SOC (State of Charge)

Written and checked by the Energy Merchants desk · Reviewed 3 October 2026

SOC stands for state of charge. It is the battery’s fuel gauge: the share of usable capacity currently stored, shown as a percentage on the inverter display or the app.

It describes the level, not the size. A 10 kWh usable battery at 40% SOC holds 4 kWh. The same reading on a 20 kWh battery means 8 kWh. For size, see usable capacity.

In practice

Installers meet SOC in three places.

• Backup reserve. Many systems let the owner hold back a minimum SOC, say 20%, for blackouts. Energy below that line is not available for daily cycling.
• VPP and tariff settings. A virtual power plant provider may ask to keep the battery above a set SOC before a forecast peak, or may discharge it down to a floor. Those rules sit in the provider’s agreement. See VPP capable.
• Commissioning. A battery is usually delivered at a low SOC to protect the cells in transport, and the first charge is part of the handover.

A quick example: a household sets a 20% reserve on a 10 kWh usable battery. At 6 pm the SOC is 70%. Usable energy for the evening is the 50 points above the reserve, which is 5 kWh, while the 2 kWh in the reserve stays put for an outage.

Common confusion

SOC is not capacity and it is not state of health. State of health describes how much the battery’s maximum capacity has faded with age, and a tired battery can show 100% SOC while holding less energy than it did new.

SOC also differs from depth of discharge. SOC says how full the battery is. Depth of discharge says how much has been taken out. They are mirror images: 30% depth of discharge is 70% SOC.

For the claim itself, see battery STCs and the battery submission guide. More terms sit in the glossary.