Usable vs nominal capacity: the number that decides your rebate.
Two batteries advertised at the same kilowatt-hours can qualify for meaningfully different rebates, store different amounts of energy, and cost different amounts per unit of both. The difference is usable versus nominal capacity — and almost every quote quotes the flattering one.
Written by Juan Flinn, Content Editor · Reviewed by the Mission Green Energy Team · Updated August 2026
One is the size of the tank.
The other is what you can pour out.
Nominal (sometimes "rated" or "total") capacity is the energy the cells hold on paper. Usable capacity is what the system will actually deliver, after the battery management system holds back a reserve to protect cell life. Usable is always the smaller number, and it is the one the federal rebate is calculated on.
Nominal capacity
The brochure headline. Useful for comparing cell content, not for comparing what you get.
Usable capacity
What the system delivers, and what the Clean Energy Council register lists. This is the number that matters.
Why they differ
The BMS reserves headroom at the top and bottom of the range so cells are not fully cycled. That reserve protects the warranty.
The rebate is paid on
usable, not nominal.
Since 1 May 2026 the federal rebate is tiered on usable capacity, so getting this wrong distorts both the rebate estimate and the size you end up choosing.
Suppose two quotes both say "28.7 kWh". One system lists 27.5 kWh usable on the CEC register; the other lists 25.8. That is 1.7 kWh of difference the brochure never mentions — and because the rebate pays per usable kilowatt-hour, the two quotes are not comparable until you convert them.
The gap between nominal and usable varies by manufacturer, typically somewhere between three and ten per cent on modern lithium-iron-phosphate systems. Some brands quote usable prominently. Others quote nominal and leave you to find the register entry.
- Ask for the CEC-listed model number, not just the brand and headline capacity.
- Look it up on the Clean Energy Regulator's approved battery register and read the usable column.
- Re-run every quote on usable before you compare price per kilowatt-hour. The ranking sometimes changes.
- Check the approval expiry date while you are there. Approvals lapse, and an install after expiry can jeopardise the rebate.
Three other numbers
worth reading twice.
Capacity is not power
Kilowatt-hours tell you how long. Kilowatts tell you how much at once. A large battery on a small inverter still only backs up a small load.
Retained capacity at year ten
Warranties guarantee a percentage of original capacity after ten years, commonly 60–80%. That percentage matters as much as the starting number.
Throughput limits
Some warranties cap total energy cycled as well as years. Heavy daily cycling can reach that cap before the decade is up.
Your questions,
answered.
Nominal capacity is the total energy the cells hold on paper. Usable capacity is what the system will actually deliver, after the battery management system reserves headroom to protect cell life. Usable is always lower, typically by three to ten per cent on modern LFP systems, and it is the figure listed on the Clean Energy Council approved battery register.
Usable capacity, as listed on the CEC approved battery register. Since 1 May 2026 the rebate is tiered on usable capacity: 100 per cent of the base rate on the first 14 kWh, 60 per cent from 14 to 28 kWh, and 15 per cent from 28 to 50 kWh.
Ask your installer for the exact model number on the quote, then search it on the Clean Energy Regulator's approved battery register. The register lists nominal and usable capacity side by side, along with the approval and expiry dates.
Because the headline figure is usually nominal and the rebate is paid on usable. Two systems advertised at the same nominal capacity can list different usable figures on the register, which changes both the rebate and the real storage you receive. Always convert quotes to usable before comparing price per kilowatt-hour.
Once you have
the right number.
Sizing follows. The rebate tiers pay 100% on the first 14 kWh usable, 60% to 28, then 15% above that — which puts a practical sweet spot at roughly 27–28 kWh usable for most all-electric homes. Our sizing guide works through it, and the rebate checker lists every current incentive with its official source.
If you are still deciding on a brand, the five-minute brand check covers the other records worth searching. Or book a free assessment and we will model it against your actual bill.