The International System of Units (abbreviated SI from “systeme internationale “, the French version of the name) is a scientific method of expressing the magnitudes or quantities of important natural phenomena. There are seven base units in the system, from which other units are derived, as follows:
| Property | Unit |
| Length | metre (m) |
| Mass | kilogram (kg) |
| Time | Second (s) |
| Electrical current | ampere (A) |
| Thermodynamic temperature | kelvin (K) |
| Amount of substance | mole |
| Luminous intensity | Candela |
The derived units most ikmportantfor hydro production as are follows:
| Property | Unit |
| Force, F | Newton (N) |
| Work | Joule (J) |
| Power, P | Watt (W) |
| Energy, E | kiloWatt-hour (kW-h) |
| Velocity, v | m/s |
| Area, A | m² |
| Flow, Q | m³/s |