Batteries are the heart of any off-grid system and provide energy storage. This means that when the sun isn't shining or the wind isn't blowing you can still used the power generated when they were.
Just a quick summary of battery basics for anyone starting out as they should be maintained and understood. 2 Volt cells make up the required voltage. Leisure and car batteries come in 6 cells to make 12 Volts and most domestic systems are a multiple of 12V (e.g. 24V, 48V). The higher the voltage the thinner cable can be used and the more efficient for inverting to 240V.
Batteries & cells can be wired up to work in series or parallel. In series the Volts add up and the Amps stay constant, so you use series configurations to increase the Voltage. In parallel the Amps add up and the Volts stay constant, so you use parallel configurations to increase the Amps which increase the storage capacity.
An Amp hour (Ah) is literally one amp for one hour, so with 100Ah you can have a one amp load for 100 hours, or a 5A load for 20 hours, or 50A load for 2 hours, and so on.
For smaller systems in boats and caravans we usually recommend 12V leisure batteries that can fit under seats or cupboards. You need to keep the weight down and they must fit inside the vehicle for transit. However when you get to static systems it's usually possible to use larger batteries. It's always nice to have a dedicated space for off grid power systems, such as a shed or external room to the dwelling. Vented batteries give off hydrogen gas which can be dangerous in closed and hot spaces, but don't worry, it can be managed.
Side note* In our original solar truck we had limited space and so the batteries were next to the wood burner. We built a box to house the batteries with 2 air gaps, fire resistant board and metal, as well as a sloped lid with active venting.
So larger batteries will obviously cost more so for a substantial domestic system you're looking at possibly 50% of the whole system cost to be the batteries, that's if you buy brand new. But in the true light of reduce, reuse and recycle, we have always used second hand forklift truck traction cells, plus they are generally about a quarter of the price! Many of the batteries used in industry have not exhausted their worth before being replaced, usually after 5 years or often if the machinery they are powering breaks.
Traction cells are great for domestic systems as they are designed to be run down to a low percentage every day (shift) and charged back up quickly overnight on a high charge. This means they can take a battering, unlike leisure batteries which require a trickle charge. To put this into context, if there are a few days of no input (no sun or wind) the supply will not be affected, if they are sized correctly, and if there is a huge amount of resource one day (very sunny or windy) they will absorb loads of that lovely energy for you to use later!
As with all components of a renewable energy system there are lots of points to weigh up and consider and unfortunately it often boils down to budget. But you can arm yourself with knowledge and reduce the demand for new items. Lead acid batteries are recyclable BUT it is done in developing countries and quite often to the detriment of the local people and ecosystems. Compared to lithium-ion batteries where the raw material is finite and although they can be recycled it is not 'economically viable yet.
So whats the most ethical decision? We have always fantasied about water energy storage. You need a hill with a reservoir at the top. You use excess power to pump water up the hill into the reservoir and run a hydro turbine when you need the power. Extremely site specific and requires constant management by either yourself or some electronic trickery.
More info like state of charge and sizing your own battery bank can be found on our downloadable battery flyer here.