What Batteries are there and what type should you choose, how powerful should the batteries be?
Well Budget is a big factor, as are the needs of the system and Space available to locate the Batteries is another!
Batteries can be located outside if some precautions are taken such as shading from direct Sunlight and prevention from very high or very low temperatures as this will shorten the lifespan.
There are many battery types some are definitely not suitable for Solar use, normal Car batteries for example are not designed for constant discharge and recharge, some batteries like Leisure Batteries, Gel Batteries can be used but will have only a couple of years lifespan if you regularly discharge them, much less of a lifespan if they are not managed very well and discharge more than 20% of their capacity before recharge.
There are older proven batteries which the Technology dates back to the war and were designed to power Submarines OPZV and OPZS which are large quite forgiving batteries if abused a little when used for Solar, the drawback is the size and weight because they have a lot of Lead in them.
There will always be new promising Batteries as Solar and Electric Car battery demands as time passes more research and development will take place.
Currently Lithium batteries (similar to what Mobile phones use) with different additives to make them more stable are the number one battery to go for, the reason is they are much lighter, it is possible to almost fully discharge them without damage to the battery and because you can use much more of their electrical capacity the physical size of the Battery needed is much smaller.
How powerful should the battery be? Well that would depend on the maximum demand that would be put on them, for example if you have batteries that are going to have to support all of your Electric requirements for up to four or six days say in an Off-Grid System where bad weather could be remain for days without good Sunshine, then a calculation of kwh or (Kilowatt Hours) per day would be needed.
If the installation is a Grid-Connected system then much smaller batteries could be used as they only need to support the system overnight and can use the Grid as a backup when required as electric demand rises or as battery capacity falls, again a calculation or history of overnight usage will help select the size of battery capacity required.
Always build in extra capacity into your battery bank, as this will help with the growing hunger we all have for electrical power as the years pass and the battery lifespan will be your reward.
Remember that batteries are a very expensive part of the system, getting this right is important and spending more money at the beginning will be the cheapest option in the longer term.