I found this for you. Looks reliable! Nothing about power of m/wave, so you may have to experiment.
http://www.bbc.co.uk/food/recipes/database/mic rowavespongewithc_74612.shtml
Microwave sponge
Ingredients
55g/2oz butter
55g/2oz caster sugar
1 piece of stem ginger, from jar, chopped
1 egg
55g/2oz self-raising flour
2 tbsp milk
Method
1. To make the sponge, blend the butter and sugar together in a food processor. Add the ginger, egg, flour and milk and pulse until combined.
2. Spoon the mixture into a greased bowl and then cover with cling film. Microwave on high for four minutes until golden brown and springy to the touch.