Tach Hour vs. Hobbs Hour
A hobbs hour is the amount of time from when the propeller starts turning, to the time it stops. A tach hour is the "time" according to engine RPMs. At full power (in cruise flight) a tach hour is close to, but a little less than, a hobbs hour. However, on the ground, a tach hour is considerably less than a hobbs hour, because the engine RPMs are low. What that means to you is that an hour-long flight in an airplane charging hobbs time costs you 1 hour times the rate of the plane. That same hour-long flight in an airplane charging tach time will most likely cost you approximately 0.8 hours times the rate of the plane. You save money.