The monthly cost of a floodlight, and other appliances
How much does it cost to watch a TV show? To bake bread in an electric oven? To charge a laptop or dry your clothes? More broadly speaking, how do all these activities add up to the usage you see on your electric bill?
Determining how much it costs to operate a given electrical appliance requires a few data points and some math. This article goes through the calculation for an exterior light, but you can always use an online calculator to save time. You can find one on our website.
Each appliance uses a different amount of electricity, from a coffee pot to a space heater. For this article, we’ll look at an outdoor floodlight that lights the driveway in the winter. Let’s assume the light is on a timer, and turns on from 4 pm to 10 pm and 6 am to 10 am every day for a daily total of 10 hours.
Next, find the power used by the appliance. Power is measured in watts or kilowatts (one kilowatt is equal to 1000 watts). Some appliances include this information on the product casing or in the appliance manual – look for W or kW. In this example, the bulb for the outdoor floodlight uses 120 watts, or 0.12 kW.
Electricity is sold by utilities in kilowatt-hours (kWh). The charge depends both on how much power an appliance requires (kilowatts) and how long it is operated (hours). Electrical energy is calculated by multiplying the power by the length of time the appliance is on.
Electrical Energy = Power in kW x time in hours
It’s important to use the correct units–kilowatts and hours–in order to match the unit used by the electric utility (kWh). For instance, if your appliance uses 50 watts, divide by 1000 to convert to kilowatts (0.05 kW). Or, if you operate an appliance for 15 minutes, convert this to hours (0.25 hours).
The outdoor floodlight uses 0.12 kW and is operated for 10 hours per day, so it uses 1.2 kWh of energy a day (0.12 kW x 10 hours), or 36 kWh a month (1.2 kWh a day multiplied by 30 days).
The last step is to translate energy into dollars. To get the current cost of electricity, check your utility’s website for rate. In Fairbanks, Alaska, the Golden Valley Electric Association publishes its rates at on their website. For the sake of this example, let’s use the 2019 residential rate of $0.21 per kWh.
Cost to run appliance = Energy in kWh x $0.21
The outdoor floodlight thus costs 25 cents each day to run, or $7.56 to operate each month.