What is the useable storage capacity of the battery?
In other words – how much does it hold that I can access? On the surface this seems easy – read the label… surely it gives you what you thought? Effectively, however you have gross and net capacities……
You may think that if a battery says that it is a 5kWh capacity that this is a value that you can point to and say with confidence “this is enough for my needs”.
There is something called depth of discharge however. This is often referred to by a percentage – e.g. the battery has a recommended depth of discharge (DoD) of 80%. This would indicate that if the battery had a total capacity of 5kWh with a reserved DoD of 20% then you would have 4kWh USEABLE storage capacity.
The best way to think about it is gross and net……… the battery has a gross rating of 5kWh but a net capacity of 4kWh – the last 20% is inactive from your perspective.
Depth of discharge limits exist due to varying battery chemistries having varied rates of degradation over time affected by the DoD limit that each battery runs. Environment factors can affect this as well – so always check in the data sheet when comparing two different batteries.
Essentially though if you think that the advertised capacity between two brands are the same, they may not be.
Look for statements such as warrantied life of 6000 cycles at 80% DoD. This means that this particular battery’s lifespan is predicted to be 6000 cycles within certain warranted performance specifications e.g. 60% of usable charge held at 10 years – see warranty – and that its chemistry really does not want the battery fully discharged but only to a maximum of 80% discharged.
An upside to this characteristic of batteries from your point of view is that if they are routinely discharged less than this then they will live longer, and depending on the size of your battery and the way you use it, it may well not be used for a full DoD cycle all the time, extending it’s practical life, particularly with larger battery systems.
To put this into practical terms when choosing a battery system, a product that has 10kWh of storage at 90% DoD means that you are getting 9kWh of accessible stored capacity. As such if another says it is 13.5kWh at 100% depth of discharge then you have a full 13.5kWh of accessible storage. Effectively 50% more accessible storage capacity than the first. So, if the price between the two is not 50% more for the second one then it offers better value for money per accessible kWh stored.
- At the very least have you checked the manufacturers data sheet for clearly stated useable capacity?
- Does the useable capacity drop off after installation depending on the location e.g. ambient temperature of location?
- Have you calculated the warranted degradation rate into your decision making? Will the battery have enough capacity to cover my needs in later years?
Written by John Rowlatt