The solar water heater has been out of water for a long time. Is it okay to add water on cloudy days?

No problem, you are doing the right thing. Remember to add water at noon in the hot sun, because it will burst the vacuum tube!

"Dr. Sun" solar energy and Sancang solar energy (without water level sensing probe and solenoid valve) are recommended, which can heat and utilize hot water quickly in rainy weather, and the after-sales rate is greatly reduced, saving time and electricity. This model takes only 2-3 minutes to heat without sun compared with ordinary single-compartment solar energy, while single-compartment solar energy takes 2-3 hours!