Electrical Power Calculator

Electrical Power Calculator for DC, AC single-phase and three-phase loads. It finds watts from voltage and current, or from a resistance, and adds the kW, hp, BTU/h and kVA values.

Electrical Power Calculator Formula

Electrical power is the rate at which a circuit uses energy, and it is measured in watts. One watt is one joule per second, which is also one volt driving one amp. There are three formulas of power in a circuit, and the Electrical Power Calculator picks one from the Calculate From menu.

To calculate electrical power from a meter reading, multiply the volts by the amps, and on AC multiply again by the power factor. Choose Voltage and Current for this case. Left on that setting, the tool works as a plain power voltage current calculator.

$$P=V\times I$$

A device on 120 V that draws 10 A uses 120 × 10 = 1,200 W, and the result panel prints it as 1.200 kW. This is the example the tool loads with, so the numbers on screen match it before you type anything.

Choose Current and Resistance when no voltage reading is available. This form explains why a wire or a resistor runs hotter as the current climbs, because the heat rises with the square of the amps.

$$P=I^2\times R$$

A current of 0.5 A through a 10 Ω resistor gives 0.5² × 10 = 2.5 W, and the second card reports the 5.000 V that sits across the load. When that heat lands in a transistor or a regulator, the Heat Sink Calculator checks the junction temperature.

Choose Voltage and Resistance for a heating element or any load with a known ohm value. The supply voltage does most of the work in this form, since it is squared before the division.

$$P=\frac{V^2}{R}$$

A 16 Ω element on 240 V gives 240² ÷ 16 = 3,600 W with 15.00 A through the load. Put the same element on 208 V and it makes 2.704 kW, which is about 25 percent less heat from the same part.

All three are one law written three ways. Putting V = I × R into P = V × I produces the other two, and the Ohm's Law Calculator handles the cases where the unknown is volts, amps or ohms instead of watts.

Open the Step-by-Step Working panel and the Electrical Power Calculator acts as a power formula calculator, printing the formula with your own numbers in it. The last line of the working repeats the answer shown in the main result.

Used as an electrical power calculator online from a phone, the voltage, current and resistance fields each have a unit button. A reading of 250 mA or 4.7 kΩ goes in as written, with no conversion to amps or ohms first.

AC Power Calculator: Real Power and kVA

In an AC circuit the current can lag or lead the voltage, so volts times amps overstates the work being done. Set Circuit Type to AC Single-Phase and the Electrical Power Calculator becomes an AC power calculator with a Power Factor field that accepts values above 0 and up to 1.

$$P=V\times I\times PF$$

Power factor is the share of the volt-amp product that reaches the load as real power. The U.S. Department of Energy's motor load fact sheet points out that a low power factor does not mean lost power, only extra current that the wiring still has to carry.

The note above the inputs suggests 1 for heaters and incandescent lamps and about 0.85 for a motor at full load. A motor nameplate lists its own full-load power factor, and the figure falls as the motor is loaded more lightly.

Take 120 V, 10 A and a power factor of 0.85 in the Electrical Power Calculator. Real power comes to 1.020 kW, and the first card shows 1.200 kVA of apparent power, 632.1 var of reactive power and a phase angle of 31.79°. The second card splits the 12.00 Ω load impedance into a 10.20 Ω resistance part and a 6.321 Ω reactance part.

What is power in an AC circuit?

Power in an AC circuit has three parts. Real power in watts does the work. Reactive power in var is stored in the magnetic field of a winding and handed back to the line on every cycle. Apparent power in VA is what the supply must carry, and its square equals real power squared plus reactive power squared.

AC entries are RMS values, the kind a meter reads and a nameplate prints. A scope reading in peak or peak-to-peak volts should go through the RMS Voltage Calculator first, and the Peak to Peak Voltage Calculator converts the other way.

Is 1 kVA equal to 1 kW?

1 kVA equals 1 kW only when the power factor is 1, as it is with a heater. kVA is apparent power, the plain product of volts and amps, and kW is what remains after multiplying by the power factor. At 0.8 a 1 kVA supply delivers 0.8 kW, and at 0.85 a 10 kVA generator carries 8.5 kW of real load.

Electrical Power Calculator 3 Phase Mode

Set Circuit Type to AC Three-Phase in the Electrical Power Calculator and a Voltage Measured menu appears. The choice matters because the multiplier in the formula depends on where the voltmeter leads were placed. A line-to-line reading uses the square root of 3, which is 1.732.

$$P=\sqrt{3}\times V_{LL}\times I\times PF$$

A line-to-neutral reading uses 3 instead, because each of the three phases carries the same current at the lower voltage. The current is the line current in both cases, taken on one conductor with a clamp meter.

$$P=3\times V_{LN}\times I\times PF$$

The two forms agree because line-to-line voltage is 1.73 times line-to-neutral voltage. On a 480Y/277 V system at 20 A and a power factor of 1, the line-to-line entry returns 16.63 kW and the line-to-neutral entry returns 16.62 kW. The small gap exists only because 277 V is a rounded figure.

