Electricity is usually modest: VIQUA Arros 9/15/22 are rated around 33W, 46W and 60W. The larger recurring expense is the scheduled lamp-unit replacement plus any prefilters.
Electricity cost
At continuous operation, 33W uses about 289 kWh/year, 46W about 403 kWh/year and 60W about 526 kWh/year. Multiply by the local electricity rate for the annual power cost.
Lamp replacement
Residential UV systems generally use a scheduled lamp or lamp-unit replacement interval because UV output declines with operating time even if the lamp still visibly glows.
Quartz sleeve cleaning
The sleeve is reusable until damaged or permanently fouled, but it should be inspected and cleaned during maintenance because scale or staining reduces UV transmission.
Prefilter cost
If the UV system depends on sediment or carbon prefiltration, those cartridge replacements belong in the UV ownership budget too.