Off-grid BESS Microgrid

What Is an Off-grid BESS Microgrid

An off-grid BESS microgrid is an autonomous power system that does not rely on the public grid and independently completes the entire generation–storage–distribution–consumption process. It consists of distributed generation sources (PV, diesel generators, and wind power where necessary), battery energy storage, power conversion systems (PCS), distribution units and an energy management system (EMS), and externally behaves as an independent source that establishes voltage and frequency on its own.

Compared with the common "PV + off-grid inverter" approach, the engineering difference lies not in the inverter itself but in power balancing capability. In a pure PV off-grid solution, PV output is uncontrollable and the load varies at any time, with no buffering stage in between; balance can only be maintained by frequent diesel generator start/stop or direct load shedding, resulting in rapid generator wear, high fuel consumption and large voltage and frequency fluctuations. Once energy storage is introduced, the storage system simultaneously takes on three engineering roles:

• Power buffering — instantaneous power imbalances are absorbed or supplemented by the battery, so both the diesel generator and PV can operate under relatively stable conditions and no longer need to track the load with sharp adjustments;
• Voltage and frequency reference — the grid-forming PCS actively establishes the voltage amplitude and frequency reference, providing grid-connection conditions for grid-following equipment such as PV and diesel generators;
• Energy time shifting — surplus PV energy during the day is stored and released at night or on cloudy and rainy days, minimizing diesel generator operating hours.

Simply put, an off-grid microgrid solves not the question of "whether there is power" but whether power can be supplied stably, economically and continuously without a grid.

System Architecture and Key Equipment

A complete off-grid BESS microgrid can be divided into four layers by function, each with different engineering responsibilities.

1. Generation layer: PV as the main source, diesel as the backup

PV is the primary source in an off-grid system, with the lowest levelized cost of electricity, no fuel dependency and low maintenance; the diesel generator provides backup for extended rainy periods. In engineering practice, the goal is generally not "zero diesel" but rather to set generator start/stop thresholds based on SOC (state of charge) and load forecasting, so that the generator intervenes only during prolonged low-irradiance periods while also taking battery cycle life into account.

PV-side energy harvesting is handled by MPPT controllers. The VGID POWER SP200/150HAPV series PV MPPT controllers use an IP65 wall-mounted design, support 16 PV inputs (SP200)/12 inputs (SP150), and offer buck-boost conversion capability with a maximum conversion efficiency of 99.26%. The significance of multiple MPPTs lies in resistance to shading and string mismatch — when the array is partially shaded by buildings, vegetation or cloud shadows, each channel tracks its own maximum power point independently, preventing one mismatched channel from dragging down the output of the entire string.

2. Storage layer: capacity determines duration, power determines load

Battery capacity is calculated on the basis of the longest continuous period without sunlight and average daily electricity consumption, rather than simply estimating by "PV installed capacity × coefficient". On the power side, the maximum starting surge of the load must be verified — the starting current of motors, water pumps and compressors can reach 5 to 7 times the rated value, and if the PCS does not have sufficient margin, it will trip immediately at startup.

In a DC-coupled architecture, PV, batteries and the PCS share a DC bus, and DCDC modules are required for voltage matching and energy routing. The VGID POWER SP120/60HCPV and SP105HCDC series DCDC modules use an isolated air duct design, achieve a maximum efficiency of 99.3%, accept a wide DC voltage input range and multiple PV inputs, support up to 15 units in parallel, and offer battery heterogeneity (SOC balancing) capability — when expanding an existing battery bank or mixing cells from different batches, this capability can significantly reduce consistency risks.

3. Conversion layer: the power conversion system (PCS) is the core of the system

The PCS performs bidirectional conversion between DC and AC and establishes voltage and frequency in off-grid mode. Off-grid scenarios place the highest demands on the PCS's grid-forming capability, because there is no large grid in the system to provide a reference, and the PCS must take on the role of a synchronous generator.

VGID POWER offers a range of grid-forming models for different power levels and installation conditions:

• SP135HAPSS series microgrid energy storage PCS — IP65 high-protection wall-mounted unit with a built-in on/off-grid transfer device and ATS dual AC input module, supporting diesel generator hybrid operation and multiple units in parallel, with a maximum bypass current of 230A. Its ATS dual AC input design allows two power sources — utility grid (if available) + diesel generator — to be switched and interlocked within the same unit, simplifying the system's primary circuit.
• SP125HCPS series microgrid energy storage PCS — aimed at small and medium-sized energy storage microgrids, supporting DC/AC coupling, with an optional built-in STS model (transfer time <10ms), maximum efficiency 98.5%, 125% overload capability and independent three-phase grid-connection control, and supporting heterogeneous battery connection.
• SP60/50/40/30HCG2 series hybrid inverters — isolated air duct and wide DC voltage, high current design, compatible with 280/314Ah cells, built-in STS seamless transfer, supporting dual MPPT, diesel generator hybrid operation and multiple units in parallel, 150% overload capability, maximum efficiency 98.2%, suitable for small and medium-power distributed installation.

