What is Balance of System?
Balance of System, BOS, is the collective term for every component of a solar plant other than the modules and the inverter. While modules generate DC power and the inverter converts it to AC, BOS components handle everything in between and around: mounting the modules, carrying the current, isolating the system for safety, protecting against surges and lightning, monitoring performance, and grounding the installation.
BOS typically includes: mounting structure (galvanised steel, aluminium, or composite), DC cables (typically 4 to 6 sqmm copper or aluminium), AC cables, combiner box at array level, junction boxes, AC and DC disconnects, lightning protection systems, earthing materials, surge protection devices (SPD), monitoring sensors and dataloggers, cable trays and walkways, and minor consumables.
BOS is distinct from civil works (foundations, pile drives, site preparation) and installation services (labour). In some accounting BOS is bundled into "BOS plus civil"; in others civil is a separate category. SECI and large tender BOQs usually itemise BOS components individually.
Why BOS matters
BOS accounts for 20 to 35 percent of total project cost. The exact share depends on plant type: rooftop projects have lower BOS share, ground-mount and utility-scale projects have higher BOS share due to extensive cabling, fencing, and structures. Underestimating BOS in BOQ leads to project cost overruns; overspecifying inflates customer pricing.
BOS quality directly drives plant performance and life. Undersized DC cables waste energy in voltage drop. Poor mounting causes panel misalignment and structural failure. Inadequate earthing creates safety hazards. Quality BOS preserves the value of the more visible modules and inverter.
For BIS, DCR, and SECI tender compliance, BOS components must be certified or specified per applicable standards. Bypassing standards on BOS in tender bids is grounds for disqualification.
How BOS is specified for a project
- Design completion. Plant layout, kWp, electrical scheme finalised.
- Mounting selection. Galvanised steel, aluminium, or alloy; foundation type.
- Cable sizing. Cross-section calculated from voltage drop and current.
- Combiner and junction boxes. Sized to string configuration.
- Disconnects. AC and DC isolation switches rated to circuit.
- Earthing. Design per IS 3043 and local soil conditions.
- Lightning protection. Per zone risk assessment.
- SPD. Surge protection at DC and AC ends.
- Monitoring. Datalogger, sensors, communication module.
- BOM and BOQ. All BOS items itemised with manufacturer and price.
Benefits of disciplined BOS specification
- Plant performance. Right cables minimise losses.
- Structural integrity. Right mounting survives wind and load.
- Safety. Earthing and SPD prevent damage.
- Compliance. BIS and tender requirements met.
- Project longevity. Match BOS warranty to module life.
- Cost transparency. Itemised BOQ supports negotiation.
- Warranty traceability. Equipment-level identification.
Limitations and challenges
Underestimation in BOQ. Many small items adding up.
Quality variation. Cheap BOS components reduce plant life.
Logistics complexity. Multiple vendors per BOS category.
Installation skill gaps. Poor cabling and earthing degrade performance.
Standard updates. BIS revisions require BOS spec updates.
BOS in Indian solar projects
| Project type | BOS focus |
|---|---|
| Residential rooftop | Lightweight mounting, weatherproof boxes, basic monitoring |
| SME commercial | Detailed cabling, AC and DC disconnect, SPD, earthing |
| Industrial captive | Heavy mounting, extensive monitoring, redundancy |
| SECI utility-scale | Full BOS itemisation, ALMM and BIS verification |
| PMKUSUM | Standardised BOS, agency-approved vendors |
| Ground-mount solar farm | Heavy galvanised steel, perimeter fencing, security |
Quick facts
| Definition | All components except modules and inverter |
|---|---|
| Cost share | 20 to 35 percent of total project |
| Major categories | Mounting, cables, switchgear, monitoring, earthing |
| BIS standards | IS 60947, IS 694, IS 3043, IS 61643 |
| Common manufacturers | Polycab, Havells, Schneider, ABB, Phoenix Contact |
| Mounting vendors | Lubi, Strolar, Yantra Solar, local fabricators |
| Warranty range | 5 to 25 years per component |
Common mistakes about BOS
- Undersized cables. Voltage drop kills yield.
- Cheap mounting structure. Structural failure risk.
- Skipping SPD. Lightning damage to inverter.
- Inadequate earthing. Safety hazard.
- No monitoring equipment. Blind to performance issues.
- Non-BIS components. Tender disqualification.
- Underestimated cable runs. Procurement shortfall.
