Is there a formula that would tell me how much energy a antenna would receive from a 7 watt transmitter at about 100 feet from source. Thanks

 

Check out the Friis Equation

https://en.wikipedia.org/wiki/Friis_transmission_equation

See J.D.Kraus, "Antennas," 2nd Ed., McGraw-Hill, 1988 for an explanation of the inputs to the equation.