Let's suppose we have a new very healthy fully charged AGM battery that is ready to put into service and it is rated at 12V 100Ah (at a 12 hour drain rate, therefore it has 600 Wh of usable energy per charge). Also suppose during its lifetime, it can be charged and discharged 500 times each, down to 50% SoC, therefore having a lifetime capacity of 500 * 0.6 KWh = 300 KWh.
Case 1: we put a 50 watt load on it for 12 hours a day, which is 600 Wh of drain, then charge it back to 100% SoC in the next 12 hours and repeat that cycle 500 times.
Case 2: we leave the 50 watt load on 24/7, however, during the daytime, it can be assumed that the battery will be charged full within 12 hours of daylight using a solar panel (assume 150 watts average over the 12 hour daylight period, enough to compensate for inefficiencies of AGM batteries and solar charge controller inefficiencies). So the battery will drain down to about 50% SoC overnight and then slowly go back to 100% SoC during the 12 daylight hours.
So how will case 2 affect the total usable lifetime capacity? Is having 100 watts of charge power and a 50 watt load simultaneously similar to no load and a 50 watt charger? Will the battery have the same # of charge discharge cycles in these 2 cases? Does a battery behave differently having a 50W load and 100W of charge power vs no load and only 50W of charge power, or can those 2 cases be considered pretty much identical?