October 20, 2023

Understanding the Efficiency of Energy Storage Systems - Key Factors and Performance Metrics

7 Min. Read

Achieving sustainable energy will require more than simply boosting renewable power generation in the US. Employing energy storage capabilities is needed to capitalize on decarbonization efforts, ensure grid stability during peak demand as well as outages, and enable a cleaner and more resilient future.

This article reviews the types of energy storage systems and examines charging and discharging efficiency as well as performance metrics to show how energy storage helps balance demand and integrate renewable energy at residential or grid levels.

Grasping Energy Storage Basics

Energy storage is the process of capturing electricity generated now for use at a later time. Through battery banks and other storage technologies, the power available for use remains reliable despite variations in generation from wind, solar, and other renewable sources, making decarbonization of electricity attainable.

Types of Energy Storage

While most common, batteries are just one energy storage technology available nowadays, all of which can be paired with software to control the charge and discharge of energy on a building or grid level. Let’s look at battery storage as well as some other energy storage options:

Battery Types

Battery capacity has grown rapidly as battery costs have decreased in recent years. As a result, battery storage is an increasingly popular solution for homeowners, businesses, and utilities looking to reduce their energy costs.

There are various types of batteries used in energy storage systems to meet specific power and duration requirements. These include:

Lithium-ion (aka Li-ion)

Li-ion batteries have a range of applications, from consumer goods to electric vehicles to scalable energy storage systems for homes and utility use. These systems typically house several connected battery cabinets depending on the energy to be stored and discharged. A small refrigerator-sized residential system can power a home for days.

Bidirectional charging capabilities can also turn an EV or a fleet of them into giant battery packs to be deployed during peak power periods or an outage. Demand for Li-ion batteries has grown exponentially in recent years, leading to shortages of raw materials and supply chain constraints. Manufacturers are testing alternative mineral and chemical compositions.


Widely used for rechargeable batteries, lead-acid batteries are known for their reliability and eco-friendly design. Most lead batteries available today are predominantly made with recycled materials, and, at end-of-life, 90% of their materials are recovered for reuse. Lead batteries are widely deployed to support photovoltaic (PV) solar power installations for both residential and commercial use and domestic premises. They are also less flammable than Li-ion alternatives.

Redox flow

A reduction-oxidation or redox flow battery uses chemical energy provided by two chemical components dissolved in electrolyte fluid flowing through the rechargeable central unit from two exterior tanks. One tank serves as the anode with a negative charge, and the other as the cathode containing a positive charge. A chemical reaction takes place inside the central cell that creates an electric current as electrons flow between the tanks.

The advantage of flow batteries is their ability to store and generate large volumes of energy for longer durations. There’s no need to add additional units; storage and output can be increased by increasing the tank sizes.


Thermal storage systems use heating and cooling to store and release energy, which reduces costs and load needs for heating and cooling at other times. Advances in thermal energy storage are enabling greater energy savings, heat pumps that are higher performing and more affordable, as well as flexibility for shifting load demands. Here are two examples of thermal energy storage systems:

  • Ice Storage Cooling: During off-peak hours, ice is made and stored in energy storage tanks. The stored ice is used to cool the building the following day rather than traditional air conditioning.

  • Molten Salt Heating: The salt is heated and stored in an insulated container, where it can be kept for several days. The salt is later pumped into a steam generator to run a turbine, which generates electricity. Solar power is one way the salt can be heated.

Pumped Hydropower

Hydroelectric energy is one of the most common forms of energy storage in use today. During off-peak hours, water is pumped to an elevated reservoir using turbines. When electricity demands require additional power, the reservoir opens to allow the retained water to flow through turbines and produce electricity.

While efficient, this renewable source of energy is limited to areas with adequate running water as well as enough space for an elevation difference between the reservoirs.

Role in Renewable Integration

Energy storage systems are critical to the integration of and efficient use of renewable energy. Renewable energy sources are not available 24/7, like an old-fashioned coal-burning generating station. Accordingly, it is critical to store solar power during the day, available wind, hydropower, and other renewable energy for use when it is not readily available. Using energy storage maximizes the use of renewable power sources by storing energy when it is cheapest and deploying it when it is most needed. This can reduce the cost of electricity and improve grid reliability.

State and local governments and utilities are capitalizing on available funds from the federal Infrastructure Investment and Jobs Act and other financing programs to increase available energy storage and other clean energy investments on a macro level.

Commercial buildings and homeowners can also use federal incentives to add battery storage banks or other energy storage tools to their own microgrids, enabling them to manage their power resources and usage on a micro-level. When integrated with solar panels, a battery energy storage system enables a homeowner or commercial property manager to optimize charging and discharging cycles, thereby reducing energy costs and their carbon footprint.

Factors Influencing Efficiency

All methods of transmitting electric power have losses along the way from generation to utilization. Approximately 5% of electricity transmitted along traditional power lines is regularly lost. It’s important to acknowledge that batteries and other energy storage solutions have losses between charging and discharging. The energy retrieved after a charge is always less than what has been put in.

Maximizing round-trip efficiency (RTE) between generation and power usage is critical to minimizing losses, from leakage in storage to dispensation. Higher RTE numbers mean less energy was lost along the storage-to-discharging path. Grid systems reportedly aim for an 80% RTE in their energy storage systems.

RTE results are impacted by the type of technology, storage duration, equipment efficiency, speed used to charge and discharge temperatures, and other factors. Managing the energy storage process requires analyzing losses periodically to maximize efficacy. A licensed electrician with experience implementing energy storage systems can help minimize energy loss and maximize system efficiency.

Performance Metrics and Assessment

There are metrics besides RTE to consider when evaluating the performance of energy storage systems. These include storage capacity, storage duration, leakage or self-discharge, and cycle lifetime (how many times it can be charged and discharged before performance declines). Two key metrics to know about are:

Coulomb Efficiency (CE)

Coulomb efficiency measures the efficiency of the battery bank in storing and releasing energy in one cycle. CE is the ratio between the charging capacity and discharge capacity after a full charge. Besides variations in results by types of energy storage systems, results differ based on the system’s age, different charge-discharge current rates, decomposition of electrolytes, nearby temperature, and other factors.

Depth of Discharge (DoD)

The DoD indicates the percentage of energy discharged relative to the overall capacity of the storage system. A battery’s lifespan can be affected by how frequently it is charged and discharged. In addition, discharging a battery entirely also affects longevity and performance. A higher DoD is ideal, showing more of the energy being stored can be used.

Exploring Options

Growing climate concerns have led to more utilities, businesses, and homeowners looking for ways to maximize their use of sustainable energy. Generating more renewable power is the first step. The second step requires adding energy storage capabilities to ensure resiliency and consistent capacity.

Weighing the energy storage systems, with or without solar integration, to determine the best storage tool for your property is complicated. You want to consult experts with specialized training in deploying and ensuring the efficiency of energy storage systems. Qmerit, a national leader in electrification, can provide that expertise and make your energy transition easy and seamless.

Qmerit offers a network of experienced, certified installers throughout North America. Our highly skilled network has installed more than 18,500 energy storage systems more than 269,000 Level 2 EV chargers, and integrated more than 53,000 solar systems, and we’ve maintained the highest level of customer satisfaction., backed by our Peace of Mind Guarantee. Qmerit’s network of certified electricians can help you determine the best battery storage or other renewable energy solutions for your needs. Contact Qmerit today to learn more.

Author: Greg Sowder Greg Sowder President, Qmerit Network