How It Is Calculated
This calculator estimates the energy usage and cost of an electrical appliance based on its power rating, usage pattern, and your electricity rate. The core calculation converts power in watts to energy in kilowatt-hours (kWh), then multiplies by time and cost.
The formulas used are:
Daily energy (kWh) = (P × H) / 1000 × (1 + W / 100)
Weekly energy (kWh) = Daily energy × D
Annual energy (kWh) = Weekly energy × 52
Monthly energy (kWh) = Annual energy / 12
Monthly cost ($) = Monthly energy × R
Annual cost ($) = Annual energy × R
Where:
- P = Appliance power in watts
- H = Hours used per day
- D = Days used per week
- W = Waste factor as a percentage (0 to 100)
- R = Electricity rate in $/kWh
All results are rounded to two decimal places.
Assumptions
- The year is assumed to have 52 weeks.
- The waste factor accounts for inefficiencies or extra usage beyond the set hours.
- The electricity rate is constant throughout the year.
- The appliance power consumption is constant while it is operating.
Worked Example
Consider an appliance with a power of 100 W, used 3 hours per day, 7 days per week, at an electricity rate of 0.12 $/kWh, and a waste factor of 0%.
First, calculate daily energy: (100 × 3) / 1000 × (1 + 0/100) = 300 / 1000 = 0.3 kWh.
Weekly energy: 0.3 × 7 = 2.1 kWh.
Annual energy: 2.1 × 52 = 109.2 kWh.
Monthly energy: 109.2 / 12 = 9.1 kWh.
Monthly cost: 9.1 × 0.12 = 1.092 ≈ 1.09 $.
Annual cost: 109.2 × 0.12 = 13.104 ≈ 13.1 $.
Thus, the results are: Daily energy usage = 0.3 kWh, Monthly energy usage = 9.1 kWh, Annual energy usage = 109.2 kWh, Monthly cost = 1.09 $, Annual cost = 13.1 $.
Practical Tips
- Use the waste factor to add a safety margin for appliances that may run longer or have startup surges.
- For accurate results, measure actual usage hours rather than estimating.
- If you need to size a circuit or check voltage drop for this appliance, consider using the Cable size calculator or Voltage drop calculator.
- To estimate total household energy consumption, use the Energy consumption calculator for multiple appliances.
- Remember that standby power can add to costs even when the appliance is off.