Calculation: Electric Power, Voltage, Current, and Resistance
Voltage current resistance and electric power formula
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The relationship between electrical parameters Electrical voltageV,amperageI,resistivityR, impedanceZ, wattage or power P
The nominal impedance Z = 4, 8, and 16 ohms (loudspeakers) is often assumed as resistance R.
Ohm's law equation (formula): V = I × R and the power law equation (formula): P = I × V. P = power, I or J = Latin: influare, international ampere, or intensity and R = resistance. V = voltage, electric potential difference or E = electro motive force (EMF = voltage).
Enter any two of the following values and click the calculation button.
The missing values will be calculated. Enter only two values.
The Formula Wheel of Electrical Engineering
V comes from "voltage" and E from "electromotive force". E means also energy, so V is chosen.
Energy = voltage × charge. E = V × Q. Some like better to stick to E instead to V, so do it.
Formula 1 − Electrical power equation: PowerP = I × V = R × I2 = V2⁄R
where power P is in watts, voltage V is in volts and current I is in amperes (DC).
If there is AC, look also at the power factor PF = cos φ and φ = power factor angle
(phase angle) between voltage and amperage.
Formula 2 − Mechanical power equation: PowerP = E ⁄ t = W ⁄ t
where power P is in watts, Energy E is in joules, and time t is in seconds. 1 W = 1 J/s.
Power = force multiplied by displacement divided by time P = F × s / t or:
Power = force multiplied by speed (velocity) P = F × v.
Electrical Energy is E = P × t − measured in watt-hours, or also in kWh. 1J = 1N×m = 1W×s
Undistorted powerful sound is not found in these formulas. Please, mind your ears!
The eardrum and microphone diaphragms are really only moved by the waves of the sound pressure. That does not do either the intensity, nor the power or the energy.
If you are in the audio recording business, it is wise not to care much about the
energy, power and intensity as the cause, care more about the effect of sound pressure
and sound pressure level on the ears and the microphones and the corresponding
audio voltage. Sound pressure and Sound power – Effect and Cause
Very loud sounding speakers will have a lot of power, but look closer at the very
important efficiency of loudspeakers. This includes the typical question: How many decibels (dB) are actually twice or three times as loud?
There is really no RMS power. The words "RMS power" are not correct. There is a
calculation of power which is the multiplication of a RMS voltage and a RMS current.
Watts RMS is meaningless. In fact, we use that term as an extreme shorthand for power in
watts calculated from measuring the RMS voltage. Please, read here: Why there is no such thing as 'RMS watts' or 'watts RMS' and never has been.
Power is the amount of energy that is converted to a unit of time. Expect to pay more when
demanding higher power.
Power is like all energy quantities
primarily a calculated value.
The word "power amplifier" is a misnomer. Power is not really something
that can be "amplified". Voltage and current can be amplified. The term
"power amplifier" although technically incorrect has become understood
to mean an amplifier that is intended to drive a load such as a loudspeaker.
We call the product of current and voltage gain just "power amplification".
The total energy within a closed system due to the conservation of energy is
neither increased nor decreased.
Tip: The electrical power triangle (power formula)
The magic triangle can be used to calculate all formulas of the "electric power law". You hide with
a finger the value to be calculated. The other two values show then how to do the calculation.
Please enter two values, the third value will be calculated.
Vl = line voltage (volts), Vp = phase voltage (volts), Il = line current (amps), Ip = phase current (amps) Z = impedance (ohms), P = power (watts), φ = power factor angle, VAR = volt-amperes (reactive)
Current (single phase): I = P / Vp×cos φ
Current (3 phases): I = P / √3 Vl×cos φ or I = P / 3 Vp×cos φ
Power (single phase): P = Vp×Ip×cos φ
Power (3 phases): P = √3 Vl×Il×cos φ or P = √3 Vp×Ip×cos φ
Power factor PF = cos φ = R/(R2 + X2)1/2, φ = power factor angle. For the purely resistive circuit, PF = 1 (perfect).
The apparent power S is calculated according to Pythagoras, the active power P and reactive power Q. S = √(P2 + Q2)
