Thanks for the great ideas. This is what i checked:a. it is not leakingb. the meter number is correct vs the invoicec. the meter reading is correct vs invoice d. the setting on the heater is correct (on 5)I did learn that the hot water unit is 135L- enough for 4 people! For one person it is too big, but explains the bill that this amount of water is being maintained for one person! Ill follow up on using hot water tap setting when only cold is needed- great point !Cheers!