... ,When the cloudy sky or clouded or sunshine shortage . Solar water heaters assistant heating tube . 100V -240V , 1000W-1500W Our special ... sunshine shortage .it is waterproof heating tube , this heating tube can put into the tank of solar water heater , When the cloudy ... than 50 centigrade , this special heating tube will turn-on electric automatic , when the water is heating up to 90 centigrade , it ...