- Bundled BOS without itemisation. Hidden margin loss.
Key takeaways
- BOS covers all solar plant components except modules and inverter.
- Accounts for 20 to 35 percent of total project cost.
- Includes mounting, cables, switchgear, monitoring, earthing.
- BIS standards apply per component category.
- Quality BOS protects plant performance and life.
- Detailed BOQ itemisation prevents cost overruns.
- Multiple specialised vendors per BOS category in India.
Frequently Asked Questions
What is Balance of System in solar?
Balance of System (BOS) is the collective term for all components of a solar plant except the modules and the inverter. BOS includes mounting structure, DC and AC cables, combiner boxes, junction boxes, disconnects, earthing, lightning protection, monitoring equipment, and minor materials. BOS typically accounts for 20 to 35 percent of total project cost.
What does BOS include?
Mounting structure (galvanised steel, aluminium, or alloy), DC cables (4 to 6 sqmm typical), AC cables, combiner box, junction boxes, AC and DC disconnects, lightning protection, earthing, surge protection devices (SPD), monitoring sensors, walkways, and cable trays. Some BOS definitions also include foundations and civil works.
What is the cost share of BOS in a project?
20 to 35 percent of total project cost. Modules are the largest single line item (40 to 50 percent), inverter is 8 to 12 percent, and BOS (mounting plus electricals plus minor materials) makes up the balance. Civil and installation services are separate.
Why is BOS often underestimated?
BOS is many small line items rather than one big component. Underestimating cable lengths, mounting steel weight, or earthing costs is common. Quality BOQ practice itemises BOS in detail to avoid surprises.
Does BOS quality affect plant performance?
Yes significantly. Wrong cable cross-section causes voltage drop and energy loss. Poor mounting affects tilt accuracy and structural integrity. Inadequate earthing risks safety incidents. Skimping on BOS to lower BOQ is a common cause of underperforming plants.
What is the BIS standard for BOS components?
Multiple BIS standards apply: IS 14286 for modules, IS 16221 for inverters, IS/IEC 60947 for switchgear, IS 694 for cables, IS 3043 for earthing, IS/IEC 61643 for SPDs. Each BOS component must meet the applicable standard.
Is BOS covered under DCR (Domestic Content Requirement)?
Selectively. DCR primarily applies to solar cells and modules. Some tenders also specify domestic manufacturing requirements for inverters and mounting structures. BOS minor components are typically not under DCR.
Who supplies BOS components in India?
Diverse vendor base: Tata Power Solar, Vikram Solar, Polycab, Havells, Schneider Electric, ABB, Phoenix Contact, Citizen, KEI Industries for cables. Mounting structures from local fabricators or specialised vendors like Lubi, Strolar, Yantra Solar.
How does BOS differ between rooftop and ground-mount?
Rooftop BOS emphasises lightweight mounting, weatherproof junction boxes, building-integration accessories. Ground-mount BOS uses heavier galvanised steel structures, more extensive cabling, larger combiner boxes, full perimeter fencing and security.
Is monitoring equipment part of BOS?
Yes typically. Solar monitoring (inverter data logger, communication modules, weather sensors, energy meters) is classified under BOS in most BOQ structures. Quality projects include monitoring; budget projects sometimes skip it.
Does BOS depreciation differ from modules?
Yes. For tax purposes, BOS components depreciate at the rate applicable to plant and machinery. Mounting structures, cables, and switchgear may have different depreciation schedules. Tax counsel can confirm specifics for individual projects.
What is the typical BOS warranty?
Mounting structures 10 to 25 years, cables 25 years, combiner boxes 5 to 10 years, monitoring equipment 2 to 5 years. Module and inverter warranties are typically longer (25-year module performance, 5 to 12 year inverter). Quality EPCs match BOS warranty to plant life.
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- MNRE technical specifications. Solar plant component requirements. mnre.gov.in
- BIS standards. IS 14286, IS 16221, IS/IEC 60947, IS 694, IS 3043, IS/IEC 61643.
- SECI tender BOM/BOQ formats. Standard BOS itemisation.
- CEA Technical Standards. Grid-connected solar requirements.
- Manufacturer datasheets. Tata Power Solar, Vikram Solar, Polycab, Havells, Schneider.
- Solar EPC field practices. BOS specifications.
- NSEFI publications. BOS quality benchmarks.
Written by QuickEstimate Editorial, QuickEstimate Editorial (Surat).
Last updated: 4 June 2026.