Q_useful and municipal pool SHW confusion

Hello,
I’m working with a team on an energy analysis for an outdoor, all year, municipal pool located in the North West. We want to compare PV, SHW, and geothermal.SAM isn’t really designed for this sort of analysis. I don’t have a typical hot water draw profile and my “storage tank” is an entire lap pool. Rather than use all of SAM’s capabilities I just want to get the total useful energy captured by the collectors. 
The documentation defines Q_useful as “Power delivered to the water tank, equal to the power received by the collector minus losses from the collector to the surroundings.” Question: Is m_dot the mass flow on the collector side or storage tank side?I’ve attached a labeled diagram for clarity. Thanks for your help!