341W is the theoretical maximum INPUT power the transformer, based on a stable condition during each half cycle of primary current and 100% efficiency. The OUTUT power available to the load will be much less because of losses in the transformer and resistive losses in the wiring. The circuit drives the transformer with a square wave which is one of the least efficient waveforms, how much energy is avalable at the output will depend largely on how well the transformer copes under load with square wave drive. If you want a guess, and that is all it is, expect maybe 100W maximum. As Audioguru points out the waveform will not drive many products, particularly inductive ones (motors, other transformers, some lighting, fans...) which do not like sharp edges on their voltage waveforms.
Brian.