Solar thermal modeling for rapid estimation of auxiliary energy requirements in domestic hot water production: On-off versus proportional flow rate control

•The long-term performance of solar water heating systems is simulated.•A criterion to compare systems with on-off and proportional control is developed.•The proportional control performance may be 50 % higher than the on-off control.•The performance of proportional control systems increases with storage volume.•The on-off control performance is not significantly affected by storage volume.