How do solar batteries work?

The 30-second take

• A home battery stores energy chemically in lithium cells — most Australian home batteries now use lithium iron phosphate (LFP) — supervised by a battery management system.
• The daily loop: solar powers the house first, surplus charges the battery, and after dark the battery discharges before any grid power is bought.
• AC-coupled batteries bolt onto an existing solar system; DC-coupled batteries share a hybrid inverter with the panels. This is why two quotes for “the same battery” can differ by thousands.
• A round trip through a battery loses roughly 10% of the energy to conversion — a normal cost of storage, and a separate specification from usable capacity.
• The spec-sheet numbers that matter: usable kilowatt-hours, continuous power output, warranted life, and whether the quote includes backup wiring.

What a solar battery actually is

Inside the cabinet on the wall are three things: a stack of lithium battery cells, a battery management system, and power electronics that move energy in and out. Most home batteries sold in Australia now use lithium iron phosphate — LFP — a chemistry chosen for thermal stability and long cycle life rather than the energy density a car needs.

The battery management system is the part nobody mentions and everybody relies on. It monitors every cell’s voltage and temperature, keeps them balanced, and decides when the pack may charge or discharge. It is also the reason a battery never gives you every kilowatt-hour on the label: the system reserves headroom at the top and bottom of the charge range to protect the cells, which is why spec sheets quote a usable figure below the nameplate one.

The daily loop

On a normal day with solar and a battery, energy moves in a fixed order of priority, all of it automatic:

• Morning. Solar generation ramps up and your appliances take it first — the house always eats before the battery does.
• Midday. Generation outruns the house. The surplus charges the battery instead of leaving for the grid at a few cents.
• Afternoon. The battery reaches full. Only now does surplus export to the grid for the feed-in tariff.
• Evening. The sun drops out and the battery takes over, discharging to run the house before a single kilowatt-hour is bought.
• Overnight. If the battery runs down, the grid quietly picks up the remainder — or, on some tariffs, refills the battery cheaply for the morning peak.

Physics takes a small commission on every cycle: of the energy that goes in, roughly a tenth is lost to conversion and chemistry on the way through. That round-trip loss is normal, and it’s one more reason the economics favour households that cycle the battery deeply rather than letting it sit full.

“House first, battery second, grid last — in both directions. The whole mechanism is that ordering, run automatically every day.”

AC-coupled or DC-coupled — why two quotes differ

Solar panels make DC power. Your house runs on AC. A battery stores DC. How a system routes energy between those three is the biggest structural difference between battery products, and the usual reason two quotes for similar capacity are thousands of dollars apart.

• AC-coupled. The battery has its own inverter and connects on the AC side, alongside whatever solar system you already have. This is the standard retrofit path: nothing about your existing panels or inverter needs to change.
• DC-coupled. Panels and battery share one hybrid inverter, and solar can charge the battery without an extra DC-to-AC-to-DC round trip. Efficient and tidy — but if your existing inverter isn’t a hybrid, the quote has to include replacing it.

Neither is simply better. A recent solar system usually points to AC coupling; a new solar-plus-battery install, or an inverter due for replacement anyway, points to a hybrid. What matters at quote stage is that the architecture is stated, because it explains where the money is going.

The spec-sheet numbers that matter

• Usable capacity, in kilowatt-hours. The energy you can actually draw per cycle — the real size of the battery, and the figure the federal rebate is calculated on.
• Continuous power output, in kilowatts. How much the battery can deliver at once. Capacity is how long the tank lasts; power is how wide the tap opens.
• Warranted life. Typically ten years or a stated energy throughput, whichever comes first, usually with a guaranteed remaining capacity at the end. Check what the warranty promises the battery will still hold, not just how long the paper lasts.
• Operating temperature range. Batteries derate in extreme heat and cold. An unshaded west wall in a hot climate is a worse home than a garage.
• Blackout behaviour. Storing energy and riding through an outage are different capabilities — backup needs a backup-capable inverter and dedicated switchboard wiring, and Australian systems must disconnect from a failed grid under AS/NZS 4777.2 either way.

What to look for when you buy

The mechanism above turns into a short buying checklist:

• Size to your evening usage, not your roof. The battery only earns on kilowatt-hours it actually cycles. Oversizing buys capacity that sits idle.
• Compare dollars per usable kilowatt-hour. It is the only way a 10 kWh quote and a 13.5 kWh quote become the same question.
• Decide backup scope before you compare prices. No backup, essential circuits, or whole home are three different jobs with different switchboard work.
• Ask whether it’s VPP-capable, and whether joining one is required by the deal or optional later.
• Ask who commissions it. Check the installer is accredited for that specific battery, and how many of them they have put on walls.

The hardware layer of this market is mature. The installation layer is where outcomes diverge: the Clean Energy Regulator’s rebate-program inspections found more than 60% of home battery installations substandard, overwhelmingly on well-known brands. When you’re ready, get matched with 2-3 vetted installers — typically 2+ years in business, with a track record of decent reviews — and have them quote the same capacity and backup scope so the answers are comparable.

Read next

• Are solar batteries worth it?
• How solar panels work
• What a battery actually keeps running
• Home batteries and storage

Sources

• cleanenergycouncil.org.au — buying solar
• standards.org.au — AS/NZS 4777.2 grid connection of energy systems via inverters
• uts.edu.au — CER battery inspection findings, April 2026