There are many metrics to use when comparing the battery bank components of an energy storage system. Comparisons can be challenging when analyzing batteries of different chemistries and their differing manufacturing standards. This article will briefly lay out the major metrics used to evaluate battery bank performance.

Nameplate Capacity

Nameplate capacity is the full chemical potential capacity of a battery or battery bank. One common way to express nameplate capacity is with amp-hours (Ah). When evaluating battery capacity using the Ah nomenclature it is imperative that the voltage of the system is considered. For instance, a 500 Ah battery bank at 24 V will provide 12 kWh of battery capacity while a 500 Ah battery bank strung at 48 V will provide 24 kWh.

As you can see in the example above, expressing nameplate capacity in kWh is a simple equalizer to compare battery capacities. For this reason, we expect to see more batteries listed by their kWh capacity than Ah capacity in the years to come. Working with batteries in kWh figures can also make for easier Comparisons to daily PV production or daily on-site energy consumption.

Nameplate capacity is a factor of discharge rate. These assumptions are listed on battery spec sheets as either a max discharge current or as a “C” value (C5 hours, C20 hours, etc). Consider comparing these rates to your power/energy needs to select the ideal battery bank.

Cycle Life

The cycle life of the battery is the number of times a battery can be charged and discharged over its lifetime. Cycle life holds an inverse relationship to the depth of discharge (DoD) of the battery, as you often see a fairly linear decrease in expected cycles as DoD is increased. While the battery is often still usable at End of Life (i.e. industry standard EoL = 80% of initial rated capacity), this is not always the case or always explicitly stated by the manufacturer.

It is important to recognize cycle life as it relates to battery chemistry when selecting a battery. For lead-acid batteries, you’ll generally see rated cycle life anywhere between 500 to 3,000 cycles, with advanced carbon lead-acid offerings pushing the envelope up to 5,500 cycles. On the other hand, lithium-ion technology is providing cycle life from 3,000 to 10,000 cycles.

Depth of Discharge

The depth of discharge (DoD) is simply the percentage of a battery’s nameplate capacity being used. For example, a battery bank with a nameplate capacity of 10 kWh at 20% DoD will only be utilizing 2 kWh of its available energy storage. The depth of discharge is a major factor in the overall life expectancy of a battery, as the deeper a battery is discharged the faster the electrolyte degrades.

It is important to carefully consider your individual use case when determining an appropriate DoD level. Let’s say you are designing a battery backup solution using the example battery above. You could potentially see a 10-year life at 80% DoD if the battery is only used 40 times a year (i.e. 400 cycles ÷ 10 years = 40 cycles/year). However in an off-grid scenario where you are cycling the battery daily, you would only expect a one year lifespan out of the system at the same DoD (i.e. 400 cycles ÷ 365 cycles/year = 1.1 years).  

Usable Capacity:

Usable Capacity = Nameplate Capacity x Depth of Discharge (DoD)

Understanding the targeted load profile and identifying your required usable capacity should always be step number one when designing an energy storage system. This also serves as a proper benchmark for comparing different options by calculating the required DoD and viewing the resulting cycle life of options at hand to best suit your needs.

Cost of Usable Capacity:

Cost of Usable Capacity = Battery Bank Cost / Usable kWh Capacity

The cost of usable capacity is another useful metric to compare battery systems. To calculate the cost of usable capacity simply divide the battery bank cost by the usable capacity figure you generated above.

Lifetime Cost of Usable Capacity (LCOE):

LCOE = Battery Bank Cost / Lifetime Usable kWh Capacity

By expanding upon the cost of usable capacity to include the lifetime throughput of a battery bank we arrive at perhaps the most telling battery metric of all, the lifetime cost of usable capacity. Sometimes expressed as the levelized cost of energy (LCOE), this metric defines a value to the energy a battery bank can store over the course of its usable life.

Careful though, just like many of the other metrics discussed in this article, LCOE is highly dependent upon your inputs and assumptions. It should be easy to see how its possible for one battery bank to derive many different LCOE figures. How? Depth of discharge and cycle life both impact the inputs to the LCOE calculation.

When performing an LCOE calculation it is important to bear in mind the warranty on your battery bank of cells. Some manufactures provide warranties measured in years. Others limit kWh throughput. Some do both. For instance, see LG Chem’s warranty here. In any case, be sure to factor this limiting metric into your calculations.

Lastly, it is important to remember that these calculations assume perfect field performance from all system components. In other words, one should consider researching various derating factors and incorporate them into a LCOE analysis.

Other Considerations

As is probably now apparent, there is unfortunately no single catch-all metric that can be used to compare battery banks. Furthermore, many of these metrics are closely correlated, making direct comparisons more challenging. And yet, as energy storage systems continue to decrease in cost, we all must become fluent in the language and metrics seen above so as to inform the development of future projects and widespread deployment of energy storage.

Lastly, keep in mind that the metrics we have discussed here are only valuable if you first have a firm understanding of the goals and constraints of the system you are designing. Sure, Battery A may offer 50% more cycles than Battery B. However, if you are designing a backup system that may only be used a handful of times per year, those extra cycles are of limited value. For this reason we suggest identifying the desired performance characteristics of your battery bank before you begin comparing chemistries and manufacturer offerings.