What Is an Energy Storage System and How Does It Work?

An energy storage system (ESS) shifts electricity use in time: it receives energy from a source, stores it and supplies it later. This article focuses on battery energy storage systems (BESS), where batteries hold the energy and the converter and controls coordinate charging and discharge. For a business, this may mean using daytime solar generation in the evening, reducing peak off-take from the grid, or providing backup for specified loads. However, these functions require different settings and sometimes additional hardware. This article explains how a system works, its components and the questions to answer before choosing a model. It helps business owners and technical teams prepare a clear brief for selecting a system.

What the system stores and where the energy comes from

The storage system does not create energy. First, it needs to be charged from an available source: power grid, solar power plant, or other compatible equipment. During battery charging, electrical energy is transformed into chemical energy, and during discharge, the reverse transformation occurs. Some energy is lost in the process, so the energy returned is less than the energy used to charge the system.

A solar power plant generates electricity when sunlight conditions are suitable. A fuel generator converts fuel energy into electrical energy. The battery shifts the use of energy already generated to a later time. It is this difference that explains why a large reserve in the battery does not replace a plan to replenish it during long outages.

The abbreviation BESS means battery energy storage system. It is one type of energy storage. This article is specifically about battery solutions for enterprises, not all possible storage technologies. A comparison of on-grid, off-grid and hybrid solar systems will help you determine which configuration the storage system will work with.

Why a battery is not a complete storage system

The battery stores energy, but controlled operation at a facility also requires other components. These can be housed in one cabinet or installed separately. Compare proposals by the full set of components and functions, not just by appearance or the number of battery modules.

Not every solar inverter can work as a bidirectional converter for the battery. Also, the presence of compatible components alone does not confirm the capabilities of the entire assembled system. Agreed electrical parameters, interoperability algorithms and configuration-specific documentation are required.

Cooling, fire protection and service access are also part of the project. Their requirements depend on the equipment and its location and cannot be determined from a universal checklist for every cabinet.

On the Sungrow equipment page, you can explore the product ranges in the catalog. Features, backup capability and compatibility must be checked for each model. The presence of a brand in the catalog does not mean that any of its products is suitable for your task.

How charging and discharge work

The operating cycle can be divided into three stages. First, the available source and permitted charging mode are established. The system then maintains the charge within its operating limits. When the agreed demand arises, the system supplies energy through the converter.

For example, a business can charge a battery with some surplus solar generation and use that stored energy on site in the evening. If there is no surplus, installing a storage system will not create one. The daily consumption and generation profiles therefore matter more than assuming that solar panels can charge everything.

The minimum charge reserved for backup is agreed separately. The energy that needs to be left in case of an outage cannot be fully spent on another task at the same time. These priorities must be clear both to the owner and to the person operating the installation.

The schedule must be considered for different working days. A low-consumption weekend, a full production shift and a seasonal peak can all produce different charging conditions. A useful discussion question is: What will the system do if the expected surplus is not present today? The answer helps separate the desired scenario from the one supported by the available sources and settings.

Power, capacity and operating time

Power in kW describes what load the system is able to support in a certain mode. Energy capacity in kWh shows the energy reserve. These values answer different questions: "how much at the same time" and "how long". A large capacity does not compensate for insufficient power of the converter.

For a preliminary understanding, the duration can be estimated as the available energy divided by the load power. It is important to immediately determine where the available energy is measured: inside the battery or already at the output of the system after accounting for losses.

Illustrative example. Suppose 160 kWh is available at the output for consumers after accounting for the relevant limits, losses and auxiliary consumption. At a constant load of 40 kW, the estimate is 160 ÷ 40 = 4 hours. For 80 kW it is 2 hours, provided the system supports that output. This is an illustrative calculation, not a model specification or forecast for a particular facility.

In a real calculation, the load varies and equipment may draw different power at startup than during normal operation. Initial state of charge, usable operating range, temperature and capacity retention over time also need to be considered. Do not automatically assume that the nameplate capacity equals the energy available to the business’s equipment.

