Annual Collector Efficiency and Q_useful dependency on load in SWH model

Hello, 
I am currently using the Solar Water Heating model in SAM 2025.4.16.I am attempting to calculate the annual collector efficiency using the following formula:
η_collector_annual = Q_useful / (A_collector × Total Incident Irradiance)

However, I noticed that when I modify the “Average daily hot water usage,” the value of Q_useful changes significantly, even though the collector type and configuration remain identical.



It appears that Q_useful in the results reflects the energy delivered to the tank (constrained by the load) rather than the potential thermal output of the collector itself.

I have attached my simulation file for reference. Could you please help clarify if I am using the correct variable for this calculation, or if there is a way to isolate the collector’s performance from the load profile?

kind regards
Joey 

Hi Joey,

SAM's solar water heating model equations are described in the draft Technical Manual for the SAM Solar Water Heating Model, which you can download from the Solar Water Heating page on the SAM website:  sam.nrel.gov/solar-water-heating.html .

Section 2.1.1 describes the useful energy equations, and Figure 1 shows a diagram of the system components.

The temperature at the collector inlet is affected by the temperature of the tank, which is dependent on the water usage, so the useful energy is not independent of the usage.

The source code for these calculations is here:  github.com/NREL/ssc/blob/bc40dcf2669371db42fffc078697385478ea1b75/ssc/cmod_swh.cpp#L653

Best regards,
Paul.