How much electricity does it hold? What size suits me? Does size matter?
It all depends which way you look at it… often the same thing looked at from a different perspective can appear to give a different answer. You need to consider what exactly you are asking the battery to do. If it is to simply maximise self-usage from solar generation throughout the day then a smaller battery e.g. 2-3 kWh with a small charge rate and discharge rate (how quickly it fills and empties) may be fine for you.
If you want to run your home at night from your own solar generation then of course you need to consider the size of your consumption after the sun goes down as well as your rate of consumption i.e. how much energy you consume at any one time. As such you should hopefully have your annual energy bill to hand, think of your habits e.g. tv, lights, cooking etc – anything that you will use in the evening – and try to calculate this total value. With most modern houses and families, it is very easy to use between 5-10 kWh during the night time. Most battery systems completely discharge by the middle of the night e.g. 3am. So, if your intention is to be self-sufficient then you probably want a bigger system than you think.
Normally, the larger “more advanced” systems will be more flexible and offer realistic rates of charge and discharge with a useful capacity that “budget” systems just do not offer – in other words they are more practical.
So as a rule, the cheaper small systems e.g. 2-3kWh are more useful for storing your own production and filling in gaps during daytime usage when the sun is not shining.
People who want to be Grid independent in the evening will need an average of around 10-15kWh minimum storage for a normal sized family home – potentially less for a smaller consuming couple of 6kWh minimum. If you have a larger home with a larger consumption pattern then not surprisingly you may want a larger battery. I have quoted for 15 to 30kWh of storage from domestic homes in the past that want to run things such as air source heat pumps in the case of a power cut. You can have pretty much any size system you need, scale is not the issue. If you have a need for it and the budget to pay for it then to a certain extent the sky is the limit.
You do however need approval from your local electricity network provide to connect batteries to your home so they may impose a limit on how big a system you can have, but that’s just in terms of the discharge rate rather than overall energy storage capacity
One key thing to be sure of though is what is the actual useable size of the battery as this is often not the same as the quoted size on marketing information – please read the next blog for more details.
Written by John Rowlatt