Voltage (electric potential difference) measures the "pressure" that drives electric current through a circuit. The SI unit is the volt (V), defined as one joule per coulomb (J/C). Named after Alessandro Volta, who invented the first electrochemical battery in 1800, the volt is a derived SI unit related to the base units by V = kg·m²·A⁻¹·s⁻³.
Working voltages span an enormous range: biological nerve signals operate at millivolts (around −70 mV at rest); household batteries are 1.5–9 V; mains electricity is 120–240 V; electric vehicles use 400–800 V battery packs; and high-voltage transmission lines carry hundreds of kilovolts. Millivolts (mV) and microvolts (µV) are essential in sensor, audio, and medical measurement contexts.
| 1 V | = 1000 mV |
| 1 kV | = 1000 V |
| 1 mV | = 1000 µV |
| AA battery | 1.5 V |
| USB power | 5 V = 5000 mV |
| Mains (US) | 120 V |