Watt Hrs vs Amp Hrs

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.

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?

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.)

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%

amp-hours are voltage dependent

watt-hours are not voltage dependent.

example:

100 amp-hours @ 12 volts = 1200 watt-hours of capacity

50 amp-hours @ 24 volts = 1200 watt-hours of capacity

a 100 amp-hour battery @ 12 volts 50% discharged would have 600 watt-hours left

1 50 amp-hour battery @ 24 volts 50% discharged would have 600 watt-hours left.

My next battery bank may have 7.2 kwh of capacity--I don't have to specify the voltage.

pianotuna wrote:
amp-hours are voltage dependent

watt-hours are not voltage dependent.

That whole article could have been written around these 2 lines instead of overcomplicating it to confusion.

You are talking about voltage in two different ways so confusing matters IMO.
1. Watts are voltage x amps, so voltage matters there
2. Watt hr is the same at 12v as at 24v, so voltage does not matter there.

road-runner wrote:

pianotuna wrote:
amp-hours are voltage dependent

watt-hours are not voltage dependent.

That whole article could have been written around these 2 lines instead of overcomplicating it to confusion.

And that would be subtly incorrect. It's not that one is voltage-dependent while the other is not; rather, they're measuring fundamentally different things, which can be related to each other by voltage.

Amp-hours are a measure of charge, which is conceptually independent of voltage. One amp-hour is (by definition) 3600 coulombs. Watt-hours, on the other hand, are a measure of energy. If you want to know how much work you can get out of a battery, that's a question of how much energy is stored in the battery, and would most correctly be measured in watt-hours or some other unit of energy. If all you have is charge, then you need take voltage into account and, at least implicitly convert the charge to energy to get a valid answer.

There are times when charge, not energy, is really what you care to measure. How long will it take to recharge a battery at 10A charge current? If it's a 100 Ah battery, the answer is 10 hours (ignoring inefficiencies), and that's true whatever the voltage of the battery is. If all we know is that it's a 1000 Wh battery, there's no way of telling without knowing the voltage and converting the energy to charge. Here, it seems that Wh are voltage-dependent while Ah are not.

If you are sticking purely with 12vDC devices, amp-hr is fine for most uses. Technically batteries don't stay exactly at 12v but for most uses, it's close enough.

If you are changing voltages or to AC, watt-hr makes more sense. Otherwise, it's critical to specify X amps @ Y volts.

But for heaven's sake, don't say I used 20 amps or 5000 watts yesterday and need to replace them in my battery bank or worse 20amp/hr.
- The first is an instantaneous measure of output. Without knowing how long you were drawing of the batteries, you have no clue how much was used.
- The second is the rate of change in output (ie: I started drawing 10amps but over an hour it gradually rose to 30amps, so the rate of change is 20amp/hr) Largely a nonsense figure as most electrical loads do not change at a steady rate.

BFL13 wrote:
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?

Only If/When using power at the same amp draw as the mfg specs the 20 HR rate

If 20he rate is 5 amps, and you are using 2 amps, then the total useable AmpHrs will be greater than spec

If you are using 10amps then the useable AmpHrs is less than spec

If you have a 100w load that is constant, as the voltage drops, amps flow/use increases to maintain that 100watts of work energy
DC resistive loads don't do that,
But inverters and DC to DC voltage converter will increase input amps use , watts, HP, btu , calories are all specific energy/power measurements that accurately describe real power use, and can be converted from one to the others

And yes calories is a measure of heat energy from a food source,
Yes it takes laboratory equipment to do to these measurements