This is normal on heavily overcast days. A solar water heater can only heat water when the sun is shining or when there is light cloud. For an immediate supply of hot water, use the electrical backup. If you have used more water that day, the solar water heater will need time to warm up the water again. For an immediate supply of hot water, use the electrical backup. If neither of the above is relevant, check that there are no obstructions in the collector circuit. The thermosyphon force that circulates the water is rather weak, and it can be stopped by small air bubbles in the collector or by bends in the pipes between the tank and the collector. Check the circulation by feeling the temperatures of the inlet and outlet pipes close to the collector. When the sun is shining the outlet temperature should be much higher than the inlet temperature. Be careful: the outlet, pipe can be very hot! If you cannot detect the thermosyphon circulation (and there are no bends in the pipes), it is likely that there are air locks in the system. Drain the water out of the collector unit and then refill it. If this does not work, contact us.