Appliance nameplates and meters often give volts and amps, while your bill charges for kilowatt-hours. This calculator bridges the two: it turns voltage, current and power factor into real power in watts, then into energy over time.
Real power in watts = volts × amps × power factor, and three-phase circuits multiply by √3, about 1.732. A 120-volt heater drawing 12.5 amps uses 1,500 watts; running continuously, that's 36 kWh a day.
Power Consumption Calculator
Real power, apparent power, and energy from voltage + current.
The formulas
- Apparent power (VA) = volts × amps, or × √3 for three-phase.
- Real power (W) = apparent power × power factor. This is what your meter bills.
- Reactive power (VAR) = the part that flows back and forth without doing work; it's the square root of apparent squared minus real squared.
The calculator also converts real power to kilowatts and horsepower (746 watts per horsepower), and to energy for an hour, a day, 30 days and a year of continuous running.
What power factor means
Resistive loads such as heaters and incandescent bulbs use all the current they draw, so their power factor is 1.0. Motors and some electronics draw current that's out of step with the voltage, so their real power is less than volts times amps. A power factor of 0.85 means only 85% of the apparent power does useful work.
Three examples
- A space heater: 120 V × 12.5 A × 1.0 = 1,500 W.
- A motor load: 240 V × 10 A × 0.85 = 2,040 W, not the 2,400 that volts times amps suggests.
- A three-phase unit: 208 V × 20 A × 0.9 × 1.732 = about 6,485 W.
When the √3 factor applies
The √3 multiplier, about 1.732, belongs only to three-phase power. Standard household circuits at 120 and 240 volts are single-phase, so you never apply it there; volts times amps times power factor is the whole real-power formula for them.
Three-phase service shows up in larger commercial and industrial buildings and in some big motors. If that is your case, the sister 3-phase power calculator works the conversion the other direction, from kilowatts to amps, and compares single- and three-phase current for the same load.
Watts to amps at 120 and 240 volts
Rearranging the formula gives amps = watts ÷ (volts × power factor). For a resistive load the power factor is 1.0, so amps = watts ÷ volts; the same wattage draws half the current at 240 volts that it draws at 120, because the voltage is doubled.
| Real power | Amps at 120 V | Amps at 240 V |
|---|---|---|
| 500 W | 4.2 A | 2.1 A |
| 1,000 W | 8.3 A | 4.2 A |
| 1,500 W | 12.5 A | 6.3 A |
| 2,000 W | 16.7 A | 8.3 A |
| 3,000 W | 25.0 A | 12.5 A |
| 4,500 W | 37.5 A | 18.8 A |
| 5,000 W | 41.7 A | 20.8 A |
Each cell is the wattage divided by the voltage: 1,500 ÷ 120 is 12.5 amps, and 1,500 ÷ 240 is 6.25, which rounds to 6.3. For a motor, divide these figures by its power factor again, since a motor draws more current than its real power alone would suggest.
From watts to kWh and cost
Energy is power times time: kWh = watts × hours ÷ 1,000. The calculator's figures assume the load runs nonstop, so the 1,500-watt heater shows 36 kWh a day and 13,140 kWh a year. Real appliances run fewer hours; for their cost, use our kWh cost calculator with your actual hours.
For scale, an average U.S. home uses about 10,356 kWh a year, about 863 a month (EIA 2024 data). At the January-to-July 2026 average of 18.19 cents per kWh, that's about $157 a month; see how many kWh per day is normal.
Reading an appliance nameplate
Nameplates label power in different units, and you convert each to real watts before using it. A plate marked in watts (W) already gives real power, so you use the number as is.
A plate that lists only volts and amps gives apparent power once you multiply them: 240 V times 20 A is 4,800 VA. For a resistive load that equals 4,800 real watts; for a motor you still multiply by the power factor to get the real watts your meter bills.
A plate marked in volt-amps (VA) is apparent power directly, so multiply it by the power factor for real watts. A plate marked in horsepower (HP) states the motor's mechanical output, where one horsepower equals 746 watts; the electrical input is somewhat higher than that, because the motor loses some energy to heat and friction.
Measuring your own loads
A plug-in watt meter reads real power and energy directly for anything on a standard outlet. For hardwired loads, an electrician's clamp meter reads amps, which you multiply by the voltage and a realistic power factor.
Frequently asked questions
How do I calculate watts from volts and amps?
Multiply volts by amps, then by the power factor: 1.0 for heaters and bulbs, lower for motors. For three-phase power, also multiply by 1.732.
What is power factor?
The fraction of the current that does useful work. A heater's is 1.0; motors are often 0.8 to 0.9.
How many kWh does a 1,500-watt heater use?
1.5 kWh per hour of running, or 36 kWh if it ran for a full day.
How much electricity does an average house use?
About 10,356 kWh a year, or about 863 kWh a month, according to EIA 2024 data.
How do I convert amps to watts at 240V?
Multiply amps by 240 and by the power factor. A resistive 240-volt load drawing 20 amps is 240 × 20 × 1.0 = 4,800 watts; a motor at the same volts and amps with a 0.85 power factor is 240 × 20 × 0.85 = 4,080 watts.
What is the difference between watts and VA?
Volt-amps (VA) is apparent power, volts times amps, the full load the wiring carries. Watts is real power, VA times the power factor, which is what your meter bills. They are equal only for a resistive load, where the power factor is 1.0.
What is the difference between kW and kWh?
A kilowatt (kW) is a rate of power, 1,000 watts at an instant; a kilowatt-hour (kWh) is energy, one kilowatt flowing for one hour. A 1,500-watt heater draws 1.5 kW while on, and running a full 24 hours it uses 1.5 × 24 = 36 kWh.
Sources & References
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