Key Differences Between String Inverters, Energy Storage PCS and Grid-Tied Inverters, and a Guide to Selecting the Right Solution for Different Scenarios

Created on：2026-08-17

Foreword

“When working on photovoltaic energy storage projects, many engineers tend to confuse string inverters, energy storage PCS units and grid-tie inverters. Whilst all three perform AC-DC conversion, their original design purposes differ. Selecting the wrong equipment can result in the energy storage system failing to charge, a lack of backup power during outages, and reduced power generation efficiency, thereby wasting the investment.”

 

Many people are unsure when to use string inverters on their own, when to combine string inverters with a PCS, and when to install a grid-tied inverter directly. Today, drawing on real-world project scenarios, we will explain the differences between the three and the conditions in which they are best suited.

I. 📊 What exactly are the differences between these three types of equipment?

1. String inverters (pure PV grid-connected inverters)

It is specifically designed to convert electricity generated by photovoltaic panels into alternating current. The DC side is connected to the photovoltaic modules, with multiple MPPT channels tracking the maximum power output of the photovoltaic system; it performs only a unidirectional conversion from DC to AC.


It does not have a battery charging circuit and cannot charge energy storage batteries. In the event of a mains power failure, the islanding protection is triggered, causing the system to shut down immediately; it is therefore unable to supply power to the external grid. It is suitable for projects that rely solely on photovoltaic power generation and do not require energy storage or a backup power supply in the event of a power cut.

2. Energy Storage PCS (Power Conversion System)

A PCS is a bidirectional device designed specifically for energy storage batteries. The DC side is connected solely to the energy storage batteries; it does not feature an MPPT and cannot be connected directly to photovoltaic panels. When connected to the grid, it can convert the battery’s DC power into AC power for use by loads; it can also rectify AC power from the grid to charge the batteries. In the event of a power cut, it can form a microgrid to provide off-grid power supply. When retrofitting energy storage to existing photovoltaic systems, the most common solution is the ‘existing string inverter + standalone PCS’ AC-coupled configuration, in which the two sets of equipment operate independently.

3. Grid-tied inverters (hybrid solar-storage systems)

This is an integrated unit that combines the MPPT function of a photovoltaic inverter with the bidirectional energy storage capability of a PCS within a single device. It features two DC interfaces: one connects to photovoltaic panels for power generation, whilst the other connects to energy storage batteries for charging and discharging. It incorporates built-in energy management logic to centrally coordinate the photovoltaic system, batteries, loads and the grid, and supports seamless switching between grid-connected and off-grid modes. Excess solar power can be fed directly to the batteries in DC form to minimise conversion losses, and the system also supports charging the batteries during off-peak grid hours.
✅A brief summary:
String inverter: Handles only solar power generation; does not manage energy storage;
Energy storage PCS: Handles only battery charging and discharging; does not manage solar power generation;
Grid-tied inverter: A single unit that simultaneously handles solar power generation, energy storage charging and discharging, and emergency backup power.

II. 🏗️ Real-world projects: where are the three approaches used respectively?

Scenario 1: Using only string inverters, without energy storage

Suitable for projects intended solely for photovoltaic power generation, with no energy storage requirements and no need for a backup power supply in the event of a power cut. Typical examples include standard residential rooftops, retail premises, and the rooftops of small and medium-sized commercial and industrial buildings. Where the roof is shaded or the modules are not uniformly oriented, multiple MPPT channels can help minimise power generation losses. If there are plans to install energy storage at a later date, the necessary electrical infrastructure can be pre-installed; however, standard string inverters cannot be connected directly to batteries.

Scenario 2: String inverter + standalone energy storage PCS solution

Circumstances in which this solution should be prioritised:
  • A solar power station has already been constructed, and the client wishes to retrofit energy storage without dismantling the existing solar equipment. The existing inverter will continue to generate electricity, whilst a new PCS and batteries will be added, connected via AC coupling, requiring minimal modifications.
  • Medium- to large-scale commercial and industrial microgrids, as well as those in industrial parks, require high-power energy storage for peak-valley arbitrage, demand response and participation in grid dispatch.
  • It is hoped that the PV and energy storage hardware will be independent of one another; should one system experience a fault, the other can continue to operate normally, facilitating easier operation, maintenance and troubleshooting.
Advantages: Power output can reach the megawatt range, and capacity expansion is flexible; Disadvantages: AC coupling; the electrical energy undergoes two conversions, resulting in a small amount of loss.
 

