Size commercial solar around self-consumption, not roof area

Export is not where the money is

Every unit you generate and use on site avoids your full import cost, including the unit rate and the network charges layered on top. Every unit you export earns a Smart Export Guarantee rate that is typically a fraction of that. The difference between those two numbers is the entire commercial argument for designing around self-consumption.

That is why an array sized to the roof can look impressive and return less than a smaller array sized to the load.

Overlay generation on real demand

Take twelve months of half-hourly consumption. Model generation for your latitude, roof orientation and pitch. Overlay the two curves. The area where they overlap is self-consumed generation, and it is the only part of the yield that earns your import rate.

A single-shift manufacturer with heavy daytime plant will show a wide overlap and can justify a large array. An office running Monday to Friday will show a narrower one and a distribution site working nights narrower still.

When storage changes the answer

Where demand sits substantially outside daylight, a battery moves midday generation into the evening and widens the overlap artificially. Batteries carry real cost and finite cycle life, so they earn their place only where the shifted units are worth more than the storage that shifted them.

Model panels alone, panels plus storage, and a phased approach on the same consumption data. If the battery does not clearly win, it should not be in version one.

Design decisions that protect yield

String layout should isolate shading from parapets, plant and flues, so one shaded module does not drag a whole string. East-west layouts flatten the generation curve and often improve self-consumption compared with a pure south array, even though total annual yield is lower. Inverter sizing should reflect the modelled curve rather than nameplate capacity.