy efficiency plays a crucial role in decarbonizing economic development. The findings of energy efficiency assessments could be used to identify opportunities for energy savings and propose cost-effective solutions to enhance energy efficiency, which is the primary goal of sustainable development. It should be noted that scholars developed a vast range of approaches for assessing energy efficiency. Scholars [52,53,54,55] outline that energy intensity refers to the amount of energy required to produce a unit of output or provide a specific service. It is a measure of the efficiency with which energy is utilized. Lower energy intensity indicates higher energy efficiency. Based on empirical findings, Dong et al. [52] outline that declining energy intensity allows increasing energy efficiency among Chinese provinces and promotes the energy capabilities of the country. Shahiduzzaman and Alam [53] empirically justify that energy intensity and carbon dioxide emissions are closely related to each other, which consequently affects the Australian energy efficiency. Applying the Granger causality test, scholars [54] conclude that energy efficiency depends on the energy consumption structure, economic structure, and energy intensity. Based on the results, scholars suggest decreasing coal energy consumption and boosting the development of green energy. Hosan et al. [55] show that energy intensity directly impacts sustainable economic growth within the energy efficiency of a country. Su et al. [56] developed the composite energy efficiency index to develop policy recommendations to improve energy efficiency in OECD countries. Scholars justify the crucial role of final energy consumption in energy independence. Furthermore, scholars [56,57] highlight that energy efficiency has a significantly positive effect on final energy consumption in a country. Paramati et al. [58] confirm that environmental technologies have a significant positive impact on energy efficiency by reducing energy consumption. These findings indicate that environmental technologies contribute to reducing overall energy consumption and improving energy efficiency in OECD countries. Studies [59,60,61,62] confirm that decreasing CO₂ emissions from fuel combustion improve the energy efficiency of a country. In addition, studies [63,64,65,66,67] show that renewable energy has the most significant impact on energy efficiency among all other dimensions. Thus, extending renewable energy allows for boosting the rapid growth of a country’s energy efficiency [68,69].

The analysis of the theoretical background on energy efficiency shows that digital business can have both linear and nonlinear effects on energy efficiency, depending on how it is implemented and utilized. Scholars [70,71,72] outline that digital technologies and solutions are used to optimize energy consumption and improve energy efficiency. Digital systems and smart meters enable real-time monitoring of energy consumption, allowing businesses to identify inefficiencies and implement corrective measures promptly. By analyzing large volumes of data generated by digital systems, businesses gain insights into energy usage patterns, identify areas of potential energy waste, and proactively schedule maintenance to prevent energy losses [73,74]. Digitalization facilitates the implementation of energy-efficient processes, such as automated controls, smart grid technologies, and demand response systems. These technologies allow optimization of energy use and reduction of waste. Pålsson et al. [75] found that e-commerce eliminates the need for physical retail spaces, and the increased reliance on transportation could result in energy consumption and associated environmental impacts. However, studies [76] outline that efficient logistics practices, such as route optimization, consolidation of shipments, and the use of electric vehicles or alternative fuels, help minimize the energy intensity of e-commerce delivery operations.

At the same time, studies [77,78,79,80] justify the nonlinear effect of digitalization on energy efficiency. Morley et al. [81] confirm that digitalization leads to increased energy consumption if the efficiency gains are offset by increased usage or new applications. For instance, the proliferation of digital devices and data centers contributes to higher overall energy consumption. Babu et al. [82] highlight that the production, use, and disposal of digital devices generates electronic waste. Improper handling and disposal of e-waste negatively impacts the environment, including energy-intensive recycling processes and resource depletion. The growth of digital business often requires expanding data centers and related infrastructure. Gunasekaran et al. [83] explain that e-commerce businesses often operate large-scale warehouses to manage inventory and fulfill online orders. These facilities require energy for lighting, heating, cooling, and operating material handling equipment. Optimizing warehouse design, implementing energy-efficient technologies, and adopting sustainable practices such as energy management systems and renewable energy integration enhance energy efficiency in these operations. Chen et al. [84] confirm the nonlinear effect of e-commerce on energy efficiency. This means that the initial effects of e-commerce on energy efficiency could be negative, but over time, as technology and practices evolve, they lead to positive energy efficiency outcomes. The U-shaped impact is represented by a curve that initially dips downward (indicating a negative impact) and then rises upward (indicating a positive impact). The U-shaped impact of e-commerce on energy efficiency underscores the importance of proactive measures, technological advancements, and sustainable practices to mitigate initial negative impacts and capitalize on the long-term potential for energy efficiency gains. It highlights the need for continuous improvement and collaboration among e-commerce businesses, policymakers, and consumers to ensure a sustainable and energy-efficient e-commerce ecosystem.