Power describes the rate at which an appliance uses energy. Running that power for a duration produces an energy quantity. For a flat-rate estimate, convert watts to kilowatts, multiply by hours, then multiply the resulting kWh by the tariff. Quantity and frequency belong in the calculation only when you want totals beyond one device and one use.

First calculate kWh

Energy per use = watts × hours ÷ 1000 × quantity. A 1500 W heater running for 3 hours uses 1500 × 3 ÷ 1000 = 4.5 kWh. If runtime is in minutes, divide minutes by 60 before applying the equation. Keep units beside each value so watts are not accidentally treated as kilowatts.

Then apply the rate and frequency

Cost per use = kWh per use × price per kWh. At 0.25 EUR/kWh, 4.5 kWh costs 1.125 EUR. One use per day for 30 days gives 135 kWh and 33.75 EUR. A 365-day projection is a comparison baseline, not a prediction that the same usage or tariff will persist.