Heat Convection Output

afa13

I need some clarification regarding the following formula:

Qconv = Pn x Qs x b x ϕ

this equation is used to determine the cooking appliance's convective heat output in kw, 'b' and 'ϕ' are coefficients easily determined. I am confused whether to use the actual capacity of the appliance for 'Pn' or its power rating which is higher and will result in a big difference. And 'Qs' is the sensible heat emitted in W and sometimes is expressed is W/kW.

Can anyone help clear this issue?