What is SOC (state of charge) and why does it matter?

SOC is the current charge level of the storage system, expressed as a percentage. For daily cycles, manufacturers typically recommend a range of 10–90%: this is the usable capacity, roughly 80% of the nominal capacity. This is decisive for sizing: from a storage system with a nominal “1 MWh”, only ~0.8 MWh can actually be moved in a daily cycle. When comparing offers, always clarify whether the stated capacity is nominal or usable — mixing the two up causes a 20% sizing error. A wider SOC window moves more energy per cycle but accelerates degradation, which is why the manufacturer’s warranty is usually tied to a specific SOC range: operating outside it, a capacity claim can be rejected. The optimal window depends on the cell chemistry and the application — daily single-cycle arbitrage requires a different setting than rarely activated peak shaving or reserve services. In practice, it is the task of the energy management system (EMS) to adapt this range to the operating profile.