Common electrical units used in formulas and equations are:

• Volt - unit of electrical potential or motive force - potential is required to send one ampere of current through one ohm of resistance
• Ohm - unit of resistance - one ohm is the resistance offered to the passage of one ampere when impelled by one volt
• Ampere - units of current - one ampere is the current which one volt can send through a resistance of one ohm
• Watt - unit of electrical energy or power - one watt is the product of one ampere and one volt - one ampere of current flowing under the force of one volt gives one watt of energy
• Volt Ampere - product of volts and amperes as shown by a voltmeter and ammeter - in direct current systems the volt ampere is the same as watts or the energy delivered - in alternating current systems - the volts and amperes may or may not be 100% synchronous - when synchronous the volt amperes equals the watts on a wattmeter - when not synchronous volt amperes exceed watts - reactive power
• Kilovolt Ampere - one kilovolt ampere - KVA - is equal to 1,000 volt amperes
• Power Factor - ratio of watts to volt amperes

Electric Power Formulas

P = V I         (1a)

P = R I²         (1b)

P = V²/ R         (1c)

where

P = power (watts, W)

V = voltage (volts, V)

I = current (amperes, A)

R = resistance (ohms, Ω)

Electric Current Formulas

I = V / R         (2a)

I = P / V         (2b)

I = (P / R)^1/2         (2c)

Electric Resistance Formulas

R = V / I         (3a)

R = V²/ P         (3b)

R = P / I²         (3c)

Electrical Potential Formulas - Ohms Law

Ohms law can be expressed as:

V = R I         (4a)

V = P / I         (4b)

V = (P R)^1/2         (4c)

Example - Ohm's law

A 12 volt battery supplies power to a resistance of 18 ohms.

I = (12 V) / (18 Ω)

    = 0.67 (A)

• download Ohm's law as pdf-file

Electrical Motor Formulas

Electrical Motor Efficiency

μ = 746 P_hp / P_{input_w}         (5)

where

μ = efficiency

P_hp = output horsepower (hp)

P_{input_w} = input electrical power (watts)

or alternatively

μ = 746 P_hp / (1.732 V I PF)         (5b)

Electrical Motor - Power

P_3-phase = (V I PF 1.732) / 1,000         (6)

where

P_3-phase = electrical power 3-phase motor (kW)

PF = power factor electrical motor

Electrical Motor - Amps

I_3-phase = (746 P_hp) / (1.732 V μ PF)         (7)

where

I_3-phase = electrical current 3-phase motor (amps)

PF = power factor electrical motor