TL;DR
- Size commercial solar to daytime load, not total kWh.
- Sweet spot: 85-95% self-consumption, minimal export.
- Ergon export caps in Mackay typically 30 kW; zero-export relays unlock bigger systems.
- Batteries make sense above 100 kW load with 4pm-9pm peak demand.
Step 1 — Get interval data
Request 12 months of 30-minute interval data from Ergon (free for NMI holders). This shows your actual load shape — weekday vs weekend, peak demand, overnight baseline. Without it, any "commercial solar quote" is guesswork.
Step 2 — Extract daytime profile
Average the 8am-4pm load on weekdays. That's your solar-addressable base. Add weekend daytime if you operate then. Divide by Mackay's ~4.8 effective production hours to estimate solar kW needed for 100% daytime offset.
Step 3 — Check export constraints
Most Mackay commercial feeders cap export at 30 kW. Options when daytime load is small:
- Size at/below the export cap (simplest).
- Install a zero-export or limited-export relay (lets you go 50-200 kW+ while capping grid feed-in).
- Add battery to soak up midday surplus for evening use.
Step 4 — Typical Mackay commercial sizes
| Business type | Daily kWh | Typical size | Installed cost (after STC) |
|---|---|---|---|
| Small retail / cafe | 100-180 | 30-50 kW | $28k-$48k |
| Office / medical | 150-300 | 40-70 kW | $38k-$68k |
| Workshop / light industrial | 400-800 | 80-150 kW | $70k-$135k |
| Cold-storage / refrigeration | 1,500+ | 150-250 kW + battery | $135k-$225k |
| Mining services / dewatering | 2,000+ | 200 kW+ three-phase | $180k+ |
Step 5 — Batteries for commercial
Commercial batteries make economic sense when: you have a high 4pm-9pm load, demand charges form a significant part of your bill, or your export is hard-capped and you're throwing away midday production. See three-phase solar + batteries for the wiring side.
Common sizing mistakes
- Sizing to total consumption rather than daytime load → export cap trips constantly.
- Ignoring planned load growth (EV fleet, refrigeration expansion).
- Treating peak demand charges as fixed — they aren't, and batteries can cut them by 20-40%.
- Not modelling public holidays and weekend shutdowns (big production waste without battery).
FAQs
Does commercial solar need council approval?
Not typically for roof-mount under 30 kW. Ground-mount systems and heritage-zoned buildings often require DA. We handle council liaison as part of engineering.
What's the payback window?
3-6 years depending on size, export limit and tariff. See the commercial payback article for worked examples.
Free 48-hour commercial feasibility
Send us your Ergon NMI — we'll return an interval-based design and ROI model.
Request feasibility