The decibel watt or dBW is a unit for the measurement of the strength of a signal expressed in decibels relative to one watt. It is used because of its capability to express both very large and very small values of power in a short range of number, e.g. 1 milliwatt = -30 dBW, 1 watt = 0 dBW, 10 watts = 10 dBW, 100 watts = 20 dBW and 1,000,000 W = 60 dBW.
Compare dBm, which is referenced to one milliwatt (0.001 W). Certain dBW value in dBm is always 30 more because 1 watt is 1000 milliwatts, and ratio of 1000 (in power) is 30 dB, e.g. 10 dBm (10 mW) is equal to -20 dBW (0.01 W).