Example 1: Room heater
A 1500 W heater runs for 3 hours per day over 30 days.
Monthly energy = 1500 x 3 x 30 / 1000 = 135 kWh
Yearly energy = 135 x 12 = 1620 kWh.
Energy Consumption (kWh)
Calculate kWh = Power (W) x Hours x Days / 1000 with monthly and yearly totals.
Introduction
Energy consumption is one of the most basic calculations used in household planning, electrical load assessment, energy audits, and cost estimation. If you know the power rating of an appliance and how long it runs, you can estimate how much electrical energy it uses over time. This is valuable for comparing equipment, checking likely utility bill impact, and planning more efficient systems.
This calculator estimates monthly and yearly electricity use in kilowatt-hours from appliance power, daily usage, and number of operating days. It is useful for both small appliance checks and broader energy-management work.
Because kWh is the same unit used on most utility bills, the result is easy to connect with cost-saving, carbon-emission, and load-planning decisions.
Formula
Energy (kWh): Energy = Power (W) x Time (hours) / 1000
Monthly kWh: Monthly = Power x Hours per day x Days / 1000
Yearly kWh: Yearly = Monthly kWh x 12
Monthly kWh: Monthly = Power x Hours per day x Days / 1000
Yearly kWh: Yearly = Monthly kWh x 12
Variable Definitions
Power = appliance or load power rating in watts
Hours per day = operating time each day
Days = number of days used in a month
Energy = electricity consumption in kilowatt-hours
Units
Power is entered in watts, time in hours, and the result is displayed in kilowatt-hours. Utility companies commonly bill electricity use in kWh, which is why this unit is especially useful for practical estimation.
Worked Examples
Example 2: Ceiling fan
A 90 W fan runs for 10 hours per day over 30 days.
Monthly energy = 90 x 10 x 30 / 1000 = 27 kWh
Yearly energy = 27 x 12 = 324 kWh.
How To Interpret kWh
The monthly result helps you estimate the billing impact of one appliance or one operating pattern, while the yearly result is more useful for comparing upgrades such as efficient lighting, better pumps, or different HVAC operating schedules. A relatively small device can still use a meaningful amount of energy when it runs for many hours each day, which is why both power and time matter.
This page is also useful when prioritizing energy-saving opportunities. If two appliances have similar power ratings but one operates much longer, the longer-running load usually offers the better saving opportunity. Converting power and time into kWh makes that comparison much easier.
Practical Notes
Actual energy use can vary because appliances do not always run at full rated power. Refrigerators, air conditioners, pumps, and some electronics cycle on and off or operate at varying load. Even so, this calculator is an excellent first estimate for budgeting, comparison, and energy-saving decisions.
If you want a closer estimate, use measured power from a meter or a realistic duty cycle instead of relying only on the nameplate value. That is especially important for compressor-based equipment, variable-speed drives, and loads with standby or cycling behavior.