Quick Facts
What Balance of System is
Balance of System (BOS) is the comprehensive term for all components of a solar plant other than the modules and inverter. The “balance” terminology comes from accounting language: the modules and inverters are the primary equipment; BOS is everything else needed to “balance” the system into a functional plant.
BOS includes:
Mounting structures: Aluminium or steel structures supporting modules.
DC cables: From modules to combiner boxes to inverters.
AC cables: From inverters to grid connection.
String Combiner Boxes (SCBs): Combining string outputs.
DC Distribution Boxes (DCDBs): Isolation and protection on DC side.
AC Distribution Boxes (ACDBs): Isolation and protection on AC side.
Transformers (for HT systems): Step-up transformers from inverter LV to HV grid.
Switchgear: Circuit breakers, isolators, contactors.
Earthing systems: Earth pits, conductors, equipotential bonding.
Lightning protection: Air terminals, down conductors, SPDs.
Surge protection devices (SPDs): For DC and AC sides.
Junction boxes: At module and other connection points.
Monitoring equipment: SCADA, sensors, met station.
Civil works: Foundations for ground-mount, walkways, fencing.
Security systems: CCTV, intrusion detection (for utility-scale).
Auxiliary equipment: UPS, control room, instrumentation.
For solar plant costing, BOS is the second-largest category after modules. The quality and design of BOS components directly affect plant reliability, safety, and lifecycle costs.
BOS in cost structure
For Indian solar plants, typical cost breakdown:
Utility-scale ground-mount (per MW):
Modules: 50% to 55% of total CAPEX.
Inverters: 7% to 10%.
BOS: 25% to 35%.
EPC services: 5% to 10%.
Total: about Rs 4 to 5 crore per MW.
Rooftop solar (per Wp):
Modules: 40% to 45%.
Inverter: 10% to 15%.
BOS: 25% to 30%.
Installation: 15% to 20%.
Total: Rs 45 to 65 per Wp depending on system size.
The BOS share varies by application. Utility-scale has more BOS components (transformers, substations, larger structures). Rooftop has simpler BOS but still significant share.
Major BOS components
Mounting structures:
For ground-mount: Pile foundations, structural members, modules clamps.
For rooftop: Roof-mounted structures, ballast, fastening systems.
Material: Galvanised steel, aluminium, or hybrid.
Cost: Rs 2 to 8 per Wp depending on application.
DC cables:
Solar-rated cables (H1Z2Z2-K typical) with UV resistance.
4 sq mm to 16 sq mm conductors typical.
Lengths from string to combiner to DCDB to inverter.
Cost: Rs 2 to 5 per Wp.
AC cables:
From inverter to ACDB to grid connection.
Sized per current and distance.
Cost: Rs 1 to 3 per Wp.
Transformers (HT only):
Step-up transformer from inverter LV (typically 415 V) to grid HV (11 kV, 33 kV).
Sized per plant capacity.
Cost: Rs 8 to 15 lakh per MVA.
Switchgear:
Circuit breakers, isolators, contactors.
DC and AC ratings appropriate.
Cost: Rs 1 to 3 lakh per inverter.
Earthing system:
Earth pits with conductors.
Equipotential bonding network.
Designed per IS 3043.
Cost: Rs 50,000 to 2 lakh per MW depending on soil conditions.
Lightning protection:
Air terminals, down conductors, SPDs.
Per IEC 62305 risk assessment.
Cost: Rs 1 to 3 lakh per MW.
Monitoring and SCADA:
Sensors, meters, computers, communication.
Real-time data logging and reporting.
Cost: 0.5% to 2% of total CAPEX.
BOS quality and reliability
BOS quality directly affects plant reliability:
Premium BOS components from established manufacturers extend life.
Quality cables (proper insulation, correct sizing) reduce failures.
Robust mounting structures handle wind, rain, lightning, hail.
Premium switchgear has higher operational reliability.
Quality earthing prevents safety issues.
Reliable monitoring catches issues early.
For utility-scale projects, BOS quality is part of lender’s diligence. Budget BOS often correlates with higher long-term costs through failures and replacements.
BOS in residential vs utility
Residential rooftop BOS:
Smaller scale and simpler.
Typically pre-engineered systems.
Less customisation.
Lower per-Wp cost.
Utility-scale BOS:
Designed for specific site conditions.
Complex with transformers, substations.
Highly customised.
Higher per-Wp cost but spread over more capacity.
For utility-scale (above 50 MW), BOS engineering becomes a specialised discipline with dedicated design teams.
Common BOS mistakes
Underspecifying cables. Cheap cables fail through UV degradation, undersizing.
Skipping proper earthing. Critical safety and equipment protection issue.
Inadequate lightning protection. Lightning damage costs significantly more than protection.
Poor mounting design. Structural failures expensive.
Mismatched BOS to local conditions. Coastal sites need corrosion resistance.
Cost-cutting on BOS. Often leads to higher lifetime costs through failures.
Best practices
For utility-scale projects, work with experienced BOS design engineers.
Use BIS-certified and IEC-compliant components throughout.
Specify quality grades with established suppliers.
Conduct thorough site survey to identify environmental requirements.
Plan BOS for the plant’s 25-year life including some replacement budget.
For coastal sites, prefer corrosion-resistant materials (HDG steel, marine-grade aluminium).
For lightning-prone areas, implement comprehensive lightning protection.
Standards and references
BOS components must meet IEC standards (IEC 62290 for inverters; IEC 61730 for safety; various for cables and switchgear). BIS certification required for Indian compliance. CEA Connectivity Regulations 2019 cover grid interconnection requirements.
Related glossary terms
Key takeaways
Balance of System (BOS) is the comprehensive term for all solar plant components except modules and inverters. BOS includes mounting structures, cables, switchgear, transformers, earthing, lightning protection, and monitoring equipment. For Indian utility-scale solar, BOS accounts for 25% to 35% of total CAPEX; for rooftop, 25% to 30%. BOS quality directly affects plant reliability, safety, and lifecycle costs. Investment in quality BOS components from established suppliers reduces long-term operational costs and supports the plant’s 25-year service life.