The amount of energy consumption differs according to RO purifiers and UV purifiers. Usually, the average power consumption of a RO purifier is about 25 watts.
It means if a family drinks 15 litres of water every day then a 25 watt machine with purification capacity of 15 ltrs/hr will cost you about 9 units per year.
25 watts * 365 days = 9125 watts = 9 units (approximately)
Though this amount of power consumption varies according to various models.
Kent RO water purifier with UV light consumes 60 watts of electricity per hour which will cost you just above 22 units per year.
And the UF water purifiers do not require electricity to run.