For QLD project home builders chasing 8-star NatHERS — display village marketing leverage, premium specifications, sustainability-precinct compliance — battery sizing is the lever that closes the last 0.5-1.0 stars. Solar alone gets a typical project home from 6.0 to 7.0-7.5 stars. The jump to 8+ comes from adding storage.
The question is: how much storage? 9.6 kWh or 13.5 kWh? When does the cost per additional star stop being worth it? This article breaks down the cost-per-star ladder of stepping up battery capacity in NCC 2022 / NCC 2025 frameworks.
How batteries contribute to NatHERS
Under NCC 2022's whole-of-home framework, battery storage contributes by capturing solar generation that would otherwise be exported (and thus partially "lost" to the home's whole-of-home calculation) and using it to offset evening loads. Without a battery, solar generation that exceeds daytime consumption is exported — useful for the grid, but it doesn't reduce the home's net regulated energy consumption as much as self-consumed generation does.
The headline parameters that determine battery contribution:
- Usable kWh capacity — what's actually available for cycling, after manufacturer-imposed depth-of-discharge limits.
- Round-trip efficiency — typically 90-92% for lithium-iron-phosphate (LFP) chemistries used in modern residential batteries.
- Daily cycle assumption — NatHERS modelling assumes one full cycle per day (charge from solar during day, discharge for evening loads).
These three together feed into the whole-of-home calculation. The output is a star contribution, which is largely a function of how much of the home's regulated equipment load can be shifted from grid-import to battery-discharge.
Cost-per-star ladder for QLD project homes
Numbers below are typical for a single-storey 4-bed project home, ~200 m², built in a Brisbane / Logan / Ipswich / Moreton Bay growth corridor. The home is assumed to start with a 6.0-star base (standard project spec) and a 6.6 kW solar system (one-star uplift), giving a 7.0-star pre-battery baseline. Cost-per-additional-star measured against that 7.0-star baseline.
| Battery capacity | NatHERS contribution | Final star rating | Builder cost (battery delta) | Cost per additional star |
|---|---|---|---|---|
| None | 0.0 | 7.0 | $0 | n/a |
| 5.0 kWh | ~+0.3 | 7.3 | $3,000-$4,000 | ~$10,000-$13,000 |
| 9.6 kWh (Sungrow standard) | ~+0.5 | 7.5 | $5,000-$6,500 | ~$10,000-$13,000 |
| 13.5 kWh | ~+0.7 | 7.7 | $8,000-$10,500 | ~$11,500-$15,000 |
| 9.6 kWh + 10 kW solar (vs 6.6 kW) | ~+1.0 (combined) | 8.0 | $7,000-$9,000 over 6.6 kW + 9.6 kWh | ~$7,000-$9,000 |
The key insight: upsizing solar is cheaper per star than upsizing battery, once you have any battery in place. Going from 6.6 kW solar + 9.6 kWh battery (7.5 stars) to 10 kW solar + 9.6 kWh battery (8.0 stars) adds 0.5 stars at ~$3,500 each. Going from 9.6 kWh battery to 13.5 kWh battery (with 6.6 kW solar) adds 0.2 stars at ~$15,000 each.
The 8-star sweet spot
The cheapest path from 7.0 stars (6.6 kW solar baseline) to 8.0 stars is:
- Upsize solar to 10 kW: +0.5 stars at ~$3,000-$4,000 incremental cost
- Add 9.6 kWh battery: +0.5 stars at ~$5,000-$6,500
Total: ~$8,000-$10,500 over a 6.6 kW solar baseline. Net result: 8.0-8.5 stars reliably, with the additional generation absorbed by battery storage and the additional battery capturing more of the evening load offset.
This is exactly the Premium package configuration: 10 kW solar + 9.6 kWh battery + Tesla Wall Connector. The EV charger adds a small additional NatHERS contribution under NCC 2025's EV-readiness pathway, pushing the package's typical outcome to 8.2-8.5 stars.
When to go larger than 9.6 kWh
Three scenarios where a larger battery (13.5 kWh+) is justified despite the diminishing per-star return:
1. All-electric homes with ducted reverse-cycle
Homes specified with no gas connection, ducted reverse-cycle for heating and cooling, and electric hot water (heat pump) have evening loads that easily exceed 9.6 kWh of stored capacity in winter. A 13.5 kWh battery captures more of the available solar generation in shoulder seasons and improves the marketing narrative around bills.
2. Display home / show home specifications
Where the display home is the "8-star benchmark" for the release, going to 13.5 kWh delivers a marketing-quotable 8.5+ star rating that survives review under NCC 2025's slightly more conservative battery methodology. Worth the premium for a single home, less worth it across a 50-home release.
3. Sustainability-precinct estates
In precincts foreshadowing 7.5-star or 8-star sub-jurisdictional minimums, the larger battery provides margin against future tightening. Not strictly required for compliance today, but de-risks 2027-onward changes.
What Trade Solar specifies as standard
Our Complete and Premium packages use the Sungrow 9.6 kWh battery as standard. It's the cost-per-star sweet spot for QLD project homes, has 10-year warranty, fits in a single wall-mounted enclosure, and is supported across both Energex and Ergon Energy networks.
For builders specifying 13.5 kWh or larger for display homes or sustainability-precinct compliance, we offer Sungrow 12.8 kWh and BYD 13.8 kWh as upgrade options on the Premium package. Specification advice and assessor-tool documentation included.
The take-home
For most QLD project homes targeting 8-star NatHERS compliance, the answer is: 10 kW solar + 9.6 kWh battery. That's the Premium package configuration. Bigger battery is rarely the right answer for cost-per-star on volume releases. Bigger solar usually is — to a point, after which you hit single-phase / three-phase boundaries and connection complexity increases.
Battery sizing is one of the few solar-specification questions where the intuition ("more is better") is actively wrong from a cost-per-star perspective. Optimal sizing is more about matching battery capacity to the home's actual evening load profile than about maxing out kWh.