KWH is actually a more accurate way of recording electrical use, compared to amp/hours, as when the battery is powering something, its voltage will drop, and in most instances the amps will need to increase to provide the device with the same overall wattage and the AH figure will read higher. Wattage accounts for the voltage drop of the battery during discharge, but some devices draw the same wattage no matter the voltage, some will actually draw less as voltage drops.
How KWH relates to a battery, well one can transfer their battery capacity to KWH, and subtract their kill a watt readings kwh and take off another 15 to 20% for inverter inefficiency, and keep in mind only 50% of the Lead Acid battery capacity should be used.
So volts x amps = watts
12.35v x 100Ah battery is 1.235 kwH, about, total. This is If the 100 AH battery is healthy and fully charged, and not discharged at a rate higher than 5 amps.
The 5 amp load is the load at which the battery earned its 100AH rating at the standard 20 hour rate. a 100Ah battery fully charged and healthy, could power a 5 amp load for 20 hours before voltage drops to 10.5v, which is considered 100% discharged.
Loads greater than 5 amps reduce total capacity, loads lesser than 5 amps will increase total available battery capacity, but not as much as loads over 5 amps reduce it.
This is the Peukert effect, and it cannot be discounted when the loads are to be larger than the rate at which the battery achieved its 20 hr capacity rating.
Battery manufacturers are not great at publishing a Peukert number. The best AGM batteries are around 1.1, Most flooded batteries are 1.15 to 1.2 when healthy. When unhealthy this might go as high as 1.4 before the battery becomes practically worthless.
Plug different loads and capacities and different peukert numbers into the following site to see just how unfair it is powering large loads on a small capacity of lead acid batteries. Remember the total AH the battery has to give result, assumes it was indeed fully charged at the start, which is something most people can never be bothered to determine.
http://www.csgnetwork.com/batterylifecalc.html
Just because a solar charge controller or automatic charging source drops to float voltage, does not mean a battery is fully charged, as so many people seem to think. dropping to float voltage means the charger held the battery at absorption voltage as long as it was programed to do so.
If it is not verified with a hydrometer, or on an AGM battery, when amps required to hold absorption voltage taper to 0.5% of capacity, only then can a battery be called/considered fully charged.
The time a battery requires at absorption voltage will change drastically depending on how it has been treated, how many accumulated cycles it has gone since the last full charge, the depth of those cycles, and how long between recharges, and the temperature and many other lesser factors, so the one size fits all charge algorithms charger marketers throw around as being perfect, are more of a one size fits none, and are
very imperfect on a regularly deeply cycled battery, especially when the battery is not 77 to 80F degrees.
SO, a solar controller which allows the user to change absorption voltage, will pay for itself in extended battery life, if the user makes an effort to find their battery's sweet spot in their usage. With automatic plug in charging sources, it is safe to bet they will stop in the 92 to 94% range and flash the green light and float the batteries at 13.2v. At 13.2v the final 6 to 8% will take 12+ hours before 100% state of charge is attained, but if absorption voltage of 14.4v+ was held it could have been accomplished in ~ 45 minutes to an hour and a half.
A charging source which can be overridden and forced to hold the battery at ~14.4 to 14.8v until 100% is truly reached, can greatly extend battery life. It will also reduce charging times if one is running a generator or has a limited time to plug into the grid. I use a 40 amp adjustable voltage power supply for all plug in/ grid based charging, and floating, but overcharging is a possibility if I leave it at 14+v for too long.
With automatic plug in smart chargers, when they flash the green light prematurely, should be restarted. they will need to be tricked by pulling battery voltage lower than 12.8v by applying loads and then restarting the charger on the next lowest amp setting. I had to do this a dozen times with a smart charger on a trolling motor battery before the battery was as fully as it could get, verified with a hydrometer.
Please take these factors into account when sizing/planning your system, and choosing your equipment. Keep in mind with 12vDC, voltage drop is an enemy, and the way to combat it is with shorter lengths of thicker copper between charging source and depleting load.
Ultimately batteries are appliances to serve us, but unless they are given a minimum of loving, they will pack up their toys and go home. When one notices the petulance starting, the 100% recharge should be considered mandatory. At this point, Absorption voltage might need to be held for 8 hours or more before specific gravity maxes out. It might not even occur at absorption voltage, at which point an Equalization charge is required, taking the battery upto as high as 16 volts, but no less than 15.5, and holding it there until specific gravity maxes out, or quits rising, or battery temperature starts increasing rapidly. This usually requires all loads be disconnected from battery as 16 volts can damage some electronics designed for 12 volts. If doing it on Solar, well expect a 100AH battery to require at least 5 amps to be brought to 16v, after it is already been held at 14.8v for a few hours. So figure 100 watts just to be able to equalize a battery on Solar, with no loads running on the battery.
A regular EQ charge can greatly extend battery longevity. The deeper the discharges and the less often the battery is returned to 100%, the more often EQ charges will be required and the longer they will take to max out Specific gravity. EQ charges are forced overcharges and are hard on a battery, so it is best to not have to perform them often, or rely on them as a hail mary capacity reset, as they don't always work, signifying the battery is on its last legs and one needs to consider replacement soon. If in a bank of batteries, a shorted cell in one battery can really degrade the remaining paralleled batteries. Removing a battery with a bad cell from a battery bank promptly, is important, so noticing a quick drop off in performance ( voltage held under load) and then verifying with a hydrometer is key to getting Okay service from the remaining batteries.
An amp hour counting battery monitor is a great tool for noticing this, but an observant human with only a voltmeter can notice it too.
If a good amount of cycles have been accumulated, then one can be OK with the battery at death's doorstep, but if the number of accumulated cycles is low, then replacement hurts, and then the battery abuser then tries to take steps needed to get better lifespan from the next set of batteries that they might have had difficulty in affording..
Better to know what the battery wanted to live a good cycle life in the first place, than learning the hard expensive inconvenient way.
Think of a battery like a balloon than when deflated, must be relatively quickly reinflated to near the bursting point each time, otherwise the skin gets less elastic and crusty more fragile , covered with small holes, and will not be able to stretch as far on the next reinflation. At some point., no matter how hard and how long one tries to refill the balloon, it just can't hold any air.
