Power and energy are different units
Watts describe the rate at which equipment uses energy. Kilowatt-hours describe the amount consumed over time. A device drawing a steady 1,000 watts for three hours uses 3 kWh. At a user-entered price of 0.20 per kWh, that is 0.60 for the day and 18.00 over 30 identical days.
The arithmetic is: watts divided by 1,000, multiplied by hours, multiplied by days, multiplied by the energy price. Try this example in the electricity calculator before replacing it with your measurements.
Rated power is not always average power
A refrigerator or thermostat-controlled heater can cycle on and off. A computer's power-supply rating is not its measured electricity draw. Using a maximum rating for every hour can therefore overstate normal consumption. Use an appropriate energy meter or documented operating measurement when available.
For equipment with different modes, calculate each mode separately. For example, use one scenario for active hours and another for standby hours, then add their energy totals. Do not enter more than 24 total operating hours for one day.
A tariff is more than one number
This tool estimates usage-based energy charges. Standing charges, minimum bills, taxes, tier boundaries and time-of-use rates are outside its simple single-rate model. If your tariff varies during the day, calculate each price period separately rather than pretending the average is exact.
Choose your currency only as a display label. The tool does not convert a quoted tariff from one currency to another. Verify the unit on the bill: a price per kWh is not a price per watt or per day.