The lifespan of a solar battery is determined by the number of cycles it can use. A battery cycle is defined as the number of times a battery can be fully charged and discharged before they reach the end of their functional life.
Cycle life specifications can vary substantially depending on their internal chemistry. Fortunately, lithium-ion batteries, which solar storage units primarily use, have the greatest number, typically having 4000-8000 cycles within their lifetime.
In practice, a solar battery may be used four times at 25% to reach one full cycle, provided the DOD of the battery in 100%.