Executive Summary Purpose The purpose of this guide is to provide both a practical overview and an in-depth understanding of the Procedure for checking the power performance of solar thermal collector fields (Power Method) of ISO 24194:2022. This guide refers to this method as “Power Check”. The presented material aims to clarify the standard, provide background information, and promote its widespread use. It complements but does not replace reading the standard. Furthermore, this guide explains how to run the Power Check with the free and open-source software SunPeek and discusses pathways to improve the Power Check. Power Check The Power Check can be utilized for power performance verification, including power performance guarantees, and ongoing performance monitoring. These applications are crucial for the successful operation of solar thermal plants. The Power Check accounts for actual solar radiation levels, system and ambient temperatures, and heat demand, unlike other evaluation methods (e.g., specific solar yield, input-output diagram). Collector efficiency parameters based on Solar Keymark or similar are factored in. The primary Power Check key performance indicator (KPI) is the measured to estimated collector field output ratio, which has proven to be a reliable quality assurance indicator for solar thermal plants. ISO 24194:2022 is the first standard to target operational solar thermal collector field performance, unlike other standards that target individual collectors and laboratory tests. The standard currently consists of the published ISO 24194:2022 and one amendment, ISO 24194/Amd 1:2024. Because of the novelty of the standard in the solar community, the need for clarification may arise. In a community effort, the open-source software SunPeek has been developed. It serves as a reference implementation of the Power Check and is presented in this guide. SunPeek provides a transparent and consistent implementation of the Power Check, making the method accessible to the solar community and stakeholders involved in solar thermal projects. Target audience This guide addresses the following stakeholders of solar thermal plants: Plant operators who want to use the Power Check for power performance verification, e.g., performance guarantees given by suppliers during the commissioning phase, or for continuous performance monitoring. This guide summarizes experiences from practical applications. For test laboratories and certification bodies, this guide outlines areas where the standard requires clarification and highlights possible improvements for revisions. For academia and industry working on software implementations, this guide discusses modelling and automation and the implementation in the open-source software tool SunPeek. For collector manufacturers, investors and other stakeholders, this guide aims to provide a deeper understanding of the procedure. This guide draws inspiration from the Guide to Standard ISO 9806:2017. It considers experiences and learnings from IEA SHC Task 68, initiating this guide, but extends beyond the task network by including industry and research experts not participating in the task activities. Also, learnings from IEA SHC Task 45 and IEA SHC Task 55, which contributed to the development of the Power Check, are incorporated.