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 power helps to penetrate any obstructions. Given this, when engineering and planning a station it is always better to consider the issues that limit the range.
a) How far the transmitting antenna can effectively see. If you stand where the antenna is mounted and look out with a pair of binoculars, wherever you can see it is possible to transmit to. This can sometimes be up to 20, 30 even 40 miles if you are looking out from a mountain top.
b) Other sources of interference or other stations operating on the same frequency. For example the antenna may be able to see 20 miles, but if another station is on the same frequency 20 miles away, it will block/interfere with the signal.
c) Transmission power. If the antenna can see 20 miles, but say 1 Watt ERP of power used, it's very likely that about 1 mile of range will result. This is quite simply because there is not enough power to propagate the signal 20 miles. If 50 Watt ERP is used, it's very likely that 20 miles of range will be achieved. This is because 50 Watt ERP is ample power to propagate a strong signal 20 miles. If 1 Million Watts of power is used, it is very likely that signal will only propagate just over 20 miles. This because the range is limited as described in point a) above.
ERP ( Effective radiated power ) is the power radiated from the antenna, all antenna systems have a degree of power gain. When calculating the gain of an antenna system the main factors are the amount of gain ( dBi ) and the amount of loss ( dB ) through coax.
A 12 Watt transmitter using a high gain antenna like the Aareff 5/8's vertical which as a gain of 4.8 dBi, and using 25 metres of RG213 low loss coax would effectively double the power to 24 Watts ERP. ( 7 miles range )
Assuming the antenna has a clear view, the frequency is clear and an average quality portable receiver is used, typical transmission power range figures are as follows: