How is the cost of electricity calculated?

• A. Power used (kW) x time (h) x Cost of Energy (c/kWh)
• B. Energy divided by time
• C. Voltage x current
• D. Resistance x time

Answer: (A)

Cost of electricity depends on two things: how much energy you use and how much energy costs per unit. Energy used by a device is found by multiplying its power by the time it runs. If a device uses P kilowatts for t hours, it consumes E = P × t kilowatt-hours. The bill is then energy times the price per kilowatt-hour, so Cost = P × t × (price per kWh). For example, a 1 kW heater running 3 hours at 10 cents per kWh costs 1 × 3 × 0.10 = 0.30 dollars. Dividing energy by time would give power, not cost; voltage × current gives power, not cost; and resistance × time isn’t a standard way to compute cost.