This question does not have a simple or a single answer. The reason is, there are so many factors that will determine how far your 1000 watts transmitter will go.
In theory, the range of a transmitted radio signal is infinite regardless of power used, basically it goes on and on forever until it encounters an obstruction. More transmitter power helps to penetrate any obstructions.
Given this, when engineering and planning a station it is always better to consider these factors that limit the range.
In this article, the team at MyTuneIn give you two different 1kW test results that will help answer your question. These two tests were done with 1000 watts transmitting power and up to four (4) dipole antennas. We used our car radio tuner and a Samsung GT-E1207Y mobile phone as receiving devices for both tests.
The Samsung phone was used to receive a signal from stations at far as 150km away before making it a sister receiving device to our car tape which also receives signals of some stations at over 150km away.
Please note that, kilometres used for our tests are 'straight line' kilometres without curves which is different from kilometres by road since radio signal does not travel like cars (by road).
We used google map to achieve the number of kilometres used in this article. Since we achieved our km by drawing a straight line from the transmitter site to our stopping points, our kilometres are less than kilometres by road.
Also, audio peak deviation for all the two setups were set at 68KHz.
Below are details of each test.
Test One: A flat land area with few dispersed storey buildings used for the 500 watts tests was used for these tests also. We fed two dipole antennas with 3dbd gain mounted at about 100 feet (30m) high through 7/8" feeder cable with 1000 watts transmitter power with 0 watts reflection for the test one. The purpose for this test was to see if we can receive a good signal with clear stereo sound from this configuration at 60km away.
Our two signal receiving sisters, car radio tuner and the Samsung GT-E1207Y mobile phone, easily picked up the station with clear stereo sound in an auto search at about 34km away.
The signal was good with clean stereo sound on both receiving devices at about 68km but with hiss at some directions on the Samsung phone.
Test Two: In this setup we added two additional dipoles to the two used in test one to make it four. And fed the four dipoles mounted at about 100 feet (30m) high with 1000 watts output power with 0 watts reflection . The gain for the four dipole antennas was 6dbd.
This was done at a flat land area with few dispersed storey buildings same place used for the 500 watts tests and test one in this article. The signal was strong with clear stereo sound on our car radio tuner and the Samsung GT-E1207Y mobile phone at about 56km away.
The signal was still clean when we got to about 79km away. This test was done to see if we can receive a good signal with clear stereo sound at 70km away.
Where we ended the 'search for the signal' weren't the end of the signal from the setups. We ended because we achieved the purpose for the tests. The above tests are only for comparison.
For instance 120km can be achieved with a good transmitter site with 1000 watts of transmitter power fed to antennas at high as 150 feet or more. The distances shown are for a reasonable signal on a car stereo and mobile phone with built-in FM reciever.
Check out how far we got for 500 watts FM radio transmitter.