What things to consider when comparing/haggling solar panels

Hi Folks

I have had a few solar panel installation quotes ranging from £8k to £15k.

Apart from the below questions, what else could I ask the installers in order to get the best deal?

The total output produced?
The size and number of panels?
The quality of the panels?
How long the warranty is for?
How long would it take for me to break even?

I had one installer say I may only need 11 panels whereas another said I would need 26.

There is a vast difference and I don't want to get ripped off.

Any advice greatly appreciated.

Thanks