Mixing the two is the usual error. In one Mike Holt forum thread, a poster was told to multiply volts by amps and to add 1.73 only when starting from 277 V, which is backwards. The Voltage Measured menu ties the right multiplier to the reading you took.

A measured case from the same DOE fact sheet makes a good test. An electrician recorded an average of 469.7 V line to line, 37 A and a power factor of 0.763 on a 40 hp motor. The Electrical Power Calculator 3 phase result is 22.97 kW, which the fact sheet prints as 22.9 kW, along with 30.10 kVA and 19.46 kvar.

At a power factor of 1, the note under the results states that the figure is the total for all three phases and gives the share in each phase. For 480 V and 20 A that share is 5.543 kW, so there is no need to multiply the answer by 3 again.

Both formulas assume a balanced load with equal current on all three lines. The Three-Phase Power Calculator takes the same inputs and adds the active and reactive current split, plus the apparent power in each phase.

DC Power Calculator Mode

Pick DC in Circuit Type and the Power Factor field disappears, since a steady current has no phase angle. The Electrical Power Calculator is then a DC power calculator that multiplies volts by amps. A 12 V circuit drawing 5 A uses 60.00 W, and the second card gives its load resistance as 2.400 Ω.

DC results are compared against 24 V, 48 V and 36 V. The same 60 W needs 5 A at 12 V but only 2.500 A at 24 V and 1.250 A at 48 V, which is why a higher battery voltage allows thinner cable for the same load.

How do I calculate DC amps?

DC amps are the watts divided by the volts. A 60 W load on a 12 V battery draws 60 ÷ 12 = 5 A, and a 1,200 W load on the same 12 V draws 100 A. With a resistance in place of a wattage, divide the volts by the ohms.

$$I=\frac{P}{V}$$

The Current Calculator is built to solve for amps. From there the Battery Life Calculator gives the hours a pack will last, and the Battery Size Calculator works back from the load to the capacity needed.

Electrical Power Calculator Result Cards

Four cards sit under the main wattage, and each one answers a different question about the load. With the default 120 V, 10 A and power factor of 1, the first card reads 1.609 hp, 4,095 BTU/h of heat and 28.80 kWh of energy over 24 hours.

The horsepower figure uses the electrical horsepower of exactly 746 W, and the heat figure uses 3.412 BTU/h per watt. Both come from the conversion factors in NIST Special Publication 811. When the power factor is below 1, this card switches to apparent power, reactive power and phase angle.

The second card describes the load as the supply sees it. The default case shows a 12.00 Ω load impedance, 8.333 A for every kilowatt and a conductance of 83.33 mS. In the two resistance modes it shows the missing third value instead, either the volts across the load or the amps through it.

The third card keeps the load fixed and changes the supply. The 12 Ω load that takes 1.200 kW at 120 V would take 4.800 kW at 240 V, 3.605 kW at 208 V and 6.394 kW at 277 V. Power follows the square of the voltage, so doubling the volts gives four times the watts.

The fourth card keeps the wattage fixed and changes the supply. Delivering the same 1.200 kW takes 5.000 A at 240 V, 5.769 A at 208 V and 4.332 A at 277 V. Below the cards, the Same Load at Other Voltages table lists volts, amps and watts side by side.

Power Calculator kW Conversions

How to calculate power in kW?

Power in kW is the watts divided by 1,000. The Electrical Power Calculator does this scaling on its own, so 1,200 W appears as 1.200 kW while 60 W stays as 60.00 W. From a meter reading, the division goes straight into the formula, and a three-phase load gets 1.732 in front of the volts.

$$P_{kW}=\frac{V\times I\times PF}{1000}$$

How much power is 1000 watts?

1,000 watts is 1 kW. It equals 1.340 electrical horsepower and 3,412 BTU/h of heat, and it draws 8.333 A at 120 V or 4.167 A at 240 V. A 1 kW load that runs for one hour uses exactly 1 kWh.

How much is 200 watts of power?

200 watts is 0.2 kW, which is 0.268 hp and 682.4 BTU/h of heat. At 120 V it draws 1.667 A, and it takes 5 hours to use 1 kWh. The table gives the same figures for other common wattages at a power factor of 1.

Load	In kW	Amps at 120 V	Time to use 1 kWh	kWh in 24 hours
40 W	0.04 kW	0.333 A	25 hours	0.96
100 W	0.1 kW	0.833 A	10 hours	2.4
200 W	0.2 kW	1.667 A	5 hours	4.8
1,000 W	1 kW	8.333 A	1 hour	24
1,500 W	1.5 kW	12.5 A	40 minutes	36
2,000 W	2 kW	16.67 A	30 minutes	48

What will 100 watts power?

100 watts will power any load whose volts times amps stays at or under 100. That is 0.833 A at 120 V, 8.33 A at 12 V or 20 A at 5 V. Ten phone chargers at 5 V and 2 A each add up to exactly 100 W, and one laptop charger at 20 V and 3.25 A takes 65 W of it.

What is 1500 watts in kW?

