Watt Hrs vs Amp Hrs

A 100AH batt full is 12.7v, and at 50% is 12.2v, say.

100 x 12.7 = 1270 Watt Hr
50 x 12.2 = 610 Watt Hr

610/1270 = 48%

Ah is a measure of charge. Wh is a measure of energy. If you can assume that the voltage is constant, then the two are directly proportional to each other.

The same issues come up with amps and watts, not to surprisingly. Amps are a measure of current, and watts of power; but sometimes amps are used as a sort of stand-in for power, under the assumption that the voltage is constant (and hence they are proportional).

A somewhat similar thing also sometimes comes up when talking about distances. If someone asks me how far it is to Albany, NY, I'd probably say it's about a three-hour drive from my house. Of course, hours are not really a measure of distance, and that answer is nonsense technically speaking. However, when the speed can be assumed to be constant, they're directly proportional. In the case of getting to Albany, the speed is definitely not constant, but it's at least fairly consistent most of the time. (The actual distance is about 150 miles, and my average speed ends up being about 50 mph.)

One of the comments mentions that Watt Hrs are good when the voltage stays the same as in the stick house.

With a battery falling in voltage as you draw from it, does that not make using AH better for seeing how much capacity is left in it?

100 amp hours @ 12 volts -- 24 volts 120 volts.

Amp hours is a shortcut. It is an assumption figure. You know how much energy you require. To say pump 100 gallons of water. Is the pump going to deliver the same gallonage at 13.6 volts as it does at 12.1 volts?

If you feed a 50 watt array of regulated LED lamps at 14.4 volts instead of 12.2 volts will the same amount of energy be extracted out of a storage battery? The lamps operate from 10 to 30 volts.

Watt hours provides a true quantity of energy. Your home meter displays kWh for a reason.

How you can benefit (or not) by utilizing the wattage measure is the question.