Industrial Energy Storage: Cost Difference Between High & Low Voltage and Crucial Grid-Tie Considerations.

Is choosing the appropriate grid connection method crucial in the construction and operation of photovoltaic (PV) power plants?

Last year, the State Grid Hubei Electric Power Company issued a notice regarding the "Detailed Rules for Distributed PV Grid Connection Services (Trial Implementation)," which significantly impacted PV companies in Hubei. A key point was that previously, only 2-6MW PV projects required 10kV high-voltage grid connection, but the new document explicitly extended this range to 400kW. This means that a large number of distributed PV projects in Hubei with capacities between 400kW and 2MW will have to switch from 380V low-voltage grid connection to 10kV high-voltage grid connection.

The most direct impact of this change is a significant increase in the grid connection cost for 400kW-2MW distributed PV projects. Compared to low-voltage grid connection, PV projects using high-voltage grid connection will face higher equipment costs and handling fees.

Similarly, in the industrial and commercial energy storage sector, there are high- and low-voltage grid connections, and choosing the appropriate grid connection method is also crucial.

What is high- and low-voltage grid connection?

The choice of grid connection method for energy storage systems must accurately match the enterprise's power demand and distribution architecture. The voltage level decision for connecting commercial and industrial energy storage to the grid is essentially a systematic engineering project based on user load characteristics, distribution capacity, and economic considerations.

Generally, high-voltage grid connection refers to the energy storage system using a transformer to step up the output voltage to a high-voltage level, typically 10kV or higher, before connecting to the high-voltage grid. This method is suitable for large commercial and industrial users or industrial parks, meeting the demand for large-capacity energy storage.

Low-voltage grid connection refers to the energy storage system directly connecting to the low-voltage grid at a low voltage level, typically 400V or below. This method is suitable for small and medium-sized commercial and industrial users.

From a technical perspective, high-voltage grid-connected systems mainly consist of energy storage batteries, inverters, transformers, and high-voltage switchgear. The energy storage batteries convert DC to AC power through an inverter, then the transformer steps up the voltage to a high-voltage level, and finally the high-voltage switchgear connects the system to the high-voltage grid. High-voltage grid-connected systems require a high level of technology and strict safety standards to ensure stable system operation and grid safety.

Low-voltage grid-connected systems mainly consist of energy storage batteries, inverters, and low-voltage switchgear. Energy storage batteries convert direct current (DC) to alternating current (AC) via an inverter, and then connect directly to the low-voltage grid through a low-voltage switchgear. Low-voltage grid-connected systems have a relatively simple structure and are easier to install and maintain, but they exert a greater impact on the grid, requiring corresponding protection measures.

From an application perspective, the two also differ. High-voltage grid connection is suitable for large industrial and commercial users or industrial parks, such as large manufacturing enterprises, data centers, and large commercial complexes. These users typically have high electricity consumption and high requirements for the stability and reliability of the power system. High-voltage grid connection can meet their large-capacity energy storage needs and provide stable power support.

Low-voltage grid connection is suitable for small and medium-sized industrial and commercial users, such as small manufacturing enterprises, small and medium-sized commercial complexes, and office buildings. These users have relatively lower electricity consumption and relatively lower requirements for the power system. Low-voltage grid connection can meet their basic energy storage needs and has the advantages of simple installation and lower cost.

Where does the cost increase with high-voltage grid connection?

In the industrial and commercial photovoltaic sector, compared to low-voltage grid connection, using high-voltage grid connection will result in higher equipment costs and handling fees.

According to Wang Shujuan, founder of "Smart Photovoltaics," taking a 5.57MW industrial and commercial distributed photovoltaic project in 2023 as an example, the total investment was 25.19 million yuan, with a capacity ratio of 1.225:1. The total DC-side capacity was 6.825MWp, and the AC-side capacity was 5.57MW.

Based on the project's cost structure, the cost of the booster station and control system was approximately 3.89 million yuan, equivalent to 0.6 yuan/W.

This equates to an additional cost of several hundred thousand yuan for a 1MW photovoltaic project, which will significantly impact investment in local distributed photovoltaic projects.

This inevitably leads some to wonder if there's a similar cost difference between high- and low-voltage grid connection for industrial and commercial energy storage.

Indeed, the difference is negligible. For industrial and commercial energy storage, the economics of grid connection methods require careful calculation. The core differences lie in three cost segments: Equipment: High-voltage access requires step-up transformers, high-voltage switchgear (each unit costs over 50,000 yuan), dedicated metering devices, and relay protection systems, increasing equipment investment by 40%-60% compared to low-voltage solutions; Construction: High-voltage access often necessitates the construction of new substations or expansion of switchyards, involving land costs, civil engineering, and high-voltage cable laying (costing over 200,000 yuan per kilometer), with increased construction complexity leading to longer construction periods; Operation and maintenance: High-voltage systems require certified professionals, with labor costs 30%-50% higher than low-voltage teams.

Choosing the grid connection voltage is not simply a matter of "choosing the lower one," but requires a comprehensive assessment of the project's boundary conditions. For large-scale systems with daily charging and discharging exceeding 800kWh, high-voltage grid connection can reduce unit costs through economies of scale. However, for small and medium-sized projects using low-voltage grid connection, cable path design needs to be optimized to avoid power loss caused by long-distance low-voltage transmission. The final decision should balance initial investment, operation and maintenance difficulty, and life-cycle benefits to maximize cost-effectiveness.

Meanwhile, national and local standards have clear requirements for the voltage level of energy storage systems connected to the grid. For example, GB/T 36547-2018 recommends 0.4kV connection for energy storage systems of 1000kW and below; the Zhejiang Province User-Side Electrochemical Energy Storage Technology Guidelines-2023 stipulates that 10kV connection should not exceed 6000kW. Furthermore, other regions have different standards and requirements for connection voltage, so local standards and power company regulations must be considered in specific projects.

In recent years, the large-scale development of distributed photovoltaic power has given rise to derivative problems such as insufficient grid capacity and lagging operation and maintenance management, opening a growth window for the industrial and commercial energy storage market. Industry data from 2024 shows that over 1,600 new industrial and commercial user-side energy storage grid-connected projects were added nationwide, totaling 3.9GW/8.8GWh, accounting for 8% of the annual incremental energy storage market (by GWh). The sheer number of projects made it one of the most active sub-sectors of energy storage that year. Over a thousand developers entered the market, resulting in explosive growth in both grid-connected and under-construction projects.

However, the decision-making process for energy storage system voltage access levels requires multi-dimensional considerations: the user's incoming line voltage level determines the grid connection benchmark; system capacity must match the grid's absorption capacity; cost calculations must cover equipment procurement, civil construction, and long-term operation and maintenance expenses; and strict adherence to local grid access regulations is essential. In practical applications, customized design based on project site conditions and grid topology is necessary. Only through refined selection can safe and efficient operation be achieved, fully leveraging the energy storage system's optimization effect on electricity costs and driving the transformation of enterprise energy management towards intelligence and low-carbon practices.