FES climb rate at 15kW power?

I would say it depends alot on the weight.

To keep hight do you need an force equal to the weight divided by L/D for current speed. For a JS3 at full load would it be something like 600/50 = 12kg
Power is force times speed = 12*9,81(Convert to Newton) * 130(best L/D)/3,6 (convert to m/s) = 12*9,81*130/3,6 = 4,3kW.

You then have about 10kW in exess power to climb: Power = weight * vertical velocity = Climb = 10k / (600 * 9,81) = 1,7m/s

This is teoretical. 15kW is the input to the converter, you loose some % in the electrical system, you also loose alot in the efficienty of the propeller. A good guess is that you have an system efficeiny of about 50%. = climbrate less than 1m/s.