1,500 watts is 1.5 kW. On 120 V that is 12.5 A, which is most of the 1,800 W that a 15 A circuit can supply at that voltage. A 1.5 kW load built for 240 V draws 6.25 A, half the current for the same heat.

What is 16A in kW?

Amps turn into kilowatts only once the voltage is fixed. 16 A is 1.92 kW at 120 V, 3.68 kW at 230 V and 3.84 kW at 240 V. On a 480 V three-phase supply, the same 16 A in each line carries 13.30 kW.

What is 500 amps in kW?

500 amps is 6 kW on a 12 V DC system, 120 kW on 240 V single-phase and 415.7 kW on 480 V three-phase, all at a power factor of 1. A motor load at 0.85 would bring the 480 V figure down to about 353 kW.

What is 4 kW in amps?

4 kW draws 33.33 A at 120 V, 16.67 A at 240 V and 4.811 A per line on 480 V three-phase. The fourth result card in the Electrical Power Calculator does this conversion for the voltages next to yours, and the table covers the common supplies.

Supply	16 A carries	500 A carries	4 kW draws
120 V single-phase	1.92 kW	60 kW	33.33 A
230 V single-phase	3.68 kW	115 kW	17.39 A
240 V single-phase	3.84 kW	120 kW	16.67 A
208 V three-phase	5.764 kW	180.1 kW	11.10 A
480 V three-phase	13.30 kW	415.7 kW	4.811 A

kWh and Units of Electricity

Watts measure power at one moment, and watt-hours measure how much is used over time. The U.S. Energy Information Administration notes that utilities generally meter electricity in kilowatt-hours. Energy is the power multiplied by the running time.

$$E_{kWh}=P_{kW}\times t_h$$

How many kWh is 1 unit?

One unit of electricity is 1 kWh. Unit is the billing word printed on electricity bills in India and the UK, while U.S. bills print kWh. Either way it is 1,000 W used for one hour, which equals 3.6 megajoules.

What is 2000 watts for 1 hour in units?

2,000 watts for 1 hour is 2 kWh, or 2 units. The same load uses 1 unit every 30 minutes and 48 units if it runs for a full day. The Energy per Day row on the first card gives that 24-hour figure for any load you enter.

What is 40 watts in units?

A 40 watt load uses 0.04 units per hour, so it takes 25 hours to use 1 unit. Left on for 24 hours it uses 0.96 units, and over a 30-day month that adds up to 28.8 units.

What is 200 units of electricity?

200 units of electricity is 200 kWh. That is a 1 kW load running for 200 hours, or a 100 W load running for 2,000 hours, which is about 83 days without a break. At a rate of 18 cents per kWh it costs $36.

How much is 1000 units of electricity?

1,000 units of electricity is 1,000 kWh, which is 1 MWh. A steady 1 kW load takes 1,000 hours, about 42 days, to use that much. At 18 cents per kWh it costs $180, and the rate printed on your own bill replaces the 18 cents.

The Electrical Power Calculator stops at the 24-hour figure. For any other run time, the Electrical Energy Calculator and the Energy Consumption Calculator take the watts and the hours and return kWh and cost.

Air Conditioner Tons to kW

A ton of air conditioning is a cooling rate, not an electrical load. NIST lists one ton of refrigeration as 12,000 BTU/h, which is 3.517 kW of heat removed. The electrical input is much smaller, and it depends on the efficiency rating of the unit.

The Department of Energy defines EER as the cooling capacity in BTU per hour divided by the power input in watts. Dividing the BTU/h by the EER therefore gives the running watts.

$$P_W=\frac{Q_{BTU/h}}{EER}$$

Size	Cooling output	Input at EER 10	Input at EER 12
1 ton	12,000 BTU/h (3.517 kW)	1.2 kW	1.0 kW
1.5 ton	18,000 BTU/h (5.275 kW)	1.8 kW	1.5 kW
2 ton	24,000 BTU/h (7.034 kW)	2.4 kW	2.0 kW

How many kW is 1.5 ton AC?

A 1.5 ton AC moves 18,000 BTU/h, which is 5.275 kW of cooling. Its electrical draw is about 1.8 kW at an EER of 10 and 1.5 kW at an EER of 12. A 1 ton AC load is 1.2 kW and 1.0 kW at the same two ratings.

Can I use 2 AC on 2 kW load?

Two 1 ton units at an EER of 12 draw 1.0 kW each, so together they fill a 2 kW limit. That leaves nothing for lights, a refrigerator or the higher current a compressor pulls as it starts. Two 1.5 ton units need 3.0 to 3.6 kW and will not fit under 2 kW at all.

How much AC bill in 1 hour?

The AC bill for 1 hour is the running kW multiplied by the rate. A 1.5 ton unit drawing 1.5 kW uses 1.5 kWh in an hour of steady running, which costs 27 cents at 18 cents per kWh. The compressor cycles off once the room reaches the set temperature, so a real hour usually costs less.

For your own unit, enter the nameplate volts and amps in the Electrical Power Calculator above with the power factor from the label. The main result is the running wattage, and the Energy per Day row shows what 24 hours of steady running would use.