Figure 1: Comparison of string inverters versus string inverters combined with energy storage PCS

Left-hand module (string inverter)

Current flow: PV modules → multi-channel MPPT → string inverter → AC grid;

Note: Unidirectional device; generates only solar power; no battery interface; shuts down immediately in the event of a grid power failure; no backup power supply.

 

Right-hand module (string inverter + energy storage PCS, AC-coupled)

Current flow: PV modules → multi-channel MPPT → string inverter → load/grid; PV modules → multi-channel MPPT → string inverter → energy storage PCS → energy storage battery; energy storage battery → energy storage PCS → load/grid;

Note: The energy storage PCS is a bidirectional device that connects only to the battery; it has no MPPT and cannot be connected directly to PV modules. It supports both grid-connected and off-grid operation, and is capable of both charging and discharging.

 

String inverters and PCS units consist of two independent AC power sources connected in parallel; the photovoltaic system and the energy storage system operate independently of one another, meaning that a fault in one does not affect the other. The photovoltaic power undergoes a single conversion via the inverter, whilst the battery charging and discharging processes undergo a secondary conversion via the PCS, resulting in a higher number of energy conversions.

Scenario 3: Selection of a hybrid grid-connected inverter

Suitable for new projects where solar and energy storage are planned simultaneously: residential solar-storage systems, small retail premises, and micro, small and medium-sized enterprises. The aim is to simplify system cabling and reduce the number of cabinets. There are three key requirements: to increase self-consumption of solar-generated electricity, to take advantage of peak-off-peak tariff arbitrage, and to provide emergency power to critical loads in the event of a power cut.


Limitations: Grid-tied inverters are better suited to small and medium power ratings; should the unit fail, both PV generation and energy storage will cease simultaneously; megawatt-scale projects generally do not utilise grid-tied inverters, continuing to employ a split-system solution comprising string inverters and a power conversion system (PCS).

 
Figure 2: Schematic diagram illustrating the operating principle of a hybrid grid inverter solution

Integrated Hybrid Grid-Connected Inverter Architecture (DC-Coupled)

Current flow: PV modules → hybrid grid-connected inverter → load/grid; PV modules → hybrid grid-connected inverter → energy storage battery; energy storage battery → hybrid grid-connected inverter (energy storage port) → load/grid;

Notes: Surplus PV energy is stored directly in the battery via the DC circuit; fewer energy conversion stages; in the event of a system-wide failure, both the PV system and energy storage system will fail simultaneously.

III. 👷 Common pitfalls to avoid during on-site construction

1. Under no circumstances should you connect a standard string inverter directly to a battery, as it lacks a charging circuit and is therefore unable to store energy; nor should you connect a PCS directly to solar panels, as the absence of MPPT results in poor power generation efficiency and poses safety risks.
2. When retrofitting existing PV systems with energy storage, do not blindly remove perfectly functional string inverters to replace them with grid-tied inverters; in most scenarios, adding a PCS is a more cost-effective solution.
3. If there is no need for energy storage or backup power during power cuts, do not force the installation of grid-tied inverters, as the equipment costs are higher and this would result in a waste of resources.

IV. 📖 Summary of Model Selection

For power generation only, with no need for energy storage: choose a string inverter;

 

For existing PV systems where energy storage is to be added at a later stage, in medium- to high-power commercial and industrial applications: choose a split-system solution comprising a string inverter and a storage PCS;

 

For new small-scale solar-plus-storage projects requiring power generation, arbitrage and emergency backup power: choose a grid-tied inverter;

 

Before undertaking a project, clarify the following: whether it is a new build or a retrofit of an existing system, whether emergency backup power is required, and the project’s power capacity. Only by matching the equipment accordingly can costs be controlled and the long-term, stable operation of the power station be ensured.

 

 

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