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. 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. 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.