The sizing method that makes sense for a business

Start with the site's electricity use during solar hours. A business that draws 80 kW fairly steadily from 8 am to 5 pm presents a very different solar opportunity from a business that averages the same daily consumption but uses most of it after 6 pm.

A commercial model should then test different array sizes against the same interval data. For each option, compare how much solar is used on site, how much is exported, the value of avoided imports and the cost of the extra capacity.

There is usually a point where adding more panels still increases generation but adds less financial value because a larger share of that generation is exported. That does not make the larger system wrong. It simply means the reason for oversizing needs to be clear.

What do common commercial solar sizes suit?

Useful size bands, not package recommendations

  • Around 30 kW: smaller offices, retail, hospitality, community facilities and businesses with moderate daytime demand.
  • Around 50 kW: larger offices, retail sites, workshops and smaller industrial premises with stronger daytime loads.
  • Around 100 kW: warehouses, schools, health facilities, aquatic centres and industrial sites with substantial daytime consumption.
  • 100 kW to 200 kW: larger warehouses and factories, often with more formal network and engineering work.
  • Above 200 kW: larger commercial and industrial sites where connection studies, protection design, export limits and project economics become increasingly site-specific.

The Victorian Government uses similar business examples for its 30 kW to 200 kW commercial solar program, but those examples are not substitutes for load analysis.

How much roof space does commercial solar need?

The Victorian Government gives about 450 to 600 square metres of clear roof area as a guide for a 100 kW solar system.

Clear roof area is not the same as total roof area. Skylights, vents, plant, access paths, fire requirements, parapets, shade and roof geometry all remove usable space. Older industrial roofs may also need structural work or roof replacement before solar is sensible.

If a proposal is based only on an aerial image, treat the layout as preliminary until the roof and structure have been assessed.

Size for the business you expect to operate

Historical interval data is the best starting point, but it does not know what the business is about to change.

Planned EV charging, electric forklifts, heat pumps, process heating, refrigeration upgrades, new production equipment or longer operating hours can all increase daytime load. If those changes are likely, include them in the model rather than installing for today's load and discovering a year later that the roof should have been used differently.

The reverse also applies. If a site is about to automate, downsize, relocate or reduce operating hours, a system based on last year's consumption can be too large.

Panel capacity and inverter capacity are not the same number

Commercial quotes normally show a DC solar array size in kilowatts and inverter capacity in kilowatts or kilovolt-amperes. Those numbers do not have to match exactly.

It is common for the panel array to be larger than the inverter's nominal AC capacity. A sensible amount of oversizing can improve inverter utilisation during lower-light periods, although the inverter will cap output when the array can produce more than it can convert.

The right ratio depends on the equipment, orientation, network agreement and financial model. Ask the designer to state both figures clearly and explain the choice.

For Victorian VEU commercial solar, the system is defined by the total rated power of the solar panels, while the program also requires at least 30 kVA of inverter capacity.

Related commercial guides

Sources and further reading

Source labels identify who published each reference, not an independent endorsement. Confirm current requirements and exact product terms before purchasing.