[Amps] load capacitor ratings?

[Will Matney]

Simple,
Determine the voltage across the 50 ohm load from the power in watts. 
Then use a load capacitor with a voltage rating equal to or up to two 
times the load voltage. The tune control is two times the plate voltage 
and so is the coupling capacitor.

Will Matney

How do I work out the voltage requirements for my load capacitor in a Pi-L?
This will be a fixed freq amp designed and set-up without the need for large
VSWR tolerances.

 

I've searched the amp archives and only found one ref to this from KM1H,
suggesting around 1kv at 1.5kw. 

 

What's the  formula please? 

 

 

Neil G0JHC