What does thermal efficiency percentage indicate about a boiler's performance?

The thermal efficiency percentage is a critical measure of a boiler's performance, as it indicates how well the boiler converts the energy contained in fuel into useful heat energy that can be used for heating or other processes. A higher thermal efficiency percentage means that more of the fuel's energy is being effectively utilized, resulting in less fuel wasted and reduced operational costs. This metric helps operators gauge the effectiveness of the boiler's design and operation, enabling them to make adjustments as necessary to optimize performance.

The other options, while related to boiler operation, do not directly assess the core function of thermal efficiency. For instance, the rate of water evaporation pertains to how quickly the boiler can convert water into steam, which is a different aspect of performance. The total fuel consumed in a day provides insight into overall usage but does not reflect how efficiently that fuel is being converted into heat. The amount of water used during operation can be important for understanding system requirements and operational limits but does not indicate the efficiency of fuel conversion into heat energy.