Calculating the ballast required for a flat roof installation

[SparkyMarky]

I've been asked about installing an array on a largish flat roof surrounded by a 50cm parapet. He also wants them laid flat which, according to my chart, leads to a 15% reduction in output. Firstly, 15% doesn't sound much to me, is this right? Secondly, and the purpose of the post, could anyone point me in the direction of information on how to calculate the necessary ballast requirement.

 

[steptoe]

bolt them down?

remember you want to put as little weight on the roof as possible,

if you must use ballast then you put as much on as the roof can hold.

 

[binky]

Flat is 10% reduction