Hopefully somebody here will be able to enlighten Graham that transformers are rated in their capability capacity in terms of VA or KVA etc, not Watts or KW etc.
Once you fully understand AC electricity and power factor influences naturally inherent with inductance and capacitance present within any loaded AC supplied circuit, it becomes very obvious as to why this is.

It is therefore never as simple as to simply suggest a transformer capable of delivering a 300w supply is what is needed because dependent on what particular type of loads it is subjected to, greatly varies the size of the transformer in VA or KVA etc, that is required to do the job safely and adequately with the necessary headroom.

My head said Michael Bay when I saw the headline :-)