Water tank monitoring

I want to create a low powered circuit that monitors the water level of a tank and sends out a message when the tank is opened. It needs to be a very low powered circuit and battery powered. The circuitry should be able to lock and unlock the tank through a mobile app. I am thinking of using GSM to send the messages. To read the values of the tank I am going to use a float sensor. Im not sure what the process should be to check whether the tank is opened. To lock and unlock the tank im thiking of using a stepper motor however im concerned of the power usage. The microcontroller I am thinking of using the ESP32, however I am not to sure how to reduce the power usage.

Any guidance on design choices or improvements will be much appreciated.