How to use the Electrical Power Calculator
Select the two electrical values you know, then enter their values in the fields. The calculator automatically determines the remaining two values.
- Select the two values you know, such as Voltage and Current.
- Enter each value using the appropriate unit.
- Read the calculated values for the remaining electrical quantities.
You can enter Voltage (V), Current (A), Resistance (Ω), or Power (W). The calculator supports all six possible pairs of known values.
Electrical power and Ohm's Law formulas
Ohm's Law describes the relationship between voltage, current, and resistance:
V = I × R I = V ÷ R R = V ÷ I Electrical power describes how much electrical energy is being transferred or used per unit of time. Power can be calculated from voltage and current, or by combining Ohm's Law with the power formula:
P = V × I P = I² × R P = V² ÷ R Rearranging these formulas also allows you to calculate voltage, current, or resistance when power is known:
V = P ÷ I I = P ÷ V R = V² ÷ P R = P ÷ I² V = √(P × R) I = √(P ÷ R) What can this calculator calculate?
You can enter any two of the four values below. The calculator uses the appropriate Ohm's Law and power formula to calculate the other two.
- Voltage + Current: calculates resistance and power.
- Voltage + Resistance: calculates current and power.
- Voltage + Power: calculates current and resistance.
- Current + Resistance: calculates voltage and power.
- Current + Power: calculates voltage and resistance.
- Resistance + Power: calculates voltage and current.
Worked example
Suppose a circuit operates at 12 volts and draws 2 amps of current. Enter 12 V for voltage and 2 A for current.
Resistance is calculated using Ohm's Law:
R = V ÷ I = 12 ÷ 2 = 6 Ω Power is calculated from voltage and current:
P = V × I = 12 × 2 = 24 W The resulting circuit values are therefore 12 V, 2 A, 6 Ω, and 24 W.
Power vs. electrical energy
Electrical power and electrical energy are related, but they are not the same thing. Power is measured in watts (W) and describes the rate at which energy is used or transferred. Electrical energy is commonly measured in watt-hours (Wh) or kilowatt-hours (kWh).
For example, a device using 100 watts for 3 hours consumes 300 watt-hours of energy. For calculations involving power and operating time, see the Watt-hour Calculator.
Power and electricity calculations
This calculator focuses on the relationship between voltage, current, resistance, and power. If you want to calculate electricity use or estimate energy consumption and cost, use the Electricity Calculator.
For battery-related calculations such as battery capacity, battery life, or charging time, see the Battery Calculator.
Frequently asked questions
Why do I only enter two values?
Voltage, current, resistance, and power are all mathematically linked through Ohm's Law and the power formula. Knowing any two of them is always enough to calculate the other two — entering a third or fourth value would be redundant, since it's already determined by the first two.
What if my current is zero?
Zero current is valid when current and resistance are the known
values. Using V = I × R and
P = I² × R, zero current produces zero voltage
and zero power for a finite resistance. However, zero current cannot
be used to calculate resistance from voltage and current because that
would require division by zero.
What units does the calculator use?
Voltage is measured in volts (V), current in amperes or amps (A), resistance in ohms (Ω), and power in watts (W). Enter values using these units before calculating.
How do I calculate watts from volts and amps?
Multiply voltage by current. The formula is
P = V × I. For example, a 12 V circuit drawing
2 A uses 24 W of electrical power.
Does this work for AC circuits?
The formulas can be applied directly to DC circuits and purely resistive AC loads. AC circuits containing inductors, capacitors, motors, or other reactive loads may require power factor and additional AC power calculations. This calculator does not account for power factor.