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To compare equipment running costs, use the electrical input power, expected operating time and your own electricity price. Keep the assumptions beside the result so you can update the comparison when your equipment settings or tariff change.
Calculate energy, then cost
Energy in kWh = power in watts × hours per day × number of days ÷ 1,000. Multiply the result by your price per kWh. This method is explained by Virginia Cooperative Extension.
For example: 100 W × 10 hours × 30 days ÷ 1,000 = 30 kWh. At a hypothetical €0.25/kWh, that costs €7.50. These numbers illustrate the calculation; the operating time is not a lighting recommendation and the price is not a current tariff.
Make the comparison useful
- Give each lamp, fan, pump and controller its own row, then add the energy totals.
- For different tariff periods, calculate each period separately using the applicable rate.
- For dimmed equipment, use power data for that setting or measured consumption, rather than assuming a dimmer percentage equals the same percentage of electrical power.
Equipment can also draw power in standby. For a device whose consumption varies, a measurement over a representative period gives a better basis than one instantaneous reading. The US Department of Energy describes standby and variable-power measurement.
This comparison covers usage at the rates entered. Account for fixed charges and any taxes excluded from those rates separately. When comparing LED grow lights, keep running cost alongside coverage and compatibility.