Im playing with electroluminescent paint, i want to change frequencies
You need a signal generator that feeds an audio power amplifier, followed by a step-up transformer.

Audio power amplifiers are generally designed to put out their maximum power into a 4&Omega; load (or less), so at 1200W, you'll be getting somewhat over 60 Vrms. In order to convert this to 240 Vrms, you'll need a 1:4 transformer. Make sure it can handle the frequency range (and power level) that you need.