mW or W when calculating decibels
The important thing to remember when dealing with dB is that it's just a ratio value, if you don't reference it to something it means nothing. For example, when referencing your signal against 1mW, you'll get an answer like 10dBm, while when working in W, you will get an answer like 10dbW. You can use whatever you prefer as long as you are consistent. That being said, values measured in dBm are a lot more common than values in dBW.