The efficiency measurement for furnaces is known as the Annual _____________ utilization rating (AFUE).

The term you are looking for is the Annual Fuel Utilization Efficiency (AFUE) rating. AFUE measures a furnace's efficiency by calculating the ratio of the fuel it converts into usable heat over the course of a year, compared to the total amount of fuel consumed. This rating is essential because it helps consumers understand how much heat is produced per unit of fuel, allowing for better energy efficiency assessments and potential savings on fuel costs.

In contrast, the other terms do not accurately reflect the specific focus on fuel utilization that AFUE emphasizes. Energy relates to broader concepts within heating systems that might not be limited to fuel efficiency. Heat could imply the output of a system but does not account for the total fuel input or efficiency over time. Load refers to the heating demand or requirements rather than efficiency ratings. Thus, fuel is the key component that directly informs the AFUE measurement, making it the correct choice.