An ideal roof would be large enough to accommodate at least 16 panels and be south facing. Even if its not, a well designed system using good quality panels and inverters can still yield fantastic results.
When a solar PV system is designed, its important to use good quality panels with a high efficiency rating. The inverter should be matched as closely to the maximum panel voltage as possible. This way the system is as efficient as possible and provides the client with a high end system capable of generating as much electricity as possible.
Cheaper panels may be appealing when purchasing a system but if they are inefficient it will cost you dearly over the next 20 years, so always check which panels are being suggested by your installer at quotation stage.
A good basic check list is as follows:
My home already has a metered electricity supply
I own the freehold of my property
My property is not a basement or flat (other than top floor).
I have free space equal to at least two garage doors on my roof,