BFL13 wrote:
"An amp hr at 12v is 12 watt hrs , an amp hr at 13v is 13 watt hrs"
Wait a sec! That is counting the volts twice or something tricky?
A Watt-hour is (volts x amps) x hour while an amp-hour has no voltage. Or something. :)
They're measuring different things, even though we in the RV world tend to treat them as equivalent measures. We likewise tend to do the same thing with amperes (current) and watts (power).
An amp-hour is a measure of charge (a certain number of electrons, if you like). A watt-hour, on the other hand, is a measure of energy or work--the amount of useful (or useless, I guess) activity that can be achieved by the charge. An amp-hour at 12V is less energy than an amp-hour at 24V (or any other greater voltage).
If the voltage is constant, then the charge is exactly proportional to energy and current proportional to power and our simplifications work out okay in practice. If there are differing voltages involved, it can get rather confused rather quickly.
A water analogy might be helpful here. An amp-hour (or other unit of charge) would be equivalent to a volume of water--say a gallon. Voltage is analogous to pressure. A gallon of water working a turbine at a pressure of 12 psi will do less work than a gallon at a pressure of 24 psi. (The equivalent of the ampere in this analogy would be the rate of flow of the water, gallons per hour or something like that.)