Why are bigger solar systems cheaper, per kW of panel installed?

Q. My brother signed up for a 10.8 KW system (Trina Tallmax + 3 phase Fronius) which is costing him about $ 10,600 excluding metering ($ 660). i.e. under $ 1,000 per kw.

I have got a quote from the same company for the same components but, as I do not have the roof space, can only accommodate 18 panels on two sides of my roof for a 5.76KW system. They have quoted $6153 excluding metering which is about 20% more than my brother's system. Both are current quotes. Is this fair?

A.  As with most things, the bigger the quantity you buy, the lower the per-unit cost. With solar this is especially true with solar due to the way the rebate works.

The rebate is paid per kW of panels. And it actually covers most of the hardware cost of the panels. So once the installer has rolled the truck and put guys on your roof, adding more kWs is cheaper, as he only has to pay for extra racking, and labour and a small part of the extra panel cost.

So you'll almost always see a smaller system cost more per kW than a larger system.