If the on/off-grid transfer on the AC bus needs to be a separate module, the optional STS200/400/600 series static transfer modules are available: transfer time <20ms, seamless on/off-grid transfer, integrated bidirectional detection on the grid side/load side, LocalEMS and multiple communication interfaces including RS485/CAN/4G, and compatibility with on/off-grid transfer standards of multiple countries.

4. Management layer: the EMS determines actual returns and battery life

The EMS is responsible for dispatch strategy: whether PV supplies the load first or charges the battery first, at what SOC the diesel generator starts, what depth of discharge to maintain, and how power is shared when multiple units operate in parallel. The strategy directly determines three core indicators: fuel consumption, battery cycle life and system availability. The EMS built into VGID POWER all-in-one products supports strategies such as peak shaving and valley filling, reverse power prevention and diesel-PV hybrid operation, and can connect to upper-level monitoring via communication interfaces.

Three Typical Configuration Forms

In engineering practice, off-grid microgrids fall into three forms by power level and degree of integration. When selecting, first determine the form, then the capacity.

Form 1: All-in-one — the first choice for small and medium-sized scenarios

The PCS, MPPT, EMS, on/off-grid transfer switch, and AC/DC distribution units are integrated into a single cabinet, and on site only four interfaces need to be connected: PV, battery, load and diesel generator. The advantages are less design work, a smaller footprint and faster commissioning.

VGID POWER's flagship model is the SP250HAG2T PV-storage on/off-grid all-in-one unit: it integrates the PCS, MPPT, EMS, PCM on/off-grid transfer switch, AC/DC distribution units, diesel generator control, surge protective devices and a built-in isolation transformer, and supports diesel generator, PV and battery connection. Its fast on/off-grid transfer time is 10ms, multi-channel DC-coupled charging efficiency is greater than 99.3%, it supports diesel generator hybrid operation and grid-forming operation, and can be expanded in capacity by paralleling multiple units. It is suitable for scenarios such as PV-storage-diesel microgrids and diesel-storage backup power for mines.

If the project has additional requirements for voltage level or electrical isolation, the SP250HAG2T series (built-in isolation transformer version) can be selected, with the isolation transformer built into the cabinet, eliminating the need for separate step-up/isolation equipment on site.

Form 2: Series all-in-one units — medium and large power scenarios

When the load reaches hundreds of kilowatts to the megawatt level, larger power units and a higher protection rating are required. The SP375/500HAG2 series PV-storage on/off-grid all-in-one units integrate the PCS, MPPT, EMS, PCM on/off-grid transfer switch and AC/DC distribution units, with IP54 protection, charging efficiency >99.3%, fast on/off-grid transfer <20ms, support for diesel generator/PV/battery connection, and a built-in EMS with peak shaving and valley filling, reverse power prevention and diesel-PV hybrid strategies, supporting multiple units in parallel.

Form 3: All-in-one cabinet — emergency power and construction sites

For scenarios requiring high-power centralized supply and rapid deployment, the SP750H54PSTD all-in-one cabinet provides 750kVA in a single cabinet, integrating the PCS, EMS, isolation transformer and AC/DC distribution units, with an optional external STS (transfer time ≤20ms), support for grid, diesel generator and battery connection plus a built-in PV connection switch, IP54 high protection, and support for paralleling multiple cabinets. It is suitable for scenarios such as emergency power, diesel-storage and construction sites.

Capacity Design: Calculate Energy First, Then Power

Capacity design for off-grid projects has two independent objectives that must be calculated separately.

Step 1: Determine battery capacity based on daily electricity consumption

Calculate the average daily electricity consumption (kWh) and the longest number of consecutive days without sunlight (days of autonomy). The usable battery capacity must satisfy:

• Usable capacity ≥ average daily electricity consumption × days of autonomy ÷ system efficiency
• Depth of discharge (DOD) is generally kept at 80%–90%; in lithium iron phosphate systems, full charge and full discharge should not be sustained over the long term
• System efficiency must account for PCS conversion losses, DCDC losses and line losses; a typical engineering value is 85%–92%

Step 2: Determine PCS power based on load characteristics

• Calculate the total rated load power and the maximum simultaneity factor to obtain the steady-state power demand;
• Separately verify the starting current of inductive loads (motors, water pumps, compressors, air conditioners), checking the largest single startup surge superimposed on the remaining steady-state loads;
• Reserve 20%–30% power margin for future load growth and power sharing margin when multiple units operate in parallel.

For example: a village in Africa consumes 240kWh per day on average, the required days of autonomy is 1 day, and system efficiency is taken as 0.9, so the usable capacity must be ≥ 240 × 1 ÷ 0.9 ≈ 267kWh; if daytime PV is sufficient to cover most of the load, the actual configuration can be moderately reduced in combination with the PV output curve. On the power side, if several water pump motors total 120kW with a maximum of 45kW for a single unit, and the check is based on 6 times the starting current with the simultaneity factor considered, the PCS selected must be above 150kW and have sufficient overload capability.

