The Basics of Battery Storage Terminology

Written By The IPS Team

On April 13, 2020

At a Glance:

Whether you’re brand new to the topic of battery backups or are well along in your research, here are the basics of battery storage terminology and metrics to keep in mind as you explore solutions for your home or business.

Home batteries on the brain? You are not alone.

From homeowners upgrading to electric vehicles to utilities using massive batteries to balance power grids, many are looking to energy storage as a technological solution with enormous economic and environmental potential.

Solar power systems are a natural pairing for battery storage systems, making the sun’s energy accessible all day and night for homes and businesses across the country. Between new battery chemistries and continual performance improvements, energy storage technologies have become more attractive than ever for a variety of markets thanks to added value streams and resiliency.

Whether you’re brand new to the topic of battery backups or are well along in your research, here are a couple of helpful terms and metrics to keep in mind as you explore energy storage solutions for your home or business.

Energy vs. Power

When talking about solar panels and energy storage devices, ‘energy’ and ‘power’ refer to two distinct and significant terms.

Power refers to the amount of electricity that a battery can put to work at any given moment. The unit for power is a watt (W), which should be a familiar unit, especially if you’ve been shopping for light bulbs recently. Perhaps even more familiar is the kilowatt (kW), which represents 1,000 watts. Kilowatts are the scale most commonly used for appliances, batteries, and many other electrical devices in our lives. Your monthly electricity usage on your bill is usually given in terms of kilowatts used during your billing period.

You may also encounter megawatt (MW) as a unit, which represents 1,000 kilowatts or 1,000,000 watts. The megawatt-scale is useful for discussing large, utility-scale power plants and energy storage projects. For example, in 2019, the United States had 899 MW of utility-scale battery storage capacity installed, with much more on the way!

Energy is power times time or said a different way, the ability to do work over time. In the context of a battery, energy refers to the amount of electricity that can be put to work over a certain duration (e.g., hours, days, weeks, etc.). A unit of energy can be expressed as Watt-hours (Wh), where 1,000 Wh equals one kilowatt-hour (kWh) or 1,000 watts of power delivered over the course of an hour. For example, a single Blue Ion 2.0 battery can provide up to 16 kWh. With a full discharge, the Blue Ion 2.0 can provide 1 kW of power for 16 hours, or perhaps 4 kW for 4 hours. Depending on your energy needs, you may need a larger battery bank to ensure you’ll be able to power your devices without interruption. For example, individuals with vital home medical devices that require continuous electricity may need larger battery banks with a greater capacity to supply power even during prolonged outages.

Regardless of the application, both power and energy need to be considered when choosing battery chemistry and the size of your battery backup system.

Battery Bank
ground-mounted solar system

Voltage and Amperage

Along with watts and kilowatt-hours, voltage and amperage are also crucial characteristics to consider when looking at battery storage systems. Voltage is given in volts (V), and amperage is given in amps (A).

To understand volts and amps, an analogy to a familiar technology is often used. In place of electrons flowing through a conductor, picture water running through a pipe. Amps are akin to the volume of water flowing through the pipe, while volts refers to the water pressure in the pipe. Amps measure the rate at which electricity flows, i.e., the current. Volts measure the difference in electric potential between two points.

For example, if evaluating solar batteries from Sun Xtender, their products are offered in two, six, and twelve-volt options. The choice usually depends on the sizing of your solar system. A knowledgeable energy storage consultant can guide you through these decisions, so your battery bank is up to the task while remaining cost-effective.

Capacity vs. Depth of Discharge

The capacity of a battery is the total energy it can store. For example, the Blue Ion 2.0 battery cabinet comes in three sizes: 8 kilowatt-hours (kWh), 12 kWh, and 16 kWh—all of which can be combined with other units to increase the system’s capacity up to 450 kWh. Battery banks are sized to ensure there is enough storage capacity to power critical systems for the desired duration.

However, there is an extra layer of complexity for determining the true capacity of a battery bank. Some batteries types like lead-acid can only be drained of a certain percentage of their stored energy safely (i.e., 50%). Going beyond that point shortens the life of the battery. The depth of discharge (DoD) is the metric for this safe drainage percentage.

The DoD of lead-acid batteries is typically around 50% while the newer lithium batteries have higher DoDs, at generally 90-100%. A depth of discharge of 100% means that the battery can be discharged entirely without any negative consequences to the equipment or premature degradation.

In terms of a battery’s true capacity, a 4 kWh lead-acid battery with a 75% DoD effectively only has 3 kWh of energy available on hand when fully charged. That 3 kWh is referred to as the relative capacity or useful capacity of a battery. So, for an 8 kWh lithium iron phosphate battery with a 100% depth of discharge, you would need two 8 kWh lead-acid batteries, each with a 50% depth of discharge, to have an equivalent capacity.

Lithium-ion Battery diagram

Efficiency

Another useful metric for comparing battery options is efficiency. When you charge a battery, there will be a slight loss during the process of converting the electricity into stored chemical energy. This means there will be slightly less energy available when you go to discharge the battery. Round-trip efficiency measures the amount of electricity required to achieve the useful capacity of the battery.

Lithium batteries again have the edge over their lead-acid counterparts when it comes to efficiency. Lead-acid battery efficiency is usually around 80-85%, while lithium-ion batteries are closer to 90-95% efficient, meaning less energy is lost when charging or discharging the battery. A battery bank should be sized to compensate for energy lost to charging/discharging inefficiencies.

Cycle

Cycles are a way to gauge the useful life of a battery. A cycle refers to each time a battery is discharged and recharged. The lifespan of a battery is estimated as a number of cycles that can be completed before performance drops below a designated level. This is called the “cycle life” of a battery. A deep cycle refers to a cycle in which the discharge continues until the battery reaches its cut-off voltage. Shallow cycling refers to cycling a battery such that the cut-off voltage is never reached.

Cycles are a helpful method for estimating battery health because the usage of the battery is generally a bigger degradation factor than just the passage of time. Some battery backup systems will only be used a few times a year, while others might be fully discharged every single day in the case of an off-grid system. Regardless of usage, proper maintenance and energy management are recommended to ensure a battery will last its rated lifespan (and beyond).

Having these terms and metrics in mind will help better understand the differences in various battery storage options. Our energy storage experts at Independent Power Systems are available to discuss the intricacies of battery storage and how it can benefit your home or business.

Reed Crossley

About Independent Power Systems

Independent Power Systems (IPS) has been designing and installing premium solar and energy storage systems for residential and commercial customers in Colorado, Massachusetts, and Montana since 1996. Our team of highly skilled engineers, electricians, and installers is committed to innovation in energy resilience and sustainability as well as helping the community achieve energy independence, do right by the environment, and save money in the process. To date, IPS has installed thousands of grid-tied, off-grid, remote wind, and microgrid systems including several challenging international projects for large commercial clients and the military.

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