Electrical Units: Watts, Amps, Volts, Ohms

The four fundamental electrical units are volts, amps, ohms, and watts, and they are tied together by Ohm's Law and the power equation. A useful way to picture them: voltage is pressure, current is flow rate, resistance is pipe diameter, and power is the work done by the water. This page explains each unit, its SI definition, and how they relate, so you can keep them straight the next time you read an appliance label or a breaker spec.

Formulas

Watt (W) = unit of power (rate of energy use)

Ampere (A) = unit of current (flow of charge)

Volt (V) = unit of voltage (electrical pressure)

Ohm (Ω) = unit of resistance (opposition to flow)

Voltage pushes current through resistance, and the result is power. Think of it like water: voltage is pressure, current is flow rate, resistance is pipe diameter, and power is how much work the water does. These four quantities are connected by Ohm's Law (V = IR) and the power equation (P = VI).

Worked Examples

Example 1: A 100W lightbulb at 120V

Draws 100/120 = 0.833 amps through a resistance of 120/0.833 = 144 Ω

Example 2: A 1,500W heater at 120V

Draws 1500/120 = 12.5 amps through a resistance of 120/12.5 = 9.6 Ω

Example 3: A 12V car circuit drawing 10A

Power = 12 × 10 = 120 watts. Resistance = 12/10 = 1.2 Ω

Common Mistakes

Confusing amps (current) with volts (voltage). Amps is how much flows, volts is how hard it pushes.
Saying "watts per hour". Watts already includes time (watts = joules per second). The correct unit is watt-hours for energy.
Treating kilowatts and kilowatt-hours as the same. kW is power (rate), kWh is energy (total).

Try the Calculator

Use the interactive Ohm's Law Calculator for instant results with any values. Every result page shows the formula applied with your specific numbers.

All Formulas

functionsWatts to Amps Formula functionsOhm's Law Formula functionsEnergy Cost Formula functionsWire Sizing Formula functionsVoltage Drop Formula functionsThree-Phase Power functionsPower Factor functionsHP to Amps Formula functionsSeries & Parallel functionsAC vs DC functionsNEC Reference

This is a formula reference page for educational use. Always consult a licensed electrician and verify compliance with the National Electrical Code (NEC) and local electrical codes before applying any of these formulas to real installations.

Standards & References

This page cites the following electrical codes and standards. Always consult the current edition of your local adopted standard for authoritative requirements.

NIST SP 811: SI Unit Definitions. Definitions of the volt, ampere, watt, ohm, coulomb, and joule under the International System of Units (SI). Authoritative reference for unit conversions and precision.
National Institute of Standards and Technology. Reference →
IEC 60038. IEC standard voltages. Defines 230/400V as the nominal low-voltage supply for 50Hz systems, which harmonized European residential supply in 1995.
International Electrotechnical Commission. Reference →

Disclaimer: The information on this page is provided for reference. Always consult a licensed electrician and the current edition of your local adopted electrical code before performing electrical work.