Electricity Calculator
Calculate electricity usage, amps, and cost based on watts and time.
Formula
Cost = (W × h / 1000) × Rate
Convert watts to kilowatts, multiply by hours, then by rate.
Worked Examples
Example 1: Running a 60 W Bulb for Five Hours
Problem:A 60 W incandescent bulb on a 120 V circuit is left on for 5 hours a day. Electricity costs $0.12 per kWh. What current does it draw and what does a day of use cost?
Solution:Current: I = P / V = 60 W / 120 V = 0.50 A Energy: kWh = 60 W x 5 h / 1,000 = 0.30 kWh Cost: 0.30 kWh x $0.12 = $0.036 per day Over a 30 day month: 0.30 x 30 = 9.0 kWh, costing $1.08.
Result:0.50 A | 0.30 kWh per day | $0.036 per day ($1.08 per month)
Example 2: Space Heater on a 15 A Circuit
Problem:A 1,500 W space heater runs on a 120 V circuit for 6 hours a day at $0.16 per kWh. Is it safe on a 15 A branch circuit, and what does it cost per month?
Solution:Current: I = 1,500 W / 120 V = 12.5 A A 15 A circuit allows 80 percent continuous load = 12.0 A, so 12.5 A exceeds the continuous limit and this heater should have a dedicated circuit. Energy: 1,500 W x 6 h / 1,000 = 9.0 kWh per day Cost: 9.0 x $0.16 = $1.44 per day Monthly (30 days): 9.0 x 30 = 270 kWh, costing $43.20.
Result:12.5 A (over the 12 A continuous limit) | 270 kWh per month | $43.20
Example 3: Refrigerator With a Realistic Duty Cycle
Problem:A refrigerator has a 150 W nameplate rating but its compressor only runs about 35 percent of the time. Electricity costs $0.14 per kWh. What is the annual cost?
Solution:Effective average power = 150 W x 0.35 = 52.5 W Annual energy = 52.5 W x 24 h x 365 d / 1,000 = 459.9 kWh Annual cost = 459.9 x $0.14 = $64.39 Using the full 150 W nameplate instead would predict 1,314 kWh and $183.96, nearly three times too high.
Result:460 kWh per year, about $64 at $0.14 per kWh
Frequently Asked Questions
How do I calculate the amps an appliance draws from its watts?
Divide power by voltage: amps = watts / volts. A 60 W lamp on a 120 V circuit draws 0.5 A, while the same lamp on a 230 V supply draws only 0.26 A. This is the rearranged power law P = V x I. For circuit planning remember that a 15 A branch circuit should carry no more than about 12 A of continuous load, which is the 80 percent derating rule.
What exactly is a kilowatt-hour on my electricity bill?
A kilowatt-hour is the energy used by a 1,000 watt load running for one hour. It equals exactly 3.6 megajoules. Utilities bill energy rather than power, so a 2,000 W heater run for 30 minutes uses 1 kWh, the same as a 100 W device run for 10 hours. Every cost figure on this page is kWh multiplied by your per-kWh tariff.
How is the running cost of an appliance calculated?
Cost equals watts divided by 1,000, multiplied by hours of use, multiplied by the rate per kilowatt-hour. A 1,500 W space heater run 6 hours a day at 16 cents per kWh costs 1.5 x 6 x 0.16 = $1.44 a day, or about $43 over a 30 day month. Add any fixed monthly service charge separately, because that part does not scale with usage.
What supply voltage should I enter for my country?
North American homes use a nominal 120 V for general outlets and 240 V for ranges, dryers and EV chargers. Most of Europe, Africa, Asia and Australia use 220 to 240 V, with 230 V the harmonised European nominal value. Japan uses 100 V. The voltage affects the amperage output but not the kilowatt-hours or the cost, which depend only on watts and hours.
What is the difference between volts, amps and watts?
Voltage is the electrical pressure that pushes charge, measured in volts. Current is the rate of charge flow, measured in amps. Power is the rate of energy transfer and equals voltage times current for a resistive load, measured in watts. Energy, what you actually pay for, is power multiplied by time and is measured in kilowatt-hours.
Why is nameplate wattage often higher than real consumption?
Nameplate wattage is the maximum draw, not the average. Refrigerators, air conditioners, freezers and washing machines cycle on and off, so a fridge rated at 150 W may only run 30 to 40 percent of the time and average 50 to 60 W. Multiply nameplate watts by an honest duty cycle before trusting a daily or monthly cost estimate.
Does power factor change the result on an AC circuit?
For resistive loads such as incandescent lamps, kettles, toasters and resistance heaters, P = V x I holds exactly and the amperage shown is accurate. Motors, LED drivers and switch-mode supplies draw current partly out of phase with voltage, so real power is V x I x power factor and the true current can exceed the amps shown here. Utilities still bill residential customers on real power in kWh.
Which household loads dominate an electricity bill?
Space heating and cooling usually come first, followed by water heating, then large appliances such as clothes dryers, ovens and pool pumps. Lighting and electronics are a much smaller share than most people expect once LED bulbs are fitted. Sizing the biggest loads first, using Electricity Calculator on each, gives a far better bill estimate than adding up small devices.
How much does standby or phantom power really cost?
Devices left in standby draw between about 0.5 W and 10 W each. Ten devices averaging 3 W run continuously use 10 x 3 x 24 x 365 / 1000 = 263 kWh a year, which is roughly $42 at 16 cents per kWh. Set-top boxes, game consoles in instant-on mode, and older external power supplies are the usual offenders.
What is the quickest way to cut the cost Electricity Calculator shows?
Attack the terms of the formula in order: reduce watts by replacing the largest loads with efficient models, reduce hours by using timers and thermostat setbacks, and reduce the rate by shifting use to off-peak periods if you are on a time-of-use tariff. Replacing a 60 W incandescent bulb with a 9 W LED cuts that fixture cost by 85 percent for the same light output.