Usually yes. How simple it is depends on your inverter type, your electrical service and who owns the existing system.
The three things we check
First, your inverter — some batteries pair with an existing string or microinverter setup through AC coupling, others expect specific hardware. Second, your main panel capacity and whether a backup subpanel is needed. Third, whether the existing system is owned, leased or under a PPA, because a third-party agreement can restrict modifications.
Backup scope drives the design
Whole-home backup and essential-loads backup are very different projects. Deciding which circuits must stay live is the first design conversation, not an afterthought.
Related services
Solar production, bill offset and payment comparisons vary by home, usage, roof, shading, utility rate schedule and financing terms. Nothing on this site is a guarantee of savings. Pricing comparisons depend on system size, equipment, financing structure and contract terms. Any comparison is only meaningful between comparable systems and comparable financing.