SI electrical units
The ampere is one of the seven SI base units. Since 2019 it is defined by fixing the elementary charge at exactly e = 1.602 176 634 × 10⁻¹⁹ C, so one ampere is one coulomb of charge passing per second. The other electrical units are derived units with special names, each expressible through the ampere and the mechanical units.
| Unit | Symbol | Quantity | In other SI units | In base units |
|---|---|---|---|---|
| ampere | A | electric current | C/s | A (base unit) |
| coulomb | C | electric charge | A·s | A·s |
| volt | V | electric potential difference (voltage) | W/A | kg·m²·s⁻³·A⁻¹ |
| ohm | Ω | electric resistance | V/A | kg·m²·s⁻³·A⁻² |
| siemens | S | electric conductance | A/V = 1/Ω | kg⁻¹·m⁻²·s³·A² |
| watt | W | power | J/s = V·A | kg·m²·s⁻³ |
| joule | J | energy, work | N·m = W·s | kg·m²·s⁻² |
| farad | F | capacitance | C/V | kg⁻¹·m⁻²·s⁴·A² |
| henry | H | inductance | Wb/A = V·s/A | kg·m²·s⁻²·A⁻² |
| weber | Wb | magnetic flux | V·s | kg·m²·s⁻²·A⁻¹ |
| tesla | T | magnetic flux density | Wb/m² | kg·s⁻²·A⁻¹ |
| hertz | Hz | frequency | 1/s | s⁻¹ |
Sources: BIPM: The International System of Units (SI Brochure), 9th edition; NIST: SI Units
Units used alongside the SI
Several units that are not SI units with special names are standard in electrical work. The kilowatt-hour is a unit of energy equal to one kilowatt sustained for one hour. The volt-ampere is the product of RMS voltage and RMS current and is used for apparent power; the SI Brochure lists the var as a special name for the volt ampere when it expresses reactive power.
| Unit | Symbol | Used for | Value in SI units | Status |
|---|---|---|---|---|
| kilowatt | kW | power | 1 kW = 1000 W | SI prefix + SI unit |
| watt-hour | Wh | energy | 1 Wh = 3600 J | non-SI (product of W and h) |
| kilowatt-hour | kWh | energy (utility billing) | 1 kWh = 3.6 × 10⁶ J = 3.6 MJ | non-SI |
| volt-ampere | VA | apparent power S = V·I | 1 VA = 1 V × 1 A | coherent SI unit V·A |
| kilovolt-ampere | kVA | apparent power (transformers, generators) | 1 kVA = 1000 VA | SI prefix + V·A |
| var | var | reactive power Q | 1 var = 1 V·A | special name listed in the SI Brochure, Table 8 |
| ampere-hour | Ah | battery charge capacity | 1 Ah = 3600 C | non-SI (product of A and h) |
| electronvolt | eV | energy of particles | 1 eV = 1.602176634 × 10⁻¹⁹ J (exact) | non-SI, listed in the SI Brochure, Table 8 |
Sources: BIPM: The International System of Units (SI Brochure), 9th edition
Decibels: dB, dBm and dBW
The bel and decibel express logarithmic ratios (1 dB = 0.1 B). For a power-like quantity the level is L = 10 log₁₀(P/P₀) dB, and the reference P₀ must always be stated. dBW uses P₀ = 1 W, so L = 10 log₁₀(P/1 W). dBm uses P₀ = 1 mW, so L = 10 log₁₀(P/1 mW), which makes dBm = dBW + 30.
For amplitude quantities such as voltage across the same impedance, power scales with the square of amplitude, so the level becomes 20 log₁₀(V/V₀) dB. Every 3 dB is very nearly a doubling of power and every 10 dB is exactly a factor of 10.
| Power | dBm | dBW |
|---|---|---|
| 1 µW | −30 | −60 |
| 10 µW | −20 | −50 |
| 100 µW | −10 | −40 |
| 1 mW | 0 | −30 |
| 10 mW | +10 | −20 |
| 100 mW | +20 | −10 |
| 500 mW | +26.99 | −3.01 |
| 1 W | +30 | 0 |
| 2 W | +33.01 | +3.01 |
| 10 W | +40 | +10 |
| 100 W | +50 | +20 |
| 1 kW | +60 | +30 |
Sources: BIPM: The International System of Units (SI Brochure), 9th edition