Electricity use is generally modest compared with water heating or dehumidification. Annual lamp replacement and sleeve cleaning are usually more important ownership tasks than the electricity bill itself.
Continuous operation
UV reactors are normally left on 24/7 because there is no storage delay between switching on and safe treatment.
Estimate annual kWh
Multiply lamp/system wattage by 8,760 hours per year and divide by 1,000. A 40-watt system would use roughly 350 kWh per year before ballast differences.
Lamp aging matters
UV output declines with lamp age even when the lamp still visibly glows. Replacement schedules should follow the manufacturer, commonly around one year.
Pretreatment protects UV performance
Turbidity, iron, manganese and hardness scale can interfere with UV transmission, so pretreatment may matter more than electrical efficiency.