What Is Power?
Power is the rate at which work is done or energy is transferred, measured in watts (W). One watt equals one joule per second. In electrical systems, DC power is simply voltage multiplied by current (P = V × I). AC power introduces the power factor to account for phase differences between voltage and current. Mechanical rotational power depends on torque and rotational speed. An engineer in Munich designing a conveyor belt motor needs to convert the mechanical torque and RPM requirements into electrical power to select the right motor and drive. A 50 N·m motor running at 3000 RPM produces about 15.7 kilowatts, or about 21 horsepower.
Understanding power across electrical and mechanical domains is essential for motor selection, energy planning, and system design. For Ohm's Law calculations including resistance, see our Ohms Law Calculator.
What This Calculator Does
This power calculator supports three modes: DC electrical power from voltage and current, AC real power from voltage, current, and power factor, and mechanical power from torque and RPM with horsepower output.
- DC mode: Inputs voltage (V) and current (I), outputs P = V × I in watts and kilowatts
- AC mode: Inputs voltage (V), current (I), and power factor, outputs P = V × I × PF in watts and kilowatts
- Mechanical mode: Inputs torque (N·m) and RPM, outputs P = τ × 2π × RPM / 60 in watts and horsepower
How the Calculations Work
DC Electrical Power
P = V × I
In a DC circuit, power is the straightforward product of voltage and current. A 12-volt battery delivering 5 amperes produces 60 watts. This is the simplest power relationship and applies to batteries, solar panels, and DC electronics.
AC Real Power
P = V × I × PF
In AC circuits with reactive loads like motors and transformers, voltage and current are not perfectly in phase. The power factor (PF) ranges from 0 to 1 and represents the fraction of apparent power that does real work. A purely resistive load has PF = 1. An induction motor might have PF = 0.85. A 120-volt circuit carrying 10 amperes at PF 0.85 delivers 1,020 watts of real power, not 1,200 watts. The NIST Guide to SI Units defines the watt and its relationship to other units.
Mechanical Power
P = τ × 2π × RPM / 60 (watts)
HP = P / 745.7
Rotational mechanical power is torque multiplied by angular velocity. Angular velocity in radians per second is 2π × RPM / 60. The result in watts can be converted to horsepower by dividing by 745.7, since one mechanical horsepower equals 745.7 watts. For detailed horsepower calculations, see our horsepower calculator.
Limitations
The DC mode assumes ideal DC sources without losses. The AC mode calculates real power only and does not show apparent power (VA) or reactive power (VAR). The mechanical mode assumes constant torque across the RPM range, which is not realistic for most engines and motors. None of the modes account for efficiency losses in power transmission. For energy cost calculations, use our Electricity Calculator. For voltage drop in wiring, see our Voltage Drop Calculator.