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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 Δ V or E = electromotive force (emf = voltage). 
Enter any two known values and press "calculate" to solve for the two others. Please, enter only two values. 

V comes from "voltage" and E from "electromotive force (emf)". E means also energy, so we choose V. Energy = voltage × charge. E = V × Q. Some like better to stick to E instead to V, so do it. For R take Z. 
The most important Formulas: Voltage V = I × R = P / I = √(P × R) in volts V Current I = V / R = P / V = √(P / R) in amperes A Resistance R = V / I = P / I^{2} = V^{2} / P in ohms Ω Power P = V × I = R × I^{2} = V^{2} / R in watts W 
The Big Power Formulas Electrical and mechanical power calculation (strength) 

AndréMarie Ampère was a French physicist and mathematician. The SI unit of measurement of electric current, the ampere, was named after him. Alessandro Giuseppe Antonio Anastasio Volta was an Italian physicist. The SI unit of measurement of electric voltage, the volt, was named after him. Georg Simon Ohm was a German physicist and mathematician. The SI unit of measurement of electric resistance, the ohm, was named after him. James Watt was a Scottish inventor and mechanical engineer. The SI unit of measurement of electric wattage (power), the watt, was named after him. 
In sound engineering there is no Impedance matching or Power matching. In audio we use only high Impedance bridging or Voltage bridging. 
Power is like all energy sizes primarily a calculated value. 
The word "power amplifier" is a misnomer  especially in audio engineering. Voltage and current can be amplified. The strange term "power amplifier" 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 "power amplification". 
A magic triangle can be used to calculate all formulas easily. You hide with
a finger the value to be calculated. The other two values show then how to do the calculation.
Calculations: Ohm's law  Ohm's magic triangle
Measurement of input impedance and output impedance
ALTERNATING CURRENT (AC) ~
V_{l} = line voltage (volts), V_{p} = phase voltage (volts), I_{l} = line current (amps), I_{p} = phase current (amps)
Z = impedance (ohms), P = power (watts), φ = power factor angle, VAR = voltamperes (reactive)
Current (single phase): I = P / V_{p}×cos φ  Current (3 phases): I = P / √3 V_{l}×cos φ or I = P / 3 V_{p}×cos φ 
Power (single phase): P = V_{p}×I_{p}×cos φ  Power (3 phases): P = √3 V_{l}×I_{l}×cos φ or P = √3 V_{p}×I_{p}×cos φ 
DC power formulas Voltage V in (V) calculation from current I in (A) and resistance R in (Ω): V_{(V)} = I_{(A)} × R_{(Ω)} The power P in (W) calculation from voltage V in (V) and current I in (A): P_{(W)} = V_{(V)} × I_{(A)} = V^{2}_{(V)} / R_{(Ω)} = I^{2}_{(A)} × R_{(Ω)} AC power formulas The voltage V in volts (V) is equal to the current I in amps (A) times the impedance Z in ohms (Ω): V_{(V)} = I_{(A)} × Z_{((Ω)} = ( I  ×  Z ) and (θ_{I} + θ_{Z}) The apparent power S in voltamps (VA) is equal to the voltage V in volts (V) times the current I in amps (A): S_{(VA)} = V_{(V)} × I_{(A)} = ( V  ×  I ) and (θ_{V} − θ_{I}) The real power P in watts (W) is equal to the voltage V in volts (V) times current I in amps (A) times the power factor (cos φ): P_{(W)} = V_{(V)} × I_{(A)} × cos φ The reactive power Q in voltamps reactive (VAR) is equal to the voltage V in volts (V) times the current I in amps (A) time the sine of the complex power phase angle (φ): Q_{(VAR)} = V_{(V)} × I_{(A)} × sin φ The power factor (FP) is equal to the absolute value of the cosine of the complex power phase angle (φ): PF =  cos φ  
Quantity  Name  Definition 
frequency f  hertz (Hz)  1/s 
force F  newton (N)  kg·m/s² 
pressure p  pascal (Pa) = N/m²  kg/m·s² 
energy E  work joule (J) = N·m  kg·m²/s² 
power P  watt (W) = J/s  kg·m²/s³ 
electric charge Q  coulomb (C) = A·s  A·s 
voltage V  volt (V)= W/A  kg·m²/A·s³ 
current I  ampere (A) = Q/s  A 
capacitance C  farad (F) = C/V = A·s/V = s/Ω  A²·s^{4}/kg·m² 
inductance L  henry (H) = Wb/A = V·s/A  kg·m²/A²·s² 
resistance R  ohm (Ω) = V/A  kg·m²A²·s³ 
conductance G  siemens (S) = A/V  A²·s³/kg·m² 
magnetic flux Φ  weber (Wb) = V·s  kg·m²/A·s² 
flux density B  tesla (T) = Wb/m² = V·s/m²  kg/A·s² 
The flow of electric charge Q is referred to as an electric current I. The amount of charge per unit time is
the change in electric current. A current flows at a constant value I. during the time t, it transports the
charge Q = I × t. For a temporally constant power, the relationship between the charge and current: I = Q / t or Q = I × t. Through this relationship, the basic units of amps and second the Coulomb in International System of Units is set. The Coulomb unit can be represented as 1 C = 1 A × s. Charge Q, (unit in amperehours Ah), discharge current I, (unit in amperes A), time t, (unit in hours h). 
In acoustics we have an "Acoustic equivalent for ohm's law"
Relationships of acoustic sizes associated with plane progressive sound waves
Conversions of many units, like power and energy
prefixes  length  area  volume  weight  pressure  temperature  time  energy  power  density  velocity  acceleration  force
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