unit 22 has a logic circuit 22 c in which a reference voltage v 1 and a working voltage v 2 are stored the working voltage v 2 is a minimum voltage to activate the power needed module 28 in the present invention the reference voltage v 1 is 300 mv and the working voltage v 2 is 6v these voltages can be any value according to the exact condition of the water heater a control method of providing the backup power via the thermoelectric cooling chip 20 and the controller 22 when the main power is off and the battery 18 is in a low battery condition is described hereunder as shown in fig3 and fig4 the battery 18 supplies the combustor 12 power for ignition at this time the thermoelectric cooling chip 20 senses a first temperatures t 1 of the cold water section 16 a and a second temperature t 2 of the hot water section 16 b through the hot junction p 1 and the cold junction p 2 to provide a dc current i with an initial voltage v the controller will take different actions according to a temperature difference u03b4t of the first temperatures t 1 and the second temperature t 2 u03b4t t 1 u2212 t 2 in a condition of the temperature difference u03b4t u03b4t t 1 u2212 t 2 being less than a minimum difference 20 u00b0 c in the present invention it means that the initial voltage v is greater than the reference voltage v 1 300 mv at this time the controller 22 commands that the battery 18 supply power to the power needed module 28 the battery 18 will soon run out.