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.
V comes from "voltage" and E from "electromotive force". E means also energy, so V is chosen.
The Big Power Formulas Electrical and mechanical power calculation
Formula 1 − Electrical (electric) 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 (mechanic) 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 times displacement divided by time P = F · s / t or:
Power = force times speed (velocity) P = F · v.
Electric (electrical) Energy is E = P × t − measured in watthours, or also in kWh.
Undistorted powerful sound is not to find in these formulas. Please, mind your ears!
The eardrums are really only moved by the waves of the sound pressure. That does not
do neither the intensity, nor the power or the energy. If you are in the audio recording
business, it is therefore wise not to care much about cause like energy, power and
intensity, care more about sound pressure and voltage.
Very loud sounding speakers should have much 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" show not correct, that there is a
calculation of a power which is the multiplication of a voltage RMS and an amperage RMS.
RMS watts 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 in a unit of time. Expect to pay more when
demanding higher power.
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)
The Big Power Formulas