Typical Application Scenarios

Key Engineering Implementation Points

1. Load classification and supply assurance boundaries

An off-grid system must clearly define "which loads must be kept supplied". Loads are usually divided into three levels: Level 1 covers critical loads such as lighting, communications, medical care and security; Level 2 covers productive loads; Level 3 covers interruptible loads (such as some air conditioners and non-critical processing equipment). Capacity configuration is based on Level 1 + Level 2, and Level 3 loads are actively shed by the EMS when battery SOC is low, preventing a total site outage.

2. Grounding arrangement and protection configuration

An off-grid system has no grounding reference provided by the grid side, so the grounding arrangement must be determined at the design stage. A common practice is to solidly ground or impedance-ground the neutral point on the AC side of the PCS, and to provide independent residual current protection and insulation monitoring. Surge protection must cover both the PV DC side and the AC side; all-in-one products usually have built-in surge protective devices, but the down conductors and grounding resistance on site still need to be calculated separately.

3. Multi-unit paralleling and power sharing

When capacity exceeds the capability of a single unit, multiple units are paralleled. Three points require attention in engineering: whether the paralleling protocols of the models are consistent, whether the communication links are redundant, and whether the power sharing strategy is equal sharing or master-slave. Several VGID POWER models (SP250HAG2T, SP375/500HAG2, SP135HAPSS, SP125HCPS, SP60HCG2) support paralleling of multiple units; when expanding, models of the same series and power rating should be preferred to avoid circulating currents and dispatch complexity caused by mixing different models in parallel.

4. Environmental adaptability and protection rating

Outdoor deployment requires selecting the protection rating according to site conditions: IP65 is suitable for wall-mounted installation positions requiring rain and dust protection (SP135HAPSS, SP200/150HAPV); IP54 is suitable for all-in-one cabinets (SP375/500HAG2, SP250HAG2T). In areas with high temperature, high humidity and high salt spray, isolated air duct designs should also be considered (SP60HCG2 and SP120/60HCPV both use isolated air ducts) to slow the aging of electronic components.

5. Commissioning and acceptance

Commissioning in the following order is recommended: first verify voltage and frequency output with a single unit under a dummy load, then connect PV to verify MPPT tracking and power limiting logic, then connect the battery to verify charge/discharge switching and SOC calibration, and finally connect the diesel generator to verify hybrid start/stop and on/off-grid transfer. Acceptance checks should focus on measured transfer time, steady-state voltage deviation, frequency fluctuation range, harmonic distortion rate and diesel generator starting success rate.

Frequently Asked Questions

Q: Does an off-grid system always need a diesel generator?

Not necessarily, but it is recommended to keep one. If local sunlight conditions are stable and the days-of-autonomy requirement is not high, a pure PV-storage configuration can be feasible in some projects; once extended rainy periods occur, a pure PV-storage system can only respond by increasing battery capacity or shedding load, and the marginal cost of increasing battery capacity is often higher than that of installing a small diesel generator. The common engineering practice is "PV-storage as the main source, diesel as the auxiliary", with the generator capacity selected according to the steady-state power of Level 1 loads, without needing to be sized for the site's peak demand.

Q: Is a larger battery capacity always safer?

No. Oversized capacity raises the initial investment on the one hand, and on the other hand the battery operating in a low SOC range or under shallow cycling over the long term can actually worsen consistency differences within the battery bank. Capacity should be accurately calculated based on days of autonomy and daily electricity consumption, with a reasonable SOC operating window reserved.

Q: What is the point of on/off-grid transfer in an off-grid system?

Many so-called "off-grid" projects are actually "weak grid" or "intermittent supply" scenarios — utility power is available during the day and lost at night, or there are fixed outages of several hours each week. Such projects must use on/off-grid models, with the PCS performing bidirectional transfer; pure off-grid models cannot feed back into the grid, nor can they automatically switch back when the grid is restored.

Q: Will PV output fluctuations cause voltage and frequency instability?

This is precisely the core value of the energy storage system. The grid-forming PCS establishes the voltage and frequency reference, the battery provides instantaneous power support, and PV fluctuations are absorbed by the storage, so the load side does not feel them. If grid-following equipment is used without storage, PV fluctuations will be transmitted directly to the load.

Q: Can the equipment be maintained remotely?

Yes. VGID POWER products generally support RS485/CAN/4G communication interfaces and can upload operating data to a monitoring platform, enabling remote viewing of SOC, charge/discharge power, alarm status and fault records. For projects with inconvenient access such as islands and mining areas, remote O&M capability is key to reducing lifecycle costs.

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For islands, villages, mining areas, oilfields, farms or emergency power supply scenarios, VGID POWER can provide a complete off-grid BESS microgrid solution covering capacity calculation, solution design, equipment selection and commissioning handover. You are welcome to contact us for solution design and a quotation.