Miles Per Watt or MPW is a term very well known to QRP operators. MPW is a calculated value based on the transmitted power that was used on a single or two-way communication and the distance that was covered between the two parties. When we say one-way communication we mean the reception of a beacon and two-way we mean the QSO between two radio amateurs where each one has the capability of transmission and reception. MPW is used as unit for the measurement of the quality/importance of a communication from the transmitting power point of view.
The only things we need to calculate MPW is the distance between the transmitting and receiving location and the transmission power. The calculation of distance is a little bit complex and can be done if we know the geographical coordinates or the grid locators of the two locations. Assuming we know the distance the MPW can be calculated with the following formula:
Just because the calculation of the distance based on geographical coordinates is difficult, web based WPM calculators are available online. The most popular is the "N9SSA Distance and MPW Calculator".
In this calculator you may enter latitude and longitude or the grid locator of each location and the power in Watts, you click the "Calculate Distance" button and you have all you need: Distance in Miles, Miles per Watt, Distance in Kilometers and Kilometers per Watt.
 N9SSA, Notes N9SSA's Miles Per Watt Calculator, http://www.hoffswell.com/n9ssa/mpwnotes.html