For QLD project home builders chasing a stronger Whole-of-Home result — display village marketing leverage, premium specifications, sustainability-precinct compliance — battery sizing is the lever that captures evening load. Solar helps Whole-of-Home; it does not lift the thermal or fabric star rating. Storage increases self-consumption.
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 | — | n/a |
| 5.0 kWh | ~+0.3 | 7.3 | Smaller battery delta | Higher cost per additional contribution |
| 9.6 kWh (Sungrow standard) | ~+0.5 | 7.5 | Standard battery delta | Typical cost per additional contribution |
| 13.5 kWh | ~+0.7 | 7.7 | Larger battery delta | Diminishing return |
| 9.6 kWh + 10 kW solar (vs 6.6 kW) | ~+1.0 (combined) | 8.0 | Step-up solar + standard battery | Better return than upsizing battery alone |
The key insight: upsizing solar is cheaper per Whole-of-Home contribution than upsizing battery, once you have any battery in place. Going from 6.6 kW solar + 9.6 kWh battery to 10 kW solar + 9.6 kWh battery usually moves Whole-of-Home more, at a lower incremental cost, than stepping the battery from 9.6 kWh to 13.5 kWh.
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 (Step-up)
- Add 9.6 kWh battery
That combination — Step-up solar plus a 9.6 kWh battery — is the usual Whole-of-Home path. Additional generation is absorbed by storage; the battery captures more of the evening load offset. Solar still does not lift the thermal star rating.
This is exactly the Step-up package configuration: 10 kW solar + 9.6 kWh battery + Tesla Wall Connector. The EV charger can add a small additional Whole-of-Home contribution under NCC 2025's EV-readiness pathway.
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 Standard and Step-up 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 Step-up 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 Step-up 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.