How to use the watts, amps and volts calculator
Pick the kind of current, type any two of watts, amps and volts, and the third appears with the formula worked out beside it. Direct current (DC) is a battery or a solar panel. Single-phase alternating current (AC) is the usual outlet in a house. Three-phase AC powers larger motors and shops, and the page asks for the line-to-line voltage there. For AC, type the power factor too: 1 for a plain heater or light, lower for a motor. The picture shows the current as dots and the element glowing with the watts. The other tabs check a breaker, convert kW, kVA and horsepower, and add up a list of loads.
The formulas
single-phase AC: watts = volts × amps × power factor
three-phase AC: watts = √3 × volts × amps × power factor
Rearranged, amps are watts divided by volts (and by the power factor in AC), and volts are watts divided by amps. The √3 is about 1.732 and comes from three lines sharing the load; the volts are measured between two of the lines. The page works every step with exact fractions and rounds only what it shows.
Worked examples
A heater is rated 1,800 W on a 120 V circuit. A plain heating element has a power factor of 1, so the amps are 1,800 ÷ 120 = 15 A. That is the example the calculator opens on.
A load at 120 V × 15 A with a power factor of 0.9 uses 1,620 W of real power, because 120 × 15 × 0.9 is 1,620 W. The wires still carry 1,800 VA. The difference is reactive power, 784.602 var.
A three-phase load on 400 volts line to line drawing 20 amps at a power factor of 1 uses √3 × 400 × 20 = 13,856.4 W, about 13.9 kW. Going the other way, 10 kW at 400 volts and a power factor of 0.9 draws 16.0375 A on each line.
Power factor and the triangle
In AC the current and the voltage can be slightly out of step, and then not all of what the wires carry does work. Watts (P) are the real power that heats, spins or lights. Volt-amps (S) are what the wires carry. Reactive power (Q, in var) is the difference. They form a right triangle: S² = P² + Q². The power factor is P ÷ S, and it is the cosine of the angle in the triangle. A power factor of 1 means a flat triangle with no reactive power; DC is always 1.
Breakers and the 80 percent rule
A 15 amp breaker at 120 volts can carry 1,800 W, and a 20 amp breaker 2,400 W. For a load that runs 3 hours or more, the usual rule in the National Electrical Code is that the breaker be rated at least 125 percent of that load. That is the same as keeping a continuous load to 80 percent of the breaker: 1,440 W on the 15 amp and 1,920 W on the 20 amp. At 240 volts the same breakers carry twice as many watts: 4,800 W and 3,840 W for a 20 amp. Your load of 1,800 watts at 120 volts draws 15 A. That fits a 15 amp breaker if it runs only briefly, but if it runs 3 hours or more it needs 18.75 A of capacity, so the 20 amp is the smallest of the five that works. The code edition your area uses decides the details, and the breaker protects the wire, which this page does not size.
kW, kVA and horsepower
kW is real power and kVA is apparent power: kVA = kW ÷ power factor, so 10 kW at 0.8 is 12.5 kVA, and on a single-phase 240 volt supply that is 52.0833 A. Horsepower comes in three common sizes: 745.6999 W for mechanical, 746 W for electrical (exact) and 735.4988 W for metric. A 5 horsepower electrical motor puts out 3,730 W. At 85 percent efficiency and a power factor of 0.8 it takes 4,388.24 W from the line, which is 5,485.29 VA and 22.8554 A at 240 volts. A real nameplate beats this estimate.
Where the numbers come from
The power formulas are standard AC relations, read at an engineering reference and the Wikipedia articles on AC power and power factor. The √3 factor is from the same engineering page. The three horsepowers come from the NIST guide to SI conversion factors: it prints 745.6999 W for 550 foot-pounds a second, 746 W exactly for electrical, and 735.4988 W for metric. The mechanical figure here is worked from the definitions to 745.69987158 W, and the metric one from 75 kilogram-force meters a second. The appliance chart is from a Virginia Cooperative Extension publication of February 2020. The 80 percent figure comes from how the code treats continuous loads. The page has no Imperial | Metric switch: volts, amps and watts are the same everywhere.
Common mistakes
- Using phase voltage for three-phase. The √3 formula takes the voltage between two lines.
- Forgetting the power factor on a motor. A plain heater is 1; a motor often is not.
- Treating watts and volt-amps as the same. They are equal only at a power factor of 1.
- Loading a breaker to 100 percent for hours. Keep a continuous load to 80 percent.
- Sizing wire from this page. It does not; ask an electrician.
Questions
How many amps is 1,800 watts?
At 120 volts and a power factor of 1, 15 A. At 240 volts it is 7.5 amps. Divide the watts by the volts.
How many watts is 1 amp?
It depends on the volts: 12 watts at 12 volts, 120 at 120, 240 at 240. Watts are volts times amps. At 12 volts and 5 amps that is 60 W.
Why does three-phase use the square root of 3?
Total power is three times the power in one phase, and the volts between two lines are √3 (about 1.732) times the volts across one phase. Put together, the factor in front of line volts and line amps is √3, not 3.
What power factor should I use?
Use 1 for a plain heater, kettle or incandescent bulb. For a motor or any load with a label that gives a power factor, type that number. If you do not know, the page shows the current at 1, which is the lowest a real load can draw.
Does the 80 percent rule make my breaker 80 percent as big?
No. The breaker is the size on its handle. The rule only limits how much of it a load that runs 3 hours or more should use.
Can I share my result?
Yes. Share makes a link with a picture for Facebook, X and Pinterest, a link you can copy, and a short video for TikTok made on your device. The link keeps only the kind of current, the volts, the amps and the power factor, for 90 days. There are no prices on this page.