Electrical formulas that explain their conditions
Every expression is paired with variable definitions, SI units, the conditions under which it applies, a worked example and the matching calculator. These are engineering relationships, not universal shortcuts.
Ohm’s Law
Ohm’s law relates voltage, current and resistance in a linear conductor: the voltage across the resistance equ...
Electrical LawsKirchhoff’s Current Law (KCL)
Kirchhoff’s current law states that the total current entering a node equals the total current leaving it, bec...
Electrical LawsKirchhoff’s Voltage Law (KVL)
Kirchhoff’s voltage law states that the algebraic sum of voltage changes around any closed loop is zero. In a...
Power EnergyElectrical Power
Electrical power is the rate at which electrical energy is transferred. For a resistive load it can be calcula...
Power EnergyElectrical Energy
Electrical energy is power multiplied by the time over which it is delivered. With power in watts and time in...
Electrical CircuitsSeries Resistance
When resistances are connected in series, the same current flows through each one and the total voltage is sha...
Electrical CircuitsParallel Resistance
Parallel branches share the same voltage, and their currents add. Because each branch provides another path fo...
Electrical CircuitsVoltage Divider
A voltage divider uses two series resistors to produce an output voltage that is a fixed fraction of the input...
Electrical CircuitsCurrent Divider
A current divider splits a total current between parallel branches. The current through one branch of a two-br...
Electrical CircuitsVoltage Drop
Voltage drop is the loss of voltage along the conductors that carry a load current. In a two-wire circuit the...
Dc ElectricityCable Resistance
The resistance of a cable depends on the material, its length and its cross-sectional area. Resistivity is a p...
ComponentsCapacitance
Capacitance is the ability of a component to store charge for a given voltage: C = Q ÷ V. The energy stored in...
ComponentsInductance
An inductor opposes a change of current by inducing a voltage proportional to the rate of change: V = L × ΔI ÷...
Ac ElectricityReactance
Reactance is the frequency-dependent opposition that inductors and capacitors present to alternating current....
Ac ElectricityImpedance
Impedance is the total opposition to alternating current, combining resistance and reactance. In a series R-L-...
Power EnergyAC Power
In an AC circuit the product of RMS voltage and RMS current gives apparent power in volt-amperes. The active p...
Power EnergyThree-Phase Power
For a balanced three-phase load, active power is the square root of three multiplied by line voltage, line cur...
Power EnergyPower Factor
Power factor is the ratio of active power to apparent power, equal to the cosine of the phase angle between vo...
Power EnergyMotor Efficiency
Motor efficiency is the ratio of mechanical output power to electrical input power, expressed as a percentage....
Power EnergyBattery Energy
Battery energy is estimated by multiplying nominal voltage by capacity in ampere-hours. The result is in watt-...
Power EnergyElectrical Consumption and Cost
Electrical consumption is the energy used over a period, obtained by multiplying power by time. Tariffs are no...
Power EnergyPower Factor Correction
Power factor correction reduces the reactive power drawn from the supply by adding a compensating reactance, u...
Electrical CircuitsLED Series Resistor
An LED needs a current-limiting element because its forward voltage changes only slightly with current. With a...
Electrical CircuitsRC Time Constant
The RC time constant is the product of resistance and capacitance and describes how quickly a capacitor charge...
ProtectionCable Sizing for Voltage Drop
Cable sizing starts from the required current-carrying capacity and from the maximum voltage drop the load can...