Choosing the right solar system size for a commercial property is a balancing act. Undersize your system and you leave savings on the table, paying the grid for energy your roof could have generated. Oversize it and you export cheap electricity at 5–10 c/kWh instead of offsetting grid consumption at 30–45 c/kWh, pushing out your payback period and tying up capital that could have been deployed elsewhere.
The optimal size depends on three factors: your annual electricity consumption, the available roof area, and how your consumption profile aligns with solar generation hours. A warehouse running daytime logistics has a fundamentally different sizing equation to a cold storage facility drawing power around the clock. This guide breaks down the recommended system sizes for the most common Australian commercial property types, with the reasoning behind each.
System size by business type
The table below provides a starting point based on typical consumption patterns and roof characteristics. Your actual sizing should be validated against 12 months of interval meter data.
| Business Type | Typical Annual kWh | Recommended kW | Roof Notes |
|---|---|---|---|
| Office (200 m²) | 40,000–60,000 | 15–30 kW | Limited roof, high self-consumption |
| Warehouse (2,000 m²) | 80,000–150,000 | 30–80 kW | Large flat roof, ideal |
| Retail Store (500 m²) | 60,000–120,000 | 25–50 kW | Variable hours, demand spikes |
| Manufacturing | 200,000–500,000+ | 50–200 kW | High baseload, demand charges |
| Cold Storage | 300,000–800,000+ | 80–200 kW | 24/7 load, battery benefit |
Offices: high self-consumption, limited roof
Office buildings are the textbook case for commercial solar. A typical 200 m² office consuming 40,000–60,000 kWh per year runs its heaviest loads between 9 am and 5 pm, which aligns almost perfectly with solar generation hours. This drives self-consumption ratios of 70–90%, meaning the vast majority of what the system generates is consumed on-site at the full retail rate rather than exported at feed-in tariff prices.
The constraint for offices is usually roof space, not consumption. Multi-storey buildings have a small roof area relative to their floor area and tenant load. A three-storey office with a 200 m² footprint might fit 15–20 kW of panels, which covers a meaningful portion of the building's daytime load but not all of it. Single-storey or two-storey suburban office parks fare better, often accommodating 25–30 kW.
HVAC is the biggest lever. Air conditioning typically accounts for 40–60% of an office's electricity consumption. Pre-cooling the building during morning solar hours (before the afternoon peak tariff band) reduces both energy charges and demand charges. If your building management system supports scheduling, this is the single highest-impact operational change you can pair with solar.
Warehouses: large roofs, ideal solar candidates
Warehouses are the most straightforward commercial solar opportunity in Australia. A 2,000 m² flat roof with minimal shading and no competing rooftop equipment can accommodate 100 kW or more of panels. Meanwhile, warehouse consumption is heavily weighted toward daytime hours when logistics operations, forklift charging, conveyor systems, and high-bay lighting are active. This combination of abundant roof space and daytime-aligned consumption makes warehouses consistently deliver the shortest payback periods in commercial solar.
Typical warehouse systems sit in the 30–80 kW range, but the roof can often support much larger installations. Whether you should fill the roof depends on your export economics. If your consumption absorbs most of the generation on-site, scaling up makes sense. If you would be exporting significant volumes at 5–8 c/kWh, the incremental return on those extra panels is marginal.
For systems exceeding 100 kW, Large-scale Generation Certificates (LGCs) become available. LGCs provide annual revenue based on metered generation, which can materially improve the economics of an oversized system. If your roof supports 120 kW+ and you have moderate daytime consumption, modelling the LGC revenue against the lower self-consumption rate is worth the analysis. See how LGCs work alongside STCs.
Retail: variable hours and demand spikes
Retail stores present a more complex sizing challenge than offices or warehouses. A 500 m² retail premises consuming 60,000–120,000 kWh per year typically operates across variable trading hours, including weekends and evenings. This means a meaningful portion of consumption falls outside solar generation hours, reducing the self-consumption ratio compared to a weekday-only office.
Peak demand in retail is driven by air conditioning, which spikes during summer trading hours when foot traffic and ambient temperatures are both high. If your peak demand occurs between 11 am and 3 pm, solar generation will help reduce it. If your store trades until 9 pm and the demand peak is in the late afternoon, solar's contribution to demand reduction is limited.
Systems in the 25–50 kW range are typical for mid-sized retail. Before committing to a system size, review your tariff structure carefully. Retail premises on time-of-use tariffs benefit most from solar during shoulder and early peak periods. If you are on a flat-rate tariff, the savings per kWh are uniform but typically lower. If you are on a demand-based tariff, the demand charge component may not reduce as much as you expect without load management or battery storage.
Manufacturing and cold storage: high baseload, demand charges
Manufacturing facilities and cold storage operations share a common characteristic: high baseload consumption that runs continuously or near-continuously. A manufacturing plant consuming 200,000–500,000+ kWh per year has loads that dwarf a typical office, and cold storage facilities can exceed 800,000 kWh annually with refrigeration compressors running around the clock.
For these businesses, demand charges often represent 30–50% of the total electricity bill. Solar helps with the energy charge component but may not reduce demand peaks, which are set by the single highest 15 or 30-minute interval in the billing period. If your compressors or production lines spike simultaneously during early morning start-up or late afternoon, solar generation may not be present to offset that peak.
Battery storage is worth serious consideration for high-baseload sites. A battery can store solar energy generated during the middle of the day and discharge it during demand peaks, shaving the kW peak that sets your demand charge. For cold storage in particular, solar alone typically covers only about 30% of total consumption due to the 24/7 load profile, but pairing solar with battery and thermal storage can push that figure significantly higher.
System sizes for manufacturing range from 50 to 200 kW depending on available roof space and consumption. Cold storage facilities are similar, with 80–200 kW being common. At these capacities, LGC revenue and demand management benefits both factor into the business case.
A simple sizing formula
For a quick first-pass estimate, use this formula:
System size (kW) = Annual kWh ÷ (Peak sun hours × 365 × 0.77)
The 0.77 is a performance ratio that accounts for real-world losses: inverter efficiency, panel temperature derating, soiling, wiring losses, and shading. Peak sun hours vary by location and correspond to the STC zones. Zone 3 (Sydney, Brisbane, Perth, Adelaide) averages approximately 4.5 peak sun hours per day. Zone 4 (Melbourne, Hobart) is closer to 3.8.
Worked example: A business in Zone 3 consuming 100,000 kWh per year needs roughly 100,000 ÷ (4.5 × 365 × 0.77) = ~79 kW. This gives a system that would generate approximately the same annual energy as the business consumes. In practice, you would likely size slightly below this to maximise self-consumption, since a system that exactly matches annual consumption will export heavily during sunny midday hours and still draw from the grid at night.
This formula is a starting point. Accurate sizing requires interval meter data (your actual half-hourly consumption profile), local weather data, and modelling of self-consumption versus export. That is exactly what Amperage does.
Size your system accurately
The estimates above are useful for scoping, but every business has a unique consumption profile. Amperage's sizing calculator uses your actual consumption data and location-specific solar generation to recommend a system size that maximises return on investment, not just panel count.