Assessing your solar water heater system's performance metrics is crucial to determine its efficacy and efficiency of operation of the solar energy. To perform this evaluation, you need to track several key metrics, which are listed below:

1. Temperature Difference: This metric measures the difference between the water heating target temperature and the current water temperature after being heated by the solar system. A higher temperature difference would generally translate to a more efficient operation of the system.

Efficiency Ratio: Also known as the exergy efficiency or the solar collector system (S.C.S.) efficiency, this metric quantifies the ratio of the thermal energy gained by the fluid circuit to the incident solar energy. The efficiency ratio of a typical solar water heater system ranges from 30% to 60% and the figure depends on the solar collector type and system type.

Thermal Energy Gained per Day: This metric is calculated by measuring the total thermal energy gained through the solar water heating system over a set timeframe. It can give you an idea about the effectiveness of the solar system.

Heat Loss: This metric refers to the amount of heat lost from the storage tank at night, which is typically calculated by using the type of coating used in the tank or by running a test using a heat loss rate calculator. Effective cooling can significantly minimize heat loss and prevent energy loss.

Pressure Drop: This metric is particularly relevant to pumps, as it is the drop in pressure resulting from the movement of the hot water through the pipes. Low pressure generally can lead to reduced productivity.

Age of the System: As your solar water heater ages, it is expected that its efficiency will decrease over time. This is mostly due to worn-out and damaged components. Thus, tracking the age of the system can allow you to anticipate potential future issues.

Cold Water Inlet Temperature: This is the source temperature that drives the temperature difference impact on heater efficiency and the production of the solar panel. Similarly, it is best if this temperature is maintained if the area for connection of an outdoor cold water inlet is frost exposed.

Production by Thermally Recovered Hot Water: This is an critical performance metric that tracks the hot water energy that gets recovered from your solar water system over a certain timeframe and also reflects on a specific reference time to evaluate the system's optimal operation to measure effectiveness.

Electrical Power Consumption: This reflects the performance of the system's productivity by comparing the installed electrical energy required to maintain the whole system, with typical efficiencies ranging between 50-65%.

Use of the Electric Booster Pump: This is used in conjunction with the previous point of electrical power consumption to track the efficiency of your solar water heater system.

Tracking these solar water heater performance metrics can provide critical data necessary for detection of operational anomalies in time prior maintenance.