Battery Rating vs Capacity Monitoring

BFL13 wrote:
How does the monitor know that 5 amps is now 63.5w instead of 68w when you change to watts instead of amps? How can the shunt know that?

The shunt is just a fixed resistor. Monitor measures the voltage drop across the shunt to get the amps. Monitor also knows the voltage. The microprocessor multiplies amps x volts to get watts. Then microprocessor accumulates this wattage over time to give watt/hours. Then watt/hours are compared to the capacity and the percent charge is displayed if requested. Capacity might be based some on what you entered and possible some on what has been measured over several cycles to see if you were honest.

The posted rating could be anything from accounting, marketing or engineering coming up with a number to give some comparable data so you can choose the right product at the right quantity. Never going to be perfectly exact.