I worked for Interstate Batteries for a couple of years when I was younger. Technology has changed a lot since I worked there but the principles are the same. More lead more reserve capacity. Discharging a battery frequently below a certain level can shorten the battery life. You will be better of with more batteries than you need. Of course that will increase the cost up front but will help extend the life of all of the batteries in the string. For power systems there are generally 2 types of batteries used, lead acid and sealed gell cell. They both have their advantages and disadvantages. Lead acid batteries are going to out gas, that is realease hydrogen gas during the chemical process between the acid and lead. You need good ventilation or you could have an explosion. The lead acid batteries require maintenance to keep them operating at peak performance. You will need to monitor acid levels and specific gravity. Specific gravity is how concentrated the acid is. It takes some work but a good lead acid battery can last a long time if properly taken care of. Sealed gell cell batteries are pretty much zero maintenance and have little or no out gassing. However once they start declining there is nothing you can do but replace it. Having a bad battery in your string will start dragging your other batteries down so monitoring any type of battery system is critical. All that being said in the world of batteries you pretty much get what you pay for. For a home system don’t skimp you will spend less money in the long run.