http://www.trojanbattery.com/product/12-agm/
Amp hours is a measure of battery capacity
Amps is a component of current. A 5 amp current draw on a battery for 4 hours removes 20 amp hours from the battery.
But a 50 amp draw on a 100 amp hour battery for one hour does not mean the battery is at 50%. Higher amp draws on a battery mean the battery has less overall capacity to give.
This is called the Peukert effect, and is often discounted or ignored when discharging a battery. It is easier to do the math without factoring in Peukert, but it leads to inaccuracy, and the larger the load on the battery the more the inaccuracy.
You should only use half of a batteries capacity, half of 420 amp hours is 210 amp hours.
AGM batteries tend to hold higher voltages under higher loads in comparison to flooded batteries
Trojan recommends a 20% charging current, or 20 amps per 100 amp hours of capacity on their AGM batteries. The battery is not going to fall flat if this current is not initially met, but it is wise to try and meet the manufacturer recommendations.
http://www.trojanbattery.com/pdf/TRJN0109_UsersGuide.pdf
20% of 420 amp hours is 84 amps. Is your Iota converter the 75 amp or 90 amp version? Plug in chargers should really be sized to meet the manufacturer recommendations of initial bulk current.
84 amps is a lot of solar too. If one figures 100 watts of solar yields about 5 to 6 amps.
There is a tendency for people to get a lot of battery capacity then not have enough charger to come close to manufacturer recommended bulk currents for that battery bank.
Any solar is better than no solar, but small amounts of solar on a large battery bank cannot be looked upon as a proper charging source, if the batteries are cycled each and every night.
The deeper the batteries are cycled, the more important it is that they receive the manufacturer recommended bulk current, and bringing the battery bank upto near 100% before the next discharge cycle will yield better longevity too.
So I recommend people size their battery banks to closer match their available solar, if Solar is to be the primary recharging source.
But if one can plug in a converter charger meeting the specs of the batteries every 10 to 14 cycles, then the solar system can be sized to merely offset the overnight usage, +10%.
AGM batteries are a lot of coin, and in comparison to flooded batteries, are much harder to determine how well they are fairing when cycled often. Dipping A hydrometer is the ultimate way of knowing, and this cannot be done on an AGM, so one is flying blind unless they have a way to measure amp hours out of, and back into a battery.
Voltage is not a good indicator of state of charge, even though many think so. If the battery has not seen any charging or discharging currents for several hours, then voltage can be indicative of state of charge, but not overall battery health.
AGM batteries tend to hold higher voltages under discharge than flooded. So the common held belief that 12.2 volts is 50% is not really accurate on an AGM either
Proper and prompt recharging is paramount to getting a good lifespan from any battery. Come as close to manufacturer recommendations as possible as often as possible, and view any other regimen as detrimental, to some degree or another.
Matching charger capability to battery capacity is wise, but even if everything is perfectly matched, the batteries will at some point fail. trying to prevent premature failure is the goal. Reaching ultimate longevity might be labor intensive and stressful so getting that extra 150 to 500 cycles might not be worth it to some.
