Electrical Unit Guide for Engineers
Electrical engineering uses a tightly connected set of units linked by fundamental laws. Mastering the relationships between volts, amps, watts, ohms, and energy units is essential for circuit analysis, power system design, and equipment specification.
Core Electrical Laws & Formulas
Ohm's Law
Power (DC / single-phase AC)
Energy
Three-Phase Power
AC RMS note: For AC circuits, voltage and current values are typically given as RMS (root mean square). Peak voltage = V RMS × √2. US household voltage is 120 V RMS; peak is ~170 V.
SI Electrical Units Reference
| Unit | Symbol | Measures | SI Derivation |
|---|---|---|---|
| Volt | V | Electric potential | J/C = kg·m²/(A·s³) |
| Ampere | A | Electric current | SI base unit |
| Watt | W | Power | V·A = J/s |
| Ohm | Ω | Resistance | V/A = kg·m²/(A²·s³) |
| Farad | F | Capacitance | C/V = A²·s⁴/(kg·m²) |
| Henry | H | Inductance | V·s/A = kg·m²/(A²·s²) |
| Hertz | Hz | Frequency | cycles/s = s⁻¹ |
| Coulomb | C | Electric charge | A·s |
Common Electrical Conversions
| From | Equals |
|---|---|
| 1 kWh | 3.6 MJ (3,600,000 J) |
| 1 horsepower (hp) | 746 W = 0.746 kW |
| 1 BTU/h | 0.293 W |
| 1 mA (milliamp) | 0.001 A |
| 1 kV (kilovolt) | 1,000 V |
| 1 MW (megawatt) | 1,000 kW = 1,000,000 W |
| 1 MWh | 1,000 kWh = 3,600 MJ |
| 1 kVA | 1 kW at unity power factor |
Voltage Standards by Country
Electrical standards vary globally. Always check local requirements before specifying equipment for international installations.
| Country / Region | Voltage | Frequency | Plug Type |
|---|---|---|---|
| United States | 120 V / 240 V | 60 Hz | Type A/B |
| Canada | 120 V / 240 V | 60 Hz | Type A/B |
| Europe (most) | 230 V | 50 Hz | Type C/F |
| United Kingdom | 230 V | 50 Hz | Type G |
| Japan | 100 V | 50/60 Hz | Type A |
| Australia / NZ | 230 V | 50 Hz | Type I |
| China | 220 V | 50 Hz | Type A/C/I |
| India | 230 V | 50 Hz | Type C/D |
Frequently Asked Questions
How do I calculate watts from volts and amps?
Use the power formula P = V × I, where P is power in watts, V is voltage in volts, and I is current in amperes. For example, a device drawing 5 amps at 120 volts consumes 600 watts. For AC circuits, multiply by the power factor: P = V × I × pf. For three-phase AC: P = √3 × VL × IL × pf.
How do I convert kWh to joules?
1 kilowatt-hour (kWh) = 3,600,000 joules (3.6 MJ). This is because 1 kW = 1,000 W = 1,000 J/s, and 1 hour = 3,600 seconds. So: 1 kWh = 1,000 J/s × 3,600 s = 3,600,000 J = 3.6 MJ.
What is the difference between kW and kWh?
kW (kilowatt) measures power — the rate of energy use at a moment in time. kWh (kilowatt-hour) measures energy — the total amount of energy used over time. A 1 kW appliance running for 1 hour uses 1 kWh of energy. Think of kW as speed and kWh as distance traveled.