Storage batteries average out the supply and demand of your solar home's electrical system.
These are rechargeable batteries, and they are necessary to give your solar electric system a smooth, continual supply of usable electricity. If you try to use solar power from a PV panel without any battery (or any regulator), the voltage will fluctuate wildly as the clouds come and go. At night time you won't be getting any electricity from the sun at all, and when the sun is really strong the panels may put out a voltage that's high enough to damage sensitive electrical equipment.
The most common type of storage battery for a home solar electric system would be a deep cycle lead acid battery — or a bank of them.
You could have a simple setup with one or two 12 volt batteries wired in parallel. If money is a problem, two car batteries will get you by; but heavier duty batteries - such as deep cycle batteries or marine batteries — would be better.
That's what I used to run on a remote property of mine in country Queensland. It wasn't remote enough to be pure "Outback" Australia, but it came pretty close. It was a long distance from any sizable towns, and nearly four hours drive inland from Brisbane. I had no mains electricity in the place because I did not wish to pay the power company the fee of more than $10,000 that they quoted to connect me to the power grid out there.
When you use 12 volt batteries, each battery is actually made up of six 2-volt cells. Eventually one or more of those individual cells will die, and when that happens the whole battery will deliver 10 volts or less, instead of the 12 volts it was supposed to provide. And since you cannot repair one dud battery cell, you end up having to throw the whole battery our and buy a new one/
If I had installed six 2-volt storage batteries in series, I would have gotten 12 volts as the output (or more likely 13.8 volts, fully charged). That is the same as a regular 12 volt battery. The big difference is that, if one of those 2-volt batteries had died, I could have replaced that lone two-volt cell and immediately be working at full power again. I wouldn't have to replace the complete battery, including the good cells, when just one cell had died.
Those 2-volt storage batteries are expensive as well. Actually they are single cells. But six of them wired in series would give me not only 12 volts, but a whole heap of storage (Amps or Watts) as well. As much as quite a few 12-volt batteries... So individual cells cost more, but they last longer and have more grunt. The individual storage cells also cope well with being drained and recharged again and again.
Many storage batteries are lead-acid types, which means they have to be topped-up regularly with distilled or de-mineralised water. Using tap water is not a good idea, under any circumstances, because the impurities in the water can short-circuit your battery and will certainly shorten the battery's life. Some lead-acid batteries are sealed and are termed sealed lead-acid batteries, or SLA batteries for short. They are sometimes referred to as gel batteries or gel cells because, instead of liquid acid, their acid is in a gel (for gelatinous or jelly-like) consistency.
Many modern automotive and deep cycle batteries are gel cell batteries. They are still lead-acid, but because they use a gel instead of liquid acid, they don't spill, and they do not need topping up with water. This makes them truly low-maintenance batteries. They are a great time saver and very convenient to use.
Car batteries are the least suitable big battery type for solar power storage. This is because a car battery is designed to deliver very high power for just a few seconds so you can crank the engine and get it started. A 12 volt car battery is not designed to be discharged more slowly over a long period of time, nor is it made to be fully charged and then deeply discharged again and again for many hundreds of times (cycles).
If you have the finances, do invest your money in proper deep cycle storage batteries that were designed for a solar power system. You'll get the best results for your buck in the long run. But if you are trying to go solar powered on a shoestring budget, then try to at least use a tractor battery, heavy duty battery or a marine battery. They will handle deep cycling a bit better than a regular automobile (car) battery.