If you perform energy models, you will almost certainly hear a reference to ASHRAE 90.1 at some point. 

ASHRAE Standard 90.1, also known as Standard 90.1 or just 90.1, is a standard for energy-efficient design of buildings. It is published by the American Society of Heating, Refrigerating, and Air-Conditioning Engineers (ASHRAE) and serves as a guide for designing and constructing buildings that use less energy and water and are more comfortable and healthy for occupants.

Standard 90.1 covers a wide range of topics related to energy efficiency in buildings, including heating and cooling systems, lighting, building envelope (the outer shell of a building that separates the interior from the exterior), and electrical systems. It also includes requirements for energy-efficient equipment such as HVAC systems, water heaters, and appliances.

The standard is updated every three years to incorporate new technologies and best practices in energy-efficient building design. It is widely adopted by building codes and is used as a reference in the design and construction of new buildings and in the retrofit of existing buildings.