Power Calculator
Connecting a new air conditioner or sizing a backup generator without a power calculator risks tripped breakers and wasted energy. Every electrical device depends on the relationship between watts, volts, and amps, and getting the numbers right keeps circuits safe and efficient.
The calculator above processes voltage, current, and power factor values for DC, single-phase AC, and three-phase AC systems. It returns the real power in watts and kilowatts, using the exact formulas described below.
How does a power calculator convert amps to watts?
The simplest relationship appears in DC circuits. Power in watts equals voltage in volts multiplied by current in amperes.
Formula: P = V × I
Example: A 12 V battery driving a 5 A load produces 60 W of power.
For AC circuits, the calculation expands because voltage and current oscillate. Multiply the product by the power factor (PF), a dimensionless number between 0 and 1.
Single-phase AC: P = V × I × PF
Example: A 120 V outlet supplying 8 A to a motor with a 0.85 power factor delivers 816 W.
Can a power calculator handle three-phase AC circuits?
Industrial motors and commercial buildings use three-phase supply. The conductors carry voltage waveforms offset by 120 degrees, so the total power formula includes √3 ≈ 1.732.
Formula: P = √3 × V_L × I_L × PF
Where V_L is line-to-line voltage and I_L is line current.
Example: A 400 V three-phase compressor drawing 15 A with a 0.88 power factor consumes:
1.732 × 400 × 15 × 0.88 = 9,145 W, or about 9.1 kW.
Calculating power with Ohm’s law
If you know resistance instead of current, two additional formulas derived from Ohm’s law let you skip measuring amperage directly.
Current-based: P = I² × R
Voltage-based: P = V² ÷ R
These work for purely resistive DC loads and for resistive AC loads where the power factor is effectively 1.
Example: A 240 V electric kettle with a 12 Ω heating element draws:
240² ÷ 12 = 57,600 ÷ 12 = 4,800 W.
Common power conversions
Electrical specifications sometimes use horsepower or BTU/hr instead of watts.
| Unit | Equivalent in watts |
|---|---|
| 1 mechanical horsepower (hp) | 746 W |
| 1 metric horsepower (PS) | 735.5 W |
| 1 BTU/hr | 0.293 W |
| 1 kilowatt (kW) | 1,000 W |
Example: A 2 hp pool pump converts to 1,492 W.
Practical examples for homes and workshops
- Smartphone charger: 5 V × 2 A = 10 W
- Desktop computer: 120 V × 3.5 A = 420 W
- Table saw (single-phase): 230 V × 12 A × 0.85 PF = 2,346 W
- Commercial HVAC (three-phase): 480 V × 25 A × 0.9 PF = 18,660 W (18.7 kW)
These figures help you choose the right wire gauge, fuse rating, or inverter capacity.
When to use apparent, real, and reactive power
Real power, measured in watts, performs actual work. Apparent power, measured in volt-amperes (VA), is the vector sum of real and reactive power. Reactive power, measured in volt-amperes reactive (VAR), sustains magnetic fields in motors and transformers but does no work.
A calculator focused on watts returns real power. When sizing an uninterruptible power supply (UPS) or transformer, check the VA rating, because it must handle the full apparent load.
Frequently Asked Questions
How many watts are in one horsepower?
One mechanical horsepower equals 746 watts. One metric horsepower (PS) equals 735.5 watts. Horsepower measures the same quantity as watts but uses a different unit scale.
What does power factor mean in AC circuits?
Power factor is the ratio of real power to apparent power, ranging from 0 to 1. A value of 1 means all current does useful work. Resistive loads like heaters have a power factor near 1, while motors often run at 0.7 to 0.9.
Can I calculate power if I only know resistance and voltage?
Yes. Using Ohm’s law, current equals voltage divided by resistance. Substituting into the power formula gives P = V² ÷ R. Alternatively, if you know current and resistance, use P = I² × R.
Why is apparent power measured in volt-amperes instead of watts?
Apparent power includes both real and reactive components. Watts measure only real power consumed. Volt-amperes describe the total load on conductors and equipment, which is important for sizing transformers and wires.
Is a power calculator suitable for both AC and DC circuits?
Yes. A reliable tool accepts both AC and DC inputs. DC calculations use P = V × I, while AC calculations add the power factor and may include the √3 multiplier for three-phase systems.
What does a power factor of 1.0 mean?
A power factor of 1.0 represents a purely resistive load where voltage and current are perfectly in phase. All supplied energy converts to useful work, with no reactive power circulating in the circuit.