What a business can use storage for

Solar self-consumption. Part of the generation is stored for another time. It is necessary to check when there is a surplus and whether there is a corresponding demand later. It is not enough to evaluate only the annual output: the coincidence of events during the day is important.

Peak load management. The battery can partially cover short periods of increased consumption. The result depends on the power, the available charge and the duration of the peak. If the peak period is longer than the reserve, the storage will not maintain the selected level indefinitely.

Power backup. The purpose is to support defined processes during the failure of the external network. The switching scheme, compatibility of loads and the possibility of autonomous operation are decisive here. The loads that require backup must be specified before the equipment is ordered.

Shifting electricity purchases to another time. Charging and use of the reserve can take place in periods with different electricity prices. But the price difference alone does not equal profit: losses, costs and terms of contracts should be taken into account. This is a separate economic evaluation, not an automatic result of battery installation.

Why an ESS alone does not guarantee operation without the grid

Operating in parallel with the grid and supplying a local network when the grid fails are different functions. For backup operation, it is necessary to confirm the appropriate mode of the converter, the safe separation of the facility from the external network and the coordinated operation of protections. A charged battery without such a configuration may not provide the required power.

In the technical task, it is worth writing down the permitted interruption for each group of loads. Lighting, server equipment and process line may have different requirements. The wording "backup for the entire business" does not explain which processes should continue to work and under what conditions.

If the facility already has a generator or a solar installation, the interaction of the sources is checked separately. You need to know who maintains the local network parameters, when charging is allowed, what happens when the supply is depleted, and how the system returns to normal.

To combine solar generation, battery and grid, consider a hybrid system for a business. Specify the required backup functions in the equipment configuration and project design, rather than relying on the name of the solution.

What to prepare before choosing equipment

Start by describing the desired result. The phrase "requires 200 kWh" already makes an assumption about the size of the battery, but does not explain the problem. It is more useful to determine the load, the required working time, the source of charge and the permissible interruptions. After that, you can reasonably compare the configurations.

1. List of processes. What must work and what can be temporarily disabled? Specify the persons responsible for each group of equipment.
2. Consumption data. Prepare available hourly or more detailed measurements, shift schedules and seasonal information. One monthly sum does not show short peaks.
3. Available sources. Record inverter models, generator parameters, input circuit and known connection limitations. Add documentation if available.
4. Site conditions. Show the site, access for delivery and service, distances to other objects. Final requirements are determined by specific equipment.
5. Operating priorities. Decide what matters most: backup, using solar surplus or managing the operating schedule. Agree when one task must take priority over another.

If the storage is planned together with a new solar power plant, the technical task must be coordinated with the solar power plant design. The scope of storage-related work is determined for each project.

When comparing offers, ask to see equipment, prep work, connections, setup and inspections separately. Clarify who is responsible for each part. This will help to compare equivalent scopes of work, rather than a cheaper cabinet with a fully prepared solution.

Agree the acceptance criteria when placing the order. For your task, it can be a test of the switch to backup, operation of specified loads, restoration of grid power and the display of operating states in the monitoring system. Ask in advance to describe the scenarios, test conditions, and how the results will be recorded. Then it is clear what exactly confirms the readiness of the system, and which functions remain outside the agreed scope.

ESS needs a clear operating procedure. Ask who receives error messages, who has the right to change modes and what actions are provided in case of insufficient charge. Save the instructions, settings, and contact details of those responsible with the facility's documentation. These are practical issues of putting the system into operation, which should be discussed along with the choice of capacity and price.

If a proposal includes expected savings, ask to see the underlying load profile, tariff assumptions and energy available for that scenario. Do not compare this estimate with the backup mode, where part of the charge is constantly left untouched.

Once the task, required power and usable energy are defined, use the Sungrow PowerStack 255CS-2H page to check one configuration’s components and parameters. Compare them with the loads, charging source and required operating modes. Separately establish which control, protection and switching components are included in the proposal for your facility. Selecting a model does not replace checking the complete configuration.