Understanding home battery storage was never easy. It’s confusing. And if you’re confused, you’re unlikely to feel confident about making a big investment like a home battery bank. You’ll hear quite a few different terms & specifications when shopping around for battery products. Optimus Energy gathered them all here in an attempt to bring some clarity. This often fraught endeavor sometimes feel like comparing apples to oranges.
Here are the useful terms & specifications related to home battery storage:
Nominal capacity: The total amount of energy that the battery can hold at a time, usually described in kilowatt-hours (kWh). Sometimes the nominal capacity of a battery is the same as the usable capacity, but not always.
Usable capacity: Take into account the amount of energy that a battery can hold after the depth of discharge, in kWh. We believe that all manufacturers/retailers should use the term ‘usable capacity’ to sell batteries.
Depth of discharge (DoD): The percentage of the battery’s capacity that you can regularly discharge without significantly damaging the battery or reducing its lifespan. Lithium batteries typically discharged to 80% or 90%, while lead batteries go down to 40% or 60%.
Maximum power: The peak amount of power that a battery can generate at a given time, for a short period in kilowatts (kW). When switching on a blender, for example, it will cause sudden surge before leveling off. The capacity of the system’s inverter may limit the maximum output power.
Continuous power: The amount of power the battery will generate in normal, non-peak operating conditions. It is the amount of power you can ordinarily expect out of the battery.
Roundtrip storage efficiency: The efficiency of charging and discharging the battery. How many kWh do you get out for every kWh you put in? For most batteries, this varies depending on battery usage, but you can usually get an average figure. Remember that this is only for the battery – the inverter will also reduce efficiency.
Cycle life: The number of charge/discharge cycles a battery has before its ‘end of life’. Some battery manufacturers give cycle life as full cycles, while others may count a partial cycle as one full cycle. Lithium batteries typically have a cycle life of 4,000-10,000 cycles, while a lead battery might go low as 1,000 cycles. For lead batteries, cycle life extends by designing the system to have a lower depth of discharge.
Design life: Sometimes it’s unclear if cycle life refers to warrantied cycle life or design cycle life. Design life is the number of cycles/years you can expect the battery to operate in a given application. Beware that this term is often marketing speak – a battery bank’s design life could run much longer than warrantied life.
End of life (EoL): End of life is the point at which you replace the battery. In most cases, end of life is associated with a decreased ability to hold a charge – which is why you’ll see figures like ‘80% retained capacity’. LG Chem‘s range of batteries has a 60% EoL figure, while Tesla’s Powerwall 2 has an EoL figure of 70%.
Ambient temperature range rating: Batteries are sensitive things and will not operate as well as they could if the outside temperature gets too hot or too cold. Operating outside of the prescribed temperature range may damage the batteries, reduce their efficiency or both (read more on Battery University), and/or void the warranty. Fortunately, most battery management systems keep batteries operating optimally. This means your battery may not operate if ambient temperatures go outside the bounds in the spec sheet.
Product warranty: This is the part of a warranty that covers all parts of the battery unit. Basically, the purpose of a product warranty is to cover everything to do with the unit. Product warranties are usually shorter than performance warranties.
Performance warranty: This is the amount of time you can expect the battery to continue to operate – within specified parameters. Most battery storage systems for home use have 5-10 year performance warranties. Always check the terms to make sure of what costs would fall in the event of a warranty claim.