How many watts any specific battery is going to require to be held fully charged, at float voltage, will vary with the specific battery, its temperature, and health. Generally the older and colder, the more wattage it will require to be held at any voltage above its resting full charge voltage, which can also vary fairly widely.
The amount of parasitic draw in any given vehicle can vary widely, so how much was used parasitically overnight, needs to be known in order to give an accurate guess as to how much wattage, and duration of that wattage, is required, to top off and keep it topped off.
AGM's have developed a reputation among the uninformed, that their greater price leads to a better battery, but in real world usage they sulfate quicker, and their only realized benefit is the lack of offgassing and additional terminal corrosion that that can cause. They are supposed to also have less self discharge, but I bet this benefit, if actually measured, on the AGMS marketed towards vehicle starting duty, would be little if any better.
Since AGMS have developed this superbattery reputation, among the uninformed and inexperienced, their purveyors have figured out how to make them as cheaply as possible, cut as many corners as possible, get them to a price point where they are perceived to be not that much more expensive than regular flooded or sealed maintenance free flooded, and thus, an 'upgrade'.
Yet AGMS are all seemingly grouped into the same category, with the lowliest cheapest agm placed into the same class as a TPPL AGM like Northstar or Odyssey, which cost twice as much, and are still not immune to abuse.
AGMS tend to weigh slightly more, have slightly less total storage capacity, and by the specs I now see, a trend that is becoming more prevalent, do not even have an advantage in higher CCA than their flooded counterparts. CCA retainment as the battery ages, is another factor so dependent on so many variables that any declaration one way or the other, with few exceptions, only reveals the dangerous mix of both ignorance and arrogance, of which there is an ever increasing supply of.
AGMS are finicky. In deep cycle use they are tortured to premature demises with 'trickle charge it' mentalities.
When they do not get truly fully charged often, they lose their capacity/performance faster, and attempting to return some of that lost capacity/performance, requires a charger willing to hold them at higher voltages, for lots longer than any so called 'smart' charger will allow for.
One should also plan on no less than a 20 amp charger for a 100 amp hour AGM battery even if they do not intend to deep cycle them, as unintentional discharges happen and low and slow recharging, to a deeply depleted AGM is equatable to tickling it to its premature demise..
My AGMS get cycled deeply, often. I find they perform badly after extended times held fully charged at float voltage, and perform better after a deeper discharge followed by a high amp recharge and then held at absorption voltage until amps taper to very low levels or stop tapering.
I find if I only shallowly cycle them, and in such usage they cannot accept higher amperage charge rates for longer, they do not retain as much voltage the next discharge.
I find the best way to restore performance of my AGMS is to discharge them to the 50% range, on occassion, then feed them no less than 40 amps until 14.7v is reached and then hold 14.7v until amps taper to very low levels or stop tapering. The worse the battery has been performing, the longer this constant voltage stage lasts, with a minimum of 3.5 hours to as long as 15. The longer it takes for amps to taper to low levels is indicative of battery health and of the last time it was truly fully charged or got there with a high rate from a well depleted state.
When deeply discharged, the higher amps for longer, forces electrolyte migration through the glass matting and into the porous plates. The high $$ AGMS like Odyssey and Lifeline mandate no less than 40 amps per 100Ah of capacity, and Lifeline/Concorde says 20. These also have the super low self discharge levels that get attributed to low $$ AGMS, which could be entirely misleading/wrong.
The UPG/ chinese AGMS self discharge levels are not much better if any than flooded batteries, and they do offer sizes like group 27 for the same price as Deka intimidators.
I think AGM'S as starting only batteries, that rarely get deeply discharged and then only slowly recharged to 95%, if that, is why there are more and more reports of AGMS not lasting, in addition to them being cheapened for the frugal 'cheapest is always best' mindset of the modern entitled and largely ignorant consumer.
It's well known in experienced boating circles that AGMS are not lasting as long in deep cycle usage, as a good pair of flooded golf cart batteries or similar, and cost more than 2x as much for that lesser lifespan. What maintenance is not required in checking and adding distilled water and cleaning corrosion from unprotected battery terminals, is then spent in worrying about being able to hold them at absorption voltage for far longer than the sun shines, or the wind generator or alternator turns. Some have to split their battery banks and charge one while discharging the other, just to have the time needed for the AGM bank to reach a true full state of charge.
When the deeply cycled AGM does not get returned often to a true full charge, they lose capacity and performance quickly, far faster than their flooded deep cycle counterparts
The only exceptional AGM lifespans in vehicles I read about, are when the batteries are not located in the engine compartment. Perhaps the offgassing of a flooded battery helps keep it somewhat cooler through evaporative cooling, in such conditions and the sealed nature of the AGM in such an environment adds to its heating and premature demise.
As for how much dashboard solar is required, more is usually better, the exceptions are when there is lots of sunlight, very low parasitic draws, and a dash solar than can hold a battery well above 13.6v for hours and hours each day, every day. The simple act of opening the door and hood can lower this voltage to what appears to be acceptable, tricking the person checking with their voltmeter.
There is a balancing act here. A solar charge controller is basically a simple voltage limiter, and they can be had for as little 10$. I've got one, clunky as all get out, but it limits voltage to 14.7v where I set it, but I could move that to 13.6v with enough button pressing and curses.
One can buy a small panel, not marketed for the dashboard solar game, and a cheap solar controller( voltage limiter) and not have to worry about either too much or not enough solar to return the battery to an ideal 100% state of charge, and keep it there whenever the sun shines on it.
One would have to have some minor wiring skills in order to do so, perhaps passing wires through firewall. This eliminates about 99.5% of people who can only barely figure out how to use a 12v power port, and might not understand it is not live 24/7 in their specific vehicle.
I recommend against AGMS for vehicles that did not come with them originally.
The Advantages of the high $$ AGMS is super high CCA, and super low self discharge. They, when still healthy, can accept huge charging currents when well depleted, possibly overheating the alternator, but they can be recharged to 80% from anywhere below that, faster than flooded batteries. 80 to 100% is still 3.5 hours though, minimum, same as flooded, regardless of charging source. The battery accepts as much as it wants at the electrical pressure(voltage) reaching the battery terminals, and lead acid batteries all take no less than 3.5 hours to get from 80% to 100% state of charge. This time only grows as the battery ages, and it ages faster when that time held at charging voltages is not also increased.
The advantage of low$$ AGMS in vehicles that did not leave factory floor with them, seems to only reside in the no offgassing, and no fume induced additional terminal corrosion, and of course bragging rights to impress the equally uniformed.
I love AGM$$, prefer them, but I can treat them according to their needs. 99% cant, and wouldn't even if they were to understand what they require to live up to their potential.
Most just assign superbattery status to them despite evidence largely pointing to the marketers and bean counters winning again at the expense of the consumer.
I'd not recommend high $$ AGMS either, except for those needing the additional ~20% of CCA, or their rather insane ability to suck up as much amperage as a high amperage charging source can deliver, when seeking absorption voltage, when they are well depleted. The super low self discharge of TPPL Agms or Mil spec concorde lifeline AGMS, is a feature/benefit few will/could ever realize.
The parasitic draws of most vehicles,negate any perceived benefit of potentially significantly lower self discharge of a high $$ AGM, and low$$ AGMS might not have any benefit over their flooded counterpart in terms of self discharge, or CCA.
