The best device to measure instantaneous energy usage
Without a doubt, it is the Kill-A-Watt meter.  You want to measure Watts.  And also Watt-hours.

Here is what they look like:



They are available from many vendors.  Worth every penny.

Every brand offers both instantaneous use, but also cumulative and max.  

Since most work the same, I suggest you shop around, assure yourself they all do about the same thing, then look at prices and speed of delivery to make your decision