Adding Batteries to Old Solar: Rules & Hidden Costs

New Tech, Old Wires

With the new federal battery rebate starting in July 2025, many homeowners with older solar systems (5+ years) are rushing to add storage. It is a great idea, but it's not always a simple plug-and-play job.

The "Current Standards" Trap

Electrical safety standards change every few years. When an electrician touches your switchboard to add a battery, they are legally required to bring that entire circuit up to current 2025 standards.

This often means:

• Switchboard Upgrade: Replacing old ceramic fuses with RCBO safety switches ($500 - $1,000).
• Earthing Upgrade: Installing a new earth stake if the old one is corroded ($200 - $400).
• Cabling: Replacing undersized cables that were fine in 2015 but aren't compliant today.

The "Deeming Period" Myth

Good news: Adding a battery does not rely on the solar "deeming period." Batteries have their own eligibility rules. As long as you are adding at least 5kWh of new capacity, you can claim the federal rebate, regardless of how old your panels are.

Inverter Compatibility

Most old inverters can't talk to a battery directly. You will likely need an "AC Coupled" battery (like a Tesla Powerwall or Sonnen) which has its own built-in inverter. This bypasses your old solar inverter entirely, making it a safe bet for retrofits.