Radar systems do not transmit equal power in all directions, they usually sweep or scan don't they? This value is probably the isotropic equivalent power.
Mind you the navy did have some pretty beefy radar systems. Some of the older types could light up Yankee stealth aircraft, much to their annoyance. The US assumed everybody would be using "modern" radar. (See Arthur C Clarke's famous short story "Superiority") These were not aircraft mounted, however.