Adding a battery to existing solar: what to check before you buy

Can you get the battery rebate with existing solar?

Yes: the federal battery discount applies to batteries installed with a new or existing solar system of up to 100 kW. Your existing panels and inverter don't need to be on today's approved product list, but they must meet your state's electrical safety rules. The new battery must be on the Clean Energy Council's approved list, be able to join a virtual power plant, and be installed by an accredited installer with a battery endorsement. See how the battery rebate is calculated.

1. Are you on a premium feed-in tariff?

This is the most expensive mistake to make. Some older systems still earn a premium rate for every kWh they export, far more than most retailers pay today (4c is typical now). A battery saves you money by storing solar you would have exported and using it in the evening instead of buying from the grid. If every exported kWh already earns 44c, storing it saves you nothing and costs you the premium, because grid power usually costs less than 44c.

• QLD: Solar Bonus Scheme (44c), paid until 1 July 2028. Closed to new applicants in 2012. Current customers can add a battery or panels without losing it, but a battery may only supply the home when the 44c system isn't generating (or needs a separate connection). Source
• SA: Premium feed-in tariff (44c), paid until 30 June 2028. For systems applied for before 30 September 2011. Installing a battery, or increasing the approved panel or inverter capacity, ends the premium tariff. Source

If you're on one of these tariffs, the usual answer is to wait until it ends before adding a battery, or to get the installer to confirm in writing how the battery will be connected so you keep it. In South Australia, adding a battery ends the premium tariff. If you're not sure, your electricity bill shows your feed-in rate.

2. Has a battery rebate already been claimed here?

Only one battery system per property (or per electricity meter) can claim the federal discount. If your home already has a battery that claimed it, a new or bigger battery won't get it again. You can add to an existing battery and claim for the extra capacity only if the original never claimed STCs, you add at least 5 kWh, and the total stays at or under 100 kWh.

This catches people who bought a home with a battery already installed. Ask the previous owner or the original installer, or contact the Clean Energy Regulator, which can tell you whether a discount was claimed for your installation. Until it's confirmed, don't count on the rebate in your budget: our estimates leave it out when you tell us a claim is possible.

3. How will the battery connect?

There are three common ways, and they affect the price and what happens to your current inverter:

• AC-coupled battery. The battery has its own inverter and sits beside your existing solar inverter, which stays. It's the usual way to add a battery to an existing system and avoids replacing equipment that still works.
• Battery with a built-in inverter. For example the Tesla Powerwall 3, which includes a solar inverter. Depending on your setup it can work alongside your current inverter or take over from it.
• DC battery on a hybrid inverter. Batteries such as Sungrow's SBR range or BYD's Battery-Box connect to a hybrid inverter that runs both the panels and the battery. That usually means replacing your current solar inverter, which adds cost but can be worth it if the old inverter is near the end of its life anyway.

Your installer should tell you which approach they're quoting and why. Also ask about export limits (your network may cap how much you can send back) and whether you're single-phase or three-phase, which affects which batteries suit.

4. Keep, repair, extend or replace?

If your system is more than about 10 years old, or isn't performing, it's worth asking whether to repair, extend or replace it before you add a battery. Inverters typically last 10 to 15 years, and older panels produce less and are much smaller than today's. The federal solar rebate rules matter here:

• A complete replacement (new panels and a new inverter, old system removed) can earn the solar rebate again, as long as nothing reused has claimed before.
• Replacing some panels while keeping components that already claimed doesn't earn any new certificates.
• An extension (adding panels, usually using the existing inverter) can earn certificates for the added capacity, if the inverter has room and the new parts are approved.

Don't let age alone push you into the bigger purchase: a sound system with a new inverter can have years left. Ask your installer to compare the options on your actual usage, including the cost of removing the old system.

Size the battery to the solar you have

A battery only saves money on days it fills up. A small older system (say 3 to 5 kW) may not have enough spare solar to fill a 14 kWh battery, especially in winter, once your daytime use comes first. Your bill shows how much you export: that's the most your battery can store. If it's not much, a smaller battery, or adding panels, may make more sense.

Will it pay for itself?

The saving on each stored kWh is roughly what you pay for grid power minus the feed-in tariff you give up, less about 10% lost charging and discharging. A battery that cycles most days saves a few hundred to several hundred dollars a year, so paybacks are often 10 years or more on savings alone. Our estimates are deliberately conservative and don't count virtual power plant payments. Joining the right retailer plan or virtual power plant (for example Amber, or a retailer's own VPP) can increase savings and shorten payback, depending on the plan's fees and how often it uses your battery. Read the terms before you sign up.

Blackout backup isn't automatic

Many people assume a battery keeps the lights on in a blackout. It only does if the system is set up for backup, which can mean extra hardware and cost. Ask whether your quote includes backup, and whether it covers essential circuits (say lights, fridge and internet) or the whole home.

Questions to ask on your quote

• Will I keep my premium feed-in tariff, if I have one?
• Is the federal battery rebate included, and has a claim been checked for my address?
• Does my current inverter stay? If not, why not?
• Is blackout backup included, and for which circuits?
• What usable capacity am I getting, and how much of my spare solar will actually fill it?
• What are the battery and workmanship warranties, and who do I call if something goes wrong?