TL;DR
- Average Mackay household uses 18-28 kWh/day. A 6.6 kW covers ~55-65%, 10 kW covers 70-85%, 13.3 kW covers 85-100%.
- Formula: (annual kWh ÷ 1,450) × 1.25 = recommended DC kW.
- Max residential DC cap in QLD is typically 13.3 kW single-phase (10 kW AC export). Three-phase homes can go higher.
- Only oversize beyond 133% DC:AC if you're charging a battery, pool or EV during the day.
Step 1 — Find your actual usage
Grab your most recent Ergon Energy bill. Look for "Total usage (kWh)" and the number of days the bill covers. Divide usage by days to get daily kWh. Multiply by 365 for annual kWh.
Step 2 — Apply the Mackay yield formula
Mackay generates about 1,450 kWh per year per kW installed (5.3 peak sun hours × 365 days × 0.82 system efficiency with losses).
Divide your annual usage by 1,450 to get the kW needed to match 100% of your consumption on paper:
- 8,500 kWh / 1,450 = 5.9 kW needed to match usage on paper
- Multiply by 1.25 for timing mismatch (you use some power at night and on rainy days): 5.9 × 1.25 = 7.4 kW
- Round up to the next standard size: 8-10 kW
Step 3 — Compare the standard sizes
| Size | Daily production (avg) | Good for | Offsets |
|---|---|---|---|
| 6.6 kW | ~26 kWh | 2-person, no pool/EV | 55-65% of bill |
| 10 kW | ~40 kWh | 3-4 person, A/C, small pool | 70-85% of bill |
| 13.3 kW | ~53 kWh | 5+ person, EV, pool, battery-ready | 85-100% of bill |
Step 4 — Factor in future load
If you're planning any of these in the next 3-5 years, size up one bracket now:
- Electric vehicle (~3,000-5,000 kWh/year added)
- Pool pump (~1,500-3,000 kWh/year)
- Electric hot water with no heat-pump (~2,500-3,500 kWh/year)
- Ducted A/C replacement
Step 5 — Don't forget the battery question
On Tariff 12E (Time of Use), solar-only sizing optimises for midday production, but you still pay 57c/kWh in the 4pm-9pm peak. That's why we usually recommend 10-13 kW with a battery for 12E households. See our Tariff 12E guide for the numbers.
FAQs
Can I install more than 13.3 kW on single phase?
Some networks allow up to 15 kW with an export-limiting device, but Ergon's current rules in Central QLD cap single-phase DC around 13.3 kW with 10 kW export. Three-phase upgrade opens 15-30 kW residential.
What if my roof is small?
High-efficiency panels (440-480 W+) let you fit 10 kW in about 28-32 m² where older panels needed 40 m². We assess every roof during design — shading, aspect and tilt matter more than area.
Should I aim for zero bill?
Usually not. Offset 80-90% of usage and live with a small bill. Chasing the final 10-15% means paying for capacity you barely use — the money is better spent on a battery if you're on 12E.
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