Assume one 1500 W electric heater runs actively for three hours once per day. The selected period is 30 days and the entered flat electricity rate is 0.25 EUR per kWh. These are example inputs, not a regional tariff or a claim that every heater draws constant power.
Calculate energy and cost for one use
1500 W × 3 h ÷ 1000 = 4.5 kWh. Multiplying by 0.25 EUR/kWh gives 1.125 EUR per use. The units cancel visibly: watts become kilowatts through division by 1000, hours produce kWh, and kWh cancels when multiplied by the price per kWh.
Extend the same use across time
At one use daily, energy per day stays 4.5 kWh. Over 30 days it is 135 kWh and 33.75 EUR. Over 365 days at unchanged use and rate it is 1642.5 kWh and 410.625 EUR. The annual figures are a mathematical projection; seasonal use and tariff changes often make the actual year different.