Watts from volts and amps, amps and ohms, or volts and ohms, with the power factor for single- and three-phase AC, plus optional kWh and cost per day, month and year.
25 amps on a 240 volt circuit - how many watts?
Watts from volts and amps, amps and ohms, or volts and ohms, with the power factor for single- and three-phase AC, plus optional kWh and cost per day, month and year.
The calculator below multiplies 25 amps by 240 V in the voltage and current mode, which assumes a resistive load with a power factor of 1. The answer is the real power in watts and kilowatts, together with the load resistance that would draw 25 amps at that voltage.
The current here is the load itself, not the rating of the breaker that protects it. A load that runs three hours or more should draw no more than 80 % of the breaker rating, so 25 amps of continuous load needs a larger breaker than 25 amps; the amps calculator on this site sizes both. Enter hours per day and a price per kWh to see what the load costs to run.
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Watts from any two electrical readings
A space heater pulling 12.5 A from a 120 V outlet turns 1.5 kW into heat, and two hours of that every day comes to 3 kWh. This calculator finds power from voltage and current, current and resistance, or voltage and resistance. For alternating current it also separates the watts the meter bills from the volt-amperes the wiring carries, on single-phase and three-phase supplies, and with hours of use and a price per kWh it adds the energy and the cost.
Five choices on the form
- What you know - voltage and current (DC or a resistive load), voltage, current and power factor on single-phase AC, the same on three-phase AC, current and resistance, or voltage and resistance.
- Voltage - in volts. A US outlet is 120 V, a dryer circuit 240 V, USB 5 V. On three-phase use the line-to-line figure, such as 208 V or 480 V.
- Current or resistance - amps from a nameplate or a clamp meter, ohms from a multimeter across a heating element.
- Power factor - AC modes only, a number from 0 to 1. Type 1 for heaters and incandescent bulbs, or the value printed on a motor nameplate.
- Hours per day and price per kWh - both optional. Hours give kWh per day, month and year; the price adds dollars.
Three supplies compared
The same pair of readings means different things depending on the supply. On DC and on a purely resistive AC load, volts times amps is the answer. With motors and electronics the power factor tells you how much of the current does work.
| Question | DC or resistive load | Single-phase AC | Three-phase AC |
|---|---|---|---|
| Real power | V × I | V × I × PF | √3 × V × I × PF |
| Apparent power | same as real power | V × I, in VA | √3 × V × I, in VA |
| Which voltage to enter | across the load | RMS, for example 120 V or 240 V | line to line, for example 208 V or 480 V |
| Pick this mode when | there is no motor or electronics, or the supply is a battery | the nameplate lists a power factor or a VA rating | the equipment is fed by three hot wires |
The formulas, with three sets of numbers
Power is the rate at which energy moves: P = V × I. Ohm's law, V = I × R, turns it into P = I² × R and P = V² ÷ R, which is why any two of the three readings are enough. Energy is power multiplied by time, so a 1 kW load running for one hour uses 1 kWh.
An element measuring 9.6 Ω on a 120 V outlet: 120² ÷ 9.6 = 1,500 W, and the current is 120 ÷ 9.6 = 12.5 A. At 2 h a day that is 3 kWh daily, 1,095 kWh a year, and at $0.17 per kWh $186.15 a year.
120 V and 10 A give 1,200 VA of apparent power. With a power factor of 0.8 the real power is 960 W, and the reactive part is √(1,200² − 960²) = 720 var.
480 V line to line, 20 A per line, power factor 0.9: 1.7321 × 480 × 20 = 16.628 kVA, times 0.9 = 14.965 kW. Leaving out the √3 would report 8.64 kW, about 42% too little.
One wattage, two outlet voltages
For a resistive load the same wattage needs half the current on a 240 V circuit and four times the resistance.
| Power | Current at 120 V | Resistance at 120 V | Current at 240 V | Resistance at 240 V |
|---|---|---|---|---|
| 60 W | 0.5 A | 240 Ω | 0.25 A | 960 Ω |
| 100 W | 0.833 A | 144 Ω | 0.417 A | 576 Ω |
| 600 W | 5 A | 24 Ω | 2.5 A | 96 Ω |
| 1,000 W | 8.333 A | 14.4 Ω | 4.167 A | 57.6 Ω |
| 1,500 W | 12.5 A | 9.6 Ω | 6.25 A | 38.4 Ω |
| 3,000 W | 25 A | 4.8 Ω | 12.5 A | 19.2 Ω |
| 4,800 W | 40 A | 3 Ω | 20 A | 12 Ω |
Where power estimates go wrong
Asked about watts, VA and kWh
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See also
Calculator verified by the LiczGrupa.pl team
Content, formulas and results have been reviewed for accuracy and relevance by our team of specialists.
Reviewed by: Natalia Skrzek