AFAIK, the 3,6,9 and 12 unit sizes ( with immersions ) employ 3x 1kW immersion heaters, not 1x 3kW. They also come with a little box of tricks to manage the 3 heaters, which IIRC are accompanied by 3x thermistors placed in ascending order which determine the state of charge. Any electrical input is deemed useful tbh, but the chemistry dictates at what temperature point the PCM changes state, so best consult with Sunamp before placing an order / designing the system.