Failing to understand jargon like the difference between ‘nominal’ and ‘useable’ capacity can leave homeowners with less battery storage than expected and even disqualify their batteries from the federal rebate. Here’s what it all means and why it matters.
Scattered among SolarQuotes’ home battery customer reviews of various brands are a few comments from disappointed Australians who didn’t get quite the amount of storage they expected. When determining the size (capacity) of battery you’ll need, it’s important to understand several related terms.
Nominal (Or Total) And Useable Capacity
Nominal or total capacity, described in kilowatt-hours (kWh), is simply the total amount of energy1 a battery can store. Useable capacity is the amount of energy that can actually be accessed for your use.
Using the example of a Sigenergy Sigenstor 5S-13, its datasheet indicates a total capacity of 13.44 kWh, and a useable capacity of 13 kWh. But the difference between nominal and useable capacities in home battery systems varies.
The difference is particularly important to understand if trying to claim the federal battery rebate, for which the nominal (total) capacity of 5 kWh to 100 kWh will be eligible, but only the first 50 kWh of usable capacity is discounted.
Why the difference?
Leaving a small amount of capacity untouched helps to avoid damage to the battery and increase its service life. The good news is you won’t have to do mental gymnastics figuring out how much you can use after it is installed. Whatever the solar battery’s useable capacity is, that’s all you’ll be able to access.
Depth-Of-Discharge (DoD)
But sometimes the useable capacity isn’t specified on a datasheet — just a total or nominal figure, or even just ‘capacity’. If that’s the case, then this is a flag to look for another term: Depth-of-discharge (DoD).
Depth-of-discharge means how much of the total capacity of a battery, expressed as a percentage, is useable. For example, if a 10 kWh battery has a DoD figure of 90%, that means it has 9 kWh of useable capacity. Usually a DoD figure (if present) for a lithium-ion based system will be 90% – 95%, but I’ve seen it as low as 80%. In that particular case, it was buried in the battery’s warranty. This is why it’s important to go beyond the datasheet and marketing materials, and closely examine the warranty document as well — and more on this in a bit.
Don’t Forget To Factor In Backup Reserve
Many Australians are buying home battery systems for their backup capabilities. Backup reserve allows you to specify how much stored energy is set aside for use during a blackout. This limits the amount of useable capacity you’ll have for day-to-day use.
Using the Sigenstor 5S-13 example again:
- Total capacity: 13.44 kWh
- Useable capacity: 13 kWh
- Backup reserve set to 20% (2.6 kWh)
… this means you’ll have 10.4 kWh to play with. But backup reserve can usually be adjusted in the system’s app — the mySigen app in this case.
Capacity Degradation
Over time, a solar battery’s storage capacity gradually degrades, and some more than others. Another good reason to carefully scrutinise a warranty document is for the performance warranty, which guarantees a battery will retain a certain amount of its nameplate capacity after a specific period or number of charging cycles/energy throughput (usually whichever comes first).
A performance warranty of at least 70% of the battery’s nameplate capacity remaining at the end of a 10-year warranty period isn’t uncommon these days, but there are some models on the SolarQuotes home battery comparison table where the figure is as low as 50% – 60%. If you install a solar battery that only just meets your capacity requirements now, it may not a few years down the track — but that doesn’t mean buying a battery that’s too big to fill.
It’s the responsibility of a solar retailer to clearly explain these terms and recommend a solution that suits your circumstances, providing among other documentation accurate details on expected system performance (and what can affect it) before you sign on the dotted line. Battery manufacturers should also be clear about these aspects in their marketing and datasheets.
But it’s not a perfect world.
Learn about other common battery-buying mistakes to avoid and everything you need to know about choosing the right home battery for you.
Footnotes
- It’s also really important to understand the difference between power and energy. ↩
About Michael Bloch
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This might seem weaponised but my positive intention is to ensure others are not deceived too, and its the truth.
Sungrow tell lies in its data sheet, with 100% DoD and 100% usable energy stated.
Neither are true.
I was hoodwinked by these, ie, believed it, only to be told after spending a sizable sum that “there is a reserve that is hidden of 5%” in Sungrow’s own words.
Hidden! Hidden alright; from purchasers and only revealed after they try to figure out why their 19.2kwh battery is only delivery 16kwhs.
Of course there is a small amount that must be retained. I knew this. But a data sheet doesnt lie does it and its illegal to make false and misleading statements to induce people to buy, isnt it?
So I assumed it wasn’t stating “nominal” but actual dischargeable capacity. Wrong.
I pointed this out to Sungrow & my retailer over a year ago. The data sheet is unchanged, both continue to market Sungrow SBR batteries with same misleading statements about capacity.
Hi Neil,
You’re not the only person I’ve had complain about Sungrow in this regard. In their defence, sometimes it’s been because a module has failed and needs replacement under warranty.
All batteries rely on a Battery Management System (BMS) to control voltage, current and temperature for lithium batteries & keep them in a safe operating window.
Part of this management is counting kilowatt hours in and out, then making an estimate on State of Charge, because there’s no hard and fast way to determine SoC simply using voltage for instance.
They make a good product and it’s well supported but I’d be happier if Sungrow were honest about a nominal capacity (x cells of x capacity = nominal) and a useable figure, like everyone else in the industry.
Yes, it’s hard to take somebody seriously when they say their battery has 15kw capacity or their solar system is 10kwh and they are espousing some opinion about home energy.
Also the w is capitalised (kW and kWh) because it references a persons name. James Watt the Scottish engineer that made the steam engine common and started the Industrial Revolution that has lead to the climate crisis we are entering now.
Why is only “W” capitalized?
This follows the SI (International System of Units) conventions:
Unit names are lowercase unless named after a person (e.g., watt → W, newton → N, volt → V)
Prefixes like kilo- use lowercase letters (so k for kilo, M for mega, etc.)
If warranties are based on say 70 % loss of useable ouput based on cycles or throughput where on the app or battery does it have the count for self checking to see how your tracking? Also is it a linear relationship between the degradation and cycles or throughput?
One thing i never see discussed with home batteries is the batteries allowed discharge rate. This is a big issue with lithium batteries if one wants to use them to power electric boat motors which can draw 50 or more amps.
I am assuming this is because the allowed discharge rate for home batteries is far in excess of what the inverters used are capable of using?
This may be a taboo topic but battery manufacturers have a rather large incentive to boost their “usable battery capacity” in whatever way possible so their battery can maximise incentives.
As the incentives are often based on “usable battery capacity” I’m guessing we’ll be seeing more cases where the depth of discharge is squashed and it’s up to us householders to figure out the actual “usable battery capacity”