Good clarification! Smarter backup batteries like Pila or Powerwall can be configured to dynamically charge and discharge, giving the benefit of outage protection and energy management. For example, charge up from the solar power you produce at home, and use that stored solar power to offset energy usage at night or when utility power is expensive. They they have a "backup reserve" setting which allows you to set a minimum charge to always be stored for an unexpected outage. Smart features like "Storm Mode" can use severe weather alerts to automatically charge up to 100% ahead of an outage to ensure you're prepared for possible outages

It can charge when energy is cheap & discharge when energy is expensive. If you have solar it can charge when solar is producing and discharge at night.