For most cooking tasks – it is. In general, microwave ovens do not need to be as powerful as stoves, because the cooking tasks you may wish to perform with them are not as elaborate as those you might need a stove for.
Also, many compact microwave models have less than 700 Watts and work just fine, so a 900-Watt model will likely perform well as long as you don’t plan to prepare elaborate meals in it.