Private businesses and policymakers are accelerating the deployment and advancement of smart grid technology innovations that can support smart energy systems. Technological advances such as the Internet of Things (IoT) provide a broad range of energy sector applications, such as transmission and distribution, energy supply, power generation, renewable energy integration, load demand management, etc. We have conducted a comprehensive and critical IoT study on business applications and smart energy systems for this objective. Four main areas of smart energy systems have been chosen, including: i) the use of IoT in business; (ii) the use of IoT in smart energy applications; (iii) the use of IoT in data transmission networks; and (iv) the use of IoT in power generation and terminal hardware/devices. Each research area is further divided into different sub-areas; for example, IoT business includes energy operational policy, energy management and planning, energy management systems, business models, and customer services. Energy forecasting, state monitoring and estimation, anomaly detection, data mining and visualization are among the IoT applications in smart energy systems. Cloud computing, edge computing, and quantum computing are provided using IoT in data transmission networks. A variety of renewable sources, pricing, and load management strategies involve the use of IoT in energy generation. Many new solutions for smart energy systems are provided with critical thinking and clear vision, and key industries for IoT revenue generation and application development are described. This study aimed to provide a clear insight into IoT devices' recent developments in smart energy systems, supported by high-quality published literature. It is noted that in 2015, the IoT worldwide energy market exceeded USD 6.8 billion and is projected to reach USD 26.5 billion by 2023, with a compound annual growth rate of 15.5 % in 2016−23.