A typical solar PV panel is between 380 to 420 watts and the smallest system I would recommend is about a 3000-watt system, so that is 7-8 panels. In reality we can build the system any size, but the infrastructure cost to install a PV system is fairly consistent up through about 40 panels, so with a very small system the cost per watt (or per panel) is higher and with less than 9 the economics alone may not be enough of a reason to make the investment.
However, other factors may come into play that would make the economics of a small system more desirable:
- An unreliable grid power supply
- No grid power available
- A desire to be green
- Build it small at first but expandable for future needs.
In general I have found that if one does not build a system large enough to produce the bulk of the power needed on an annual basis the satisfaction with the investment is cool at best. So build it as big as you can afford and the space will allow for, and as close to 100% of your historical consumption.
