dBW and dBm

The first method of expressing 13dBW in watts is the most general.
The calculator!

You divide by 10 to get 1.3, enter 1.3, press Inv (Inverse; or an equivalent on your calculator) and press log.

I got 19.95


A second method is to estimate, which we will find particularly easy in this example.

Note that   13  =  10 + 3
We know that 10dBW is 10watts, and 3dB is approximately a factor
of 2.

10watts  x  2  =  20 watts