Choosing batteries for your 24V system
Although other rechargeable battery technologies do exist - Nickel Cadmium batteries have been around for decades, and Lithium ion and Lithium polymer batteries are increasingly used in gadgets where weight is important - lead-acid batteries are the almost invariable choice for systems where you need a significant amount of power. Cost is the main factor, but lead-acid batteries are also pretty reliable and simple, so they'll be around for a long time to come yet.
Type of battery
Assuming you've settled on lead acid, the next question is whether you go for flooded or sealed types. Advantages of flooded batteries are the price - often about half the price for a given battery capacity - and the fact that they happily stand quite heavy charge and discharge currents. On the other hand if you turn them upside-down they can leak battery acid all over the place - not too pleasant - and they do gradually boil off the water in the acid, especially when charged hard. Sealed Lead Acid (SLA) batteries get round the sloppy acid problem by turning it into a gel, and Absorbed Glass Matt (AGM) batteries trap it in a matt that is wound around the lead plates in the battery. Both types of sealed battery are pretty much maintenance-free, but on the other hand they tend not be as good at handling big discharge or charge currents as traditional flooded batteries.
In general, SLAs and AGMs are best reserved for remote power situations where high reliability over a number of years with little maintenance is a high priority. Typically, such systems have a low power draw, and are recharged with a gentle trickle charge, perhaps from a solar panel. Flooded batteries are the usual choice for systems on boats and caravans. There are of course a multitude of flooded batteries out there too - and they aren't all the same. In general, 'deep-cycle' or 'traction' batteries have thicker lead plates than 'leisure' batteries (and cost a lot more) but stand deep discharge better and will last longer. Car starter batteries have even thinner plates than leisure batteries and won't last long in an off-grid power system.
So you've decided on leisure batteries as the best choice for you (most people do) - or maybe you're going to splash out on some AGMs or hefty traction batteries. Next question is, how big should your battery bank be? Well, firstly work out how much power you use every day, and then decide how long you need to last between recharges. If you use equipment that draws on average 48W (2Amp at 24V) for 10 hours a day, that's 20 Amp-hours (Ah) per day. If you want to last 5 days before needing to recharge, multiply that number by 5: 100Ah. Now double it, because it's not good for batteries to be discharged more than about 50% of their rated value. Voila - a 200Ah battery bank would be good.
24 volt batteries are as rare as hens teeth, and the usual thing to do is wire two 12 volt batteries in series to give you 24 volts. But don't go out and buy 2 100Ah 12V batteries and wire them in series and think you have 200Ah - sorry mate, you ain't. By wiring them in series you double the voltage, but you don't double the current you draw from them. So you still have 100Ah, but at 24 volts. If you had wired them in parallel you would have 200Ah, but at 12 volts. What we want is 200Ah at 24 volts, so we need to buy four batteries, and wire them in pairs in series, that are then wired in parallel.
Looking after batteries
Batteries do benefit greatly from a bit of TLC. If you have traditional flooded batteries you should keep an eye on the acid level, and top up with distilled water if it looks like getting a bit low. Sealed batteries can't be topped up - but on the other hand they suffer more from being overcharged. Make sure you use a regulator that is appropriate for both the charging current and the battery type on any solar panel or wind turbine that you connect to the battery. Modern solar and wind turbine regulators are very sophisticated and will prevent any overcharging.
Deep discharge is bad for any type of battery, and keeping your batteries well topped up with power will significantly extend their life. Batteries do slowly discharge on their own even if nothing is connected to them, so regular recharging is a must. It's one reason why solar panels are so good for batteries - because they are slowly trickling power back into the batteries all the time the sun is out, the batteries stay nice and happy and ready for action. It's far better for the batteries to have nice regular topping up from the panels (or a wind turbine) than a big heavy occasional blast from a generator or diesel engine.