C&I, BESS and Mini-Grids
What is a BESS?
BESS Key Components
Integration with Solar PV and the Grid
A BESS in a C&I solar installation is usually integrated either in AC-coupled or DC-coupled configurations, and it interfaces with both the onsite PV system and the utility grid:
AC-Coupled Integration
In an AC-coupled setup, the solar PV system and the BESS each have their own power conversion devices and are connected on the AC side of the electrical system (e.g. both feed into the facility’s AC distribution panel or minigrid).
The PV array feeds a solar inverter to produce AC power, which can supply loads or charge the battery via the battery’s inverter (essentially AC is converted back to DC for storage).
AC coupling is often used when adding storage to an existing PV system because it is simpler to retrofit. One drawback is that charging the battery from PV involves two conversion steps (DC to AC to DC), incurring some efficiency loss.
DC-Coupled Integration
In a DC-coupled system, the PV array and the battery are tied together on the DC side, typically sharing a common DC bus and using a single hybrid inverter or a DC/DC converter.
This setup is efficient for new PV+storage systems because it avoids extra conversion steps – the PV can charge the battery without going through an AC stage.
A DC-coupled BESS can also be designed to allow charging from the grid (if a bi-directional hybrid inverter is used).
Grid Connection and Operation
Charge/Discharge Cycles & Operating Modes
A BESS goes through continuous charge and discharge cycles as it operates, and these cycles are tailored to the facility’s needs. Modern lithium-ion batteries can endure thousands of cycles before their capacity significantly degrades, so a well-managed daily cycling regimen can last for a decade or more.
Peak Shaving(Demand Charge Management – C&I):
In this mode, the battery discharges during periods of highest power demand to reduce the peak load drawn from the grid.
By supplying stored energy at peak times, the BESS prevents the facility’s grid demand from spiking, thus lowering demand charges on the utility bill. The battery is then recharged during off-peak times or when there is excess solar generation.
Peak shaving with a BESS allows users to “store power during off-peak periods and discharge during peak times to reduce electricity costs”.
This is especially valuable for C&I customers, as demand charges can comprise a significant portion of their energy costs.
Load Shifting(Energy Time-Shifting or Arbitrage-C&I):
Load shifting involves storing energy when it’s abundant or cheaper (for example, midday when solar PV output is high, or at night when off-peak grid rates apply) and using it later when energy is scarce or expensive.
In a solar C&I scenario, this often means capturing surplus solar production around noon that would otherwise be curtailed or exported at low value, and discharging that energy in the late afternoon/evening to supply the facility’s loads.
It can also mean charging from the grid during low-tariff periods and discharging during high-tariff periods. This “energy arbitrage” smooths out the supply-demand curve and minimizes energy purchase when prices are high.
The BESS thereby helps the facility avoid buying expensive peak-hour electricity by using cheaper stored energy.
Backup Power(Energy Resilience and UPS – C&I):
A BESS can operate as a backup power source in case of grid outages or emergencies. In this mode, the battery (often in conjunction with on-site solar and possibly a generator) will kick in to supply power to critical loads whenever the grid fails. The transition can be fast enough to prevent downtime (similar to an uninterruptible power supply).
Solar Minigrids:
In the case of off-grid facilities (such as rural villages or lodges), the BESS and PV together can form a self-sustaining minigrid, powering the facility 24/7.
The solar panels keep recharging the batteries during daylight, while the batteries supply power at night. In the context of off-grid facilities the term “minigrid” refers to the BESS plus the local electrical reticulation network.