How much does a Photovoltaic installer pay?

This will entirely depend on how big the installation company is...

Big companies will secure large discounts from suppliers, small ones less.

Big companies take bigger overheads to run though, so the price a consumer pays may not differ a huge amount. Most businesses expect to make 30% profit to run reasonably well, whether that is on the mark up from discounted supplies or on high labour costs is almost irrelevant.

Supply companies tend to work on a 30-50% discount system for big installers in the UK construction industry depending on how much business they are turning over.

It could even be higher for PV panels, but don't concentrate on the panel price, worry about the quality of service.