Separate resilience from bill management
Backup and economic dispatch are different operating objectives. A resilience design identifies essential circuits and outage duration. A cost-focused design examines the tariff and demand profile. One battery may serve both, but the priorities must be explicit.
Account for charging
Stored electricity comes from somewhere. Charging from the grid introduces energy costs and losses. Time-varying rates may create an opportunity, but a rate spread alone does not demonstrate an attractive project.
Power and energy are different limits
Power capacity determines how much equipment the system can supply at once. Energy capacity helps determine how long it can do so. Starting loads, reserve settings and operating limits also matter.
Backup requires the complete system
A battery installation is not automatically a whole-building backup solution. Islanding capability, transfer equipment, electrical design, controls and commissioning determine what operates during an outage.
Further reading
U.S. Department of Energy — Solar Integration: Solar Energy and Storage Basics ↗