State of charge (SoC) is the level of charge of an electric battery relative to its capacity. The units of SoC are percentage points (0% = empty; 100% = full). An alternative form of the same measure is the depth of discharge (DoD), the inverse of SoC (100% = empty; 0% = full). SoC is normally used when discussing the current state of a battery in use, while DoD is most often seen when discussing the lifetime of the battery after repeated use.

In a battery electric vehicle (BEV), hybrid vehicle (HV), or plug-in hybrid electric vehicle (PHEV), SoC for the battery pack is the equivalent of a fuel gauge.It is important to mention that state of charge, presented as a gauge or percentual value at any vehicle dashboard, especially in plug-in hybrid vehicles, may not be representative for a real level of charge. In that particular case, some noticeable amount of energy stored in the electric battery is not shown at the dashboard, and it is reserved for hybrid-work operations. It permits a vehicle to accelerate with electric motors mainly using battery energy, while engine serves as a generator is used to recharge the battery to the minimum level needed for such operation. Examples of such cars are Mitsubishi Outlander PHEV (all versions/years of production), where 0% of the state of charge presented to the driver is a real 20-22% of charge level (assuming zero level as the lowest level of charge permitted by car producer). Another one is BMW i3 REX (Range Extender version), where about 6% of SOC is reserved for PHEV-alike operations. Teslas have stated that their SoC should be less than 95%, with some saying between 30%-80%. There is some data to back this up as well.