Home Energy Management Systems provide a comprehensive solution for managing various energy aspects of a home, including generation, storage, and consumption. HEMS is designed to integrate with a wide variety of devices such as solar panels, battery storage systems, smart appliances, lighting, heating, and cooling systems. 

It aims to analyse and optimize energy usage patterns to reduce energy costs and improve efficiency, adjusting the operation of connected devices based on real-time data, grid demand, energy prices, and user preferences. Additionally, HEMS usually comes with detailed monitoring capabilities, providing real-time data and historical insights into energy consumption and generation.

Smart home systems like Nest or Hive may focus more narrowly, often primarily on climate control and security, though they may extend to other home automation features. 

These systems can integrate with a variety of smart home devices (thermostats, cameras, smoke detectors, etc.), but might not integrate directly with energy generation sources like solar panels or battery storage systems. Systems like Nest focus on learning user behaviour and preferences to optimize heating and cooling schedules for comfort and efficiency. They provide feedback on energy usage but may not offer detailed energy production and consumption analytics.

The key differences lie in the scale and focus of the two systems. HEMS aims to provide a holistic approach to energy management across multiple devices and sources, whereas smart home systems like Nest or Hive may have a narrower focus, such as optimizing heating and cooling or enhancing home security.