Short definition
An off-grid system supplies electrical loads without a connection to the public power grid. It forms an independent electrical network, also referred to as an island grid or stand-alone power system. Possible components include PV, a battery energy storage system (BESS), grid-forming inverters, generators and local loads.
Off-grid systems at a glance
- At least one component must form the island grid and stabilize its voltage and frequency.
- Available generation must be continuously adjusted to consumption, storage capacity and system limits.
- PV power must be actively curtailed or disconnected if it cannot be absorbed.
- A local EMS can provide supervisory operation, while fast grid regulation and protection functions remain within the responsible equipment.
How does an off-grid system work?
In a public power grid, voltage and frequency are defined by the interconnected grid. In an off-grid system, this grid-forming function is performed locally, for example by a suitable BESS inverter or generator. The grid-forming component establishes the AC voltage and responds immediately to short-term changes in generation and consumption.
Conventional grid-following PV inverters require an existing, stable grid signal and therefore cannot normally establish an island grid on their own. They also do not necessarily adjust PV generation automatically to the absorption capacity of the island grid. PV power must be limited whenever the loads and BESS cannot fully absorb the available generation. Depending on the system architecture, this can be achieved through external active-power setpoints, compatible frequency-dependent curtailment or the disconnection of individual generation units.
If a BESS is installed, it can compensate for short-term power imbalances. It charges when surplus PV power is available and supplies the loads when generation is insufficient. Key operating parameters include the state of charge, available charging and discharging power, usable capacity and any energy reserve that must be maintained.
During an extended energy deficit, a generator can be started. Its start-up, connection and assumption of load must be coordinated. Depending on the operating strategy, synchronization with the existing island grid, minimum run times, minimum loading requirements and possible charging of the BESS must also be considered.
A local EMS provides supervisory energy and operational coordination. It processes measurements and operating states and determines parameters such as state-of-charge limits, generator start requests, PV power limits and load priorities. Fast voltage and frequency regulation and the management of transient events remain the responsibility of grid-forming inverters, generator controllers and protection devices.
Where are off-grid systems used?
Typical applications include:
- Remote buildings, agricultural operations, telecommunications sites and technical facilities where a grid connection is unavailable or economically impractical.
- Commercial and industrial sites in regions with insufficient or unavailable grid infrastructure.
- Mobile or temporary power systems for construction sites, events and research stations.
- Small stand-alone systems for individual loads such as pumps, lighting, measurement equipment and communications technology.
Benefits, limitations and technical requirements
An off-grid system enables an electrical supply that is independent of the availability of a public power grid. Combining PV and BESS can reduce generator fuel consumption and increase the share of locally generated renewable energy. The actual benefits depend on the load profile, generation potential, system configuration and operating strategy.
System sizing must account not only for average energy demand but also for inrush currents, short-term load peaks, overnight consumption, conversion losses and extended periods of poor weather. During energy shortages, load management can limit or disconnect non-critical loads so that critical loads remain supplied.
If autonomous recovery after a complete loss of voltage is required, at least one component must be capable of a black start. It must be able to establish the island grid without an existing external voltage. Additional generators and loads are then connected in a defined sequence.
The electrical protection concept must also be appropriate for the system architecture. Particularly in inverter-dominated island grids, the magnitude and time profile of fault currents can differ substantially from those in conventional power systems. Protection coordination, selectivity, grounding and disconnection conditions must therefore be designed for the specific project.
A grid-connected system with backup power capability is not a pure off-grid system. However, it can disconnect from the public grid during an outage and temporarily establish a local island grid. Grid disconnection, switching, synchronization, reconnection and protection systems must be suitable and coordinated for this purpose.
Implementation with EcoPhi
EcoPhi can consolidate measurements and operating states from PV, BESS, generators, energy meters and controllable loads and use them for supervisory operational coordination. Depending on the project, functions such as state-of-charge management, generator start requests, PV curtailment, load prioritization, alarm management and local data storage can be implemented.
The specific functionality depends on the EcoPhi hardware used, the available device interfaces and the control capabilities of the connected components. Operationally relevant functions can be executed locally if the respective system configuration is designed accordingly. EcoPhi does not provide grid formation, fast voltage and frequency regulation or electrical protection and switching functions. These tasks remain with the designated inverters, generator controllers, switchgear and protection systems.
Short summary
An off-grid system is an independent electrical supply system. Safe operation requires suitable grid-forming equipment, controlled adjustment of generation, sufficient power and energy reserves and an appropriate protection concept. An EMS can coordinate the connected components at a supervisory level.
Frequently asked questions about off-grid systems
Does every off-grid system require a BESS?
No. An off-grid system can, for example, be supplied exclusively by a generator. In PV-based systems, however, a BESS often performs important functions related to power balancing, time-shifting of energy and grid formation.
Can a conventional PV inverter establish an island grid?
A conventional grid-following PV inverter cannot normally establish an island grid on its own. It requires a grid-forming component and a suitable control mechanism that reduces or disconnects its output when the island grid has limited absorption capacity.
What does black-start capability mean?
Black-start capability is the ability to establish an electrical network from a completely de-energized state. It is required if the off-grid system must restart after a complete shutdown without support from an external grid.
What role does an EMS perform in an off-grid system?
An EMS typically provides supervisory operation, including state-of-charge management, generator start requests and the coordination of flexible loads. Fast voltage and frequency stabilization and electrical protection functions are performed by purpose-designed equipment.