TL;DR
- On Tariff 11 (flat): solar-only is the better investment. Payback 3.5-5 yrs.
- On Tariff 12E: solar + battery is the better investment. Combined payback 5.5-7 yrs.
- 10 kW solar + 10 kWh battery: $19k-$23k installed (before Battery Booster). Rebates take $3-5k off for eligible households.
- Battery gives backup power during outages only if you specify a backup-capable system (not all do).
The head-to-head
We built this table from real Mackay installs over 2024-2025, normalised to a 4-person household using 24 kWh/day (9 kWh in 4pm-9pm peak):
| Scenario | Tariff | Install cost | Annual savings | Payback |
|---|---|---|---|---|
| 10 kW solar-only | Tariff 11 | $10,000 | $2,700 | 3.7 yrs |
| 10 kW solar-only | Tariff 12E | $10,000 | $1,900 | 5.3 yrs |
| 10 kW + 10 kWh battery | Tariff 11 | $20,000 | $3,000 | 6.7 yrs |
| 10 kW + 10 kWh battery | Tariff 12E | $20,000 | $3,600 | 5.5 yrs* |
*Includes Federal Battery Booster. Without it, payback extends to ~7.5 yrs.
Why 12E changes the answer
Peak period at 57c/kWh means that every 10 kWh avoided in the evening is worth $5.70. A battery doing 300 effective peak-shift cycles per year saves ~$1,700 beyond what solar alone captures. On Tariff 11, avoiding the same 10 kWh saves only about $3.20 (flat rate ~32c/kWh).
Backup: the underrated bonus
Mackay's cyclone season and occasional grid faults make blackout backup valuable. A backup-capable battery (Tesla Powerwall 3, Sungrow SBR with backup module, Enphase IQ with System Controller) keeps fridges, A/C, router and selected circuits running for 10-24 hours per charge. Without a backup gateway, the battery goes silent during an outage — specify it upfront.
When solar-only still wins
- You're on Tariff 11 and not planning to move tariffs.
- You consume most power during daylight (pool pump, work-from-home, running shift).
- Your capex budget is hard-capped and you need to choose one.
If that's you, install hybrid-ready hardware and keep the battery option open for later. See our sizing guide for hybrid-inverter choices.
FAQs
How long do home batteries actually last?
Quality LFP (lithium iron phosphate) batteries like Sungrow SBR, BYD Premium HVS and Tesla Powerwall 3 are warranted for 10 years with 60-70% end-of-warranty capacity retention. Real-world life is usually 12-15 years.
Will adding a battery affect my solar's performance?
Done correctly, no. A hybrid inverter or AC-coupled battery adds a control layer; solar still produces the same. Badly designed systems can cause export-limit conflicts — that's why we prefer single-vendor hybrid setups where possible.
Want a tariff-specific comparison for your bill?
We model both scenarios using your Ergon data.
Get the numbers