TL;DR
- Tariff 12E peak: ~57c/kWh, 4pm-9pm daily. Off-peak: ~23c/kWh.
- Solar produces ~0 kWh during the peak window — you still pay full peak rate without a battery.
- A 10 kWh battery stores midday solar and covers a typical 4-person Mackay household's peak usage.
- Post-Battery Booster, payback on battery is 7-9 years. On flat Tariff 11 it's closer to 11-13 years.
What Ergon changed — and why it matters
Ergon Energy introduced Tariff 12E (Time-of-Use) as the default residential tariff for new solar connections in Central QLD from 30 June 2021. The structure is simple but punishing for solar-only households:
| Period | Hours | Rate (approx) |
|---|---|---|
| Peak | 4pm - 9pm daily | 57c/kWh |
| Off-peak | 9pm - 4pm next day | 23c/kWh |
| Feed-in tariff (export) | All day | 8-12c/kWh |
The critical detail: solar production in Mackay typically drops below household demand from 4pm onwards. By 5pm your panels produce ~20% of daytime peak output, and by 6pm they're essentially zero. You're buying grid power at 57c while your bill-saving solar system has gone to sleep.
The maths without a battery
Take an average 4-person Mackay home using 24 kWh/day (9 kWh during peak, 15 kWh off-peak):
- Solar-only, 10 kW system on 12E: Offsets daytime usage and exports ~20 kWh/day at 10c/kWh. Peak usage (9 kWh) still bought at 57c/kWh = $5.13/day peak purchase. Net saving vs no solar: ~$5.80/day.
- Solar + 10 kWh battery on 12E: Battery charges from excess solar and discharges during peak, avoiding the $5.13/day peak purchase. Net saving: ~$10-$11/day.
How to size a battery for 12E
- Add up kWh used between 4pm-9pm on your Ergon bill (or estimate from appliances: A/C, oven, dishwasher, lights).
- Multiply by 1.1 to allow for battery efficiency (~90%).
- That's your minimum usable capacity.
Most Mackay homes land at 8-14 kWh of peak consumption. A 10-13 kWh usable battery is the standard recommendation. We use Sungrow SBR, Tesla Powerwall 3, BYD Premium HVS and AlphaESS SMILE-G3 depending on inverter pairing and backup needs.
The rebate stack on batteries
- Federal Battery Booster (2025): up to 30% off installed cost, capped by system size.
- QLD Battery Booster (state): means-tested, stacks with federal.
- VPP sign-on credits: Optional — some retailers pay $500-$1,500 for VPP enrolment. Read the T&Cs: you give them dispatch rights during grid events.
When a battery still doesn't make sense
- You're on flat Tariff 11 (not 12E) and don't expect to move across.
- You consume most power during daylight (shift worker at home, pool running midday).
- Your roof can't support enough solar to reliably fill a battery (< 6.6 kW).
FAQs
Can I switch off 12E back to Tariff 11?
Ergon lets existing customers stay on Tariff 11, but new solar connections since July 2021 are typically placed on 12E. Some retailers allow a one-time swap back. Talk to your retailer first — the swap isn't always reversible.
Does a battery give me backup during cyclones?
Only if specified with a backup gateway (e.g. Tesla Gateway, Sungrow SBR with BACK-UP-SG, Enphase IQ Battery with System Controller 3). Not every battery ships with backup capability — tell us upfront and we'll specify accordingly.
Is VPP worth joining?
For 12E households, a $500-$1,500 sign-on bonus plus 10-20 discharge events per year is generally positive. Check the event-day export cap (some VPPs drain your battery to 10% before a storm).
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