[ERROR: corrected Feb 19, 2017 Used dBm, but always called 0dBm to be 1watt]
Lets suppose you want a burst of data every so often. Legal in that ISM band.
Lets do 100,000 bits per second, but only a 10mS burst. You'll get 1,000 bits.
OK. The bandwidth needs to be approximately 100,000Hz. Approximately. Depends upon type of modulation. Use OOK...on_off_Keying, where you simply turn on/off your PowerAmplifier transistor, controlled by the serial bit stream. And take a couple microseconds to ramp up or ramp down, so your transmitted energy remains (mostly) inside the ISM band.
How much power is needed? We'll work with 'dBm', dB referred to 1milliWatt. 0dBm is 0.223volts RMS across 50 Ohms, or 0.632vPeakPeak, using P=V^2/R.
First let's compute the necessary energy into the receiver antenna.
-174 dBm/Hertz is the Boltzmann/Nyquist/Johnson noise floor (comes from K*T)
50 dB from 10*log10(100,000 bits/second) in OOK modulation
20 dB to achieve very low bit error rate
5 dB sloppy receiver noise figure, and other flaws
5 dB sloppy matching from Antenna to Receiver electronics (LNA)
becomes -174dBm + 80dB
-174 + 80 = -94dBm (I know 0dBm/50ohms is 0.632vpp;-100dBm/50ohms is 6.32uVpp)
(This is 6dB stronger, so 12.64uVpp)
What is loss between Transmitter and Receiver? Assume the 2 antennas have unity gain, meaning there is no focusing of the radiated energy.
Assume 1meter (300MHz)frequency. Assume 10,000 meter separation, or 6 miles.
The path loss is 22dB + 10*log10[ (separation/wavelength)^2 ] ==102dB loss.
Thus [ERROR:1 watt] 1milliWatt, radiated uniformly, becomes -102dBm, at 6 miles.
As other people mentioned, the urban environment is not line-of-sight, so another 20 dB margin is wise. If not more. You are not line-of-sight.
How healthy is datalink? Compare receiver floor(RF) to TXpower+PathLoss(TXPL).
Looks like you need RF:(-94) - TXPL:(-102) = +6dB additional signal, because the received power(TXPL) is 6db weaker than what receiver needs(RF). Our datalink is 6dB short of what a robust link requires.
Summing Multipath+Linkshortage: 20+6 == +20dB atop 1milliWatt, or +26dBm which is 400 milliWatt.
Working at 433MHz requires a few more dB of power, say 1 Watt.