Should I cover food while reheating it in a microwave?

For reasons of time, I like to cook more than one portion of a meal and then reheat a portion in the microwave whenever it's needed.

Aiming for maximum energy efficiency, I wonder if it's a good idea to cover the food with a big ceramic or glass bowl?

---

Just in case this is somehow relevant, here are my two main thoughts which led me to ask this question:

• 
  

  Covering might be good because the developing steam is concentrated in a small volume and gives off more of its heat to the food.

  
  


• 
  

  Covering might be bad because the bowl partly reflects the microwaves, and therefore a part of the energy never reaches the food.