Power Calculator

What is an electrical power calculator?

Electrical power describes the rate at which electrical energy is converted to other forms (heat, light, motion). The fundamental relationship is P = V * I (power equals voltage times current). Combined with Ohm's Law (V = I * R), this gives four interrelated formulas known as the power wheel, letting you calculate any quantity from any two known values.

Power is measured in watts (W). In DC circuits, the calculation is straightforward multiplication. In AC circuits, real power (watts) differs from apparent power (volt-amps) due to the phase angle between voltage and current, known as the power factor. This calculator handles DC power calculations.

The power formulas

• P = V * I — Power from voltage and current (most direct measurement).
• P = I squared * R — Power from current and resistance (useful for wire heating calculations).
• P = V squared / R — Power from voltage and resistance (useful for resistor power ratings).
• P = V * V / R = I * I * R — All forms are algebraically equivalent via Ohm's Law.

How to use this tool

Enter any two of the four electrical quantities (voltage, current, resistance, power) and the calculator instantly computes the other two. All units are displayed clearly. This saves you from memorizing or rearranging formulas manually.

Practical applications

Power calculations are essential for selecting components: choosing resistor wattage ratings, sizing power supplies, determining wire gauges, and calculating battery life. A resistor dissipating more power than its rating will overheat and fail. A power supply must provide at least as many watts as the circuit demands.

Worked example

A device draws 2 amps at 120 volts. Electrical power is P = V × I = 120 × 2 = 240 watts. Over an 8-hour day it uses 240 × 8 = 1,920 watt-hours, or 1.92 kWh. If you only know the current and resistance, use P = I²R; if you know voltage and resistance, use P = V²÷R. All three forms come from combining P = VI with Ohm's law.

Common mistakes

The classic mistake is confusing power (watts) with energy (watt-hours); power is the instantaneous rate, energy is power accumulated over time. Another is using RMS and peak values interchangeably in AC circuits — mains power calculations use RMS voltage and current. For AC with reactive loads, real power also depends on the power factor, so P = VI overstates the true consumption of motors and transformers.

Frequently asked questions

What is the difference between watts and volt-amps?

In DC circuits, they are identical. In AC circuits, watts represent real (useful) power, while volt-amps (VA) represent apparent power. The ratio is the power factor. A purely resistive load (like a heater) has a power factor of 1.0. Motors and switching power supplies have lower power factors, meaning they draw more current than you might expect from their wattage alone.

How do I choose the right wattage rating for a resistor?

Calculate the actual power dissipation using P = V squared / R, then choose a resistor rated for at least twice that power. Standard through-hole resistors are typically 1/4 watt (250 mW). If your calculation shows 200 mW, use a 1/2 watt resistor. This safety margin accounts for ambient temperature and prevents premature failure.

What is the difference between watts and volt-amps?

In a DC or purely resistive circuit they are equal. In AC circuits with reactive components, volt-amps (VA) measure apparent power — the product of RMS voltage and current — while watts measure real power actually consumed. The ratio between them is the power factor, which is why equipment like UPS units is rated in VA.