Balance of System (BOS)

Balance of System (BOS) refers to all the components, hardware, electrical equipment, and infrastructure required to operate a solar photovoltaic (PV) system other than the solar modules themselves. While panels generate electricity, BOS components enable mounting, wiring, power conversion, protection, and grid interconnection—making them essential to the performance, reliability, and safety of any PV installation.

BOS includes racking, inverters, wiring, combiner boxes, switches, protection devices, monitoring systems, and—depending on the system—batteries, transformers, and communication hardware. For solar designers using modern software tools such as Solar Designing, BOS plays a major role in layout planning, stringing, cost estimation, shading optimization, and construction readiness.

Key Takeaways

Balance of System (BOS) includes all components of a solar project excluding the PV modules.
BOS covers structural, electrical, safety, monitoring, and interconnection equipment.
BOS directly impacts performance, safety, installation cost, and long-term reliability.
Proper BOS planning supports NEC compliance, shading optimization, and installation efficiency.
Modern platforms like SurgePV simplify BOS estimation, layout design, and engineering.

What Is Balance of System (BOS)?

Balance of System encompasses all components outside of the solar module. These components support energy collection, power conversion, electrical safety, structural stability, and connectivity to the grid or load.

Typical BOS categories include:

Structural BOS → racking, mounting rails, clamps, roof attachments
Electrical BOS → inverters, wiring, breakers, fuses, combiner boxes
Protection & Safety BOS → disconnects, grounding, surge protectors
Monitoring BOS → sensors, data loggers, metering equipment
Energy Storage BOS (when applicable) → batteries, battery management systems
Communication BOS → gateways, monitoring hubs, controllers

Related foundational concepts include Mounting Structure, Stringing & Electrical Design, and Solar Inverter.

How BOS Works

While solar panels produce DC electricity, BOS components ensure that:

1. Panels are structurally mounted

Racking, mounting hardware, ballast, rails, and attachments secure modules in place and maintain optimal orientation.

2. DC electricity is safely collected

Wiring, combiner boxes, connectors, and junction boxes route power to the inverter.

3. Power is converted to usable AC

Inverters convert DC → AC and synchronize with the grid—see Inverter Sizing.

4. System protection is enforced

Breakers, fuses, rapid shutdown devices, grounding, and surge protection ensure safety and NEC compliance.

5. Energy is monitored

Data loggers, meters, and IoT gateways provide performance visibility.

6. AC power connects to the building or grid

Switchgear, distribution panels, and transformers handle AC distribution.

Together, BOS supports every stage from power generation to consumption and grid export.

Types / Variants of BOS

1. Structural BOS

Includes the physical mounting components:

Rails
Racking systems
Clamps
Roof attachments
Ballast trays
Used heavily in rooftop and ground-mount systems.

2. Electrical BOS

Critical for safe and functional electrical performance:

Inverters
AC/DC wiring
Combiners
Disconnect switches
Conduits
Breakers
Rapid shutdown equipment

3. Mechanical BOS

Hardware for stability and weather resilience, particularly in ground-mount or tracker systems.

4. Monitoring & Communication BOS

Equipment for tracking system performance:

Data loggers
SCADA devices
Gateways
Monitoring sensors

5. Energy Storage BOS (Optional)

For systems with batteries:

Battery racks
Battery wiring
BMS (Battery Management System)
Storage enclosures

How BOS Is Measured

Typical BOS quantifications include:

Material Cost Contribution

BOS often represents 40–60% of total system cost in residential systems and 25–45% in commercial/utility projects.

Electrical Ratings & Limits

Voltage (V)
Current (A)
Power ratings (kW/kVA)
Conductor sizing
Protection device ratings

Structural Load Capacity

Measured in:

Wind load (psf)
Snow load (psf)
Uplift tolerances

Efficiency Impacts

Inverter efficiency, voltage drop, shading losses, and wiring resistance all influence BOS performance.

Installation Labor

Labor effort is also considered part of BOS cost modeling.

Typical Values / Ranges

Values vary based on AHJ, NEC code requirements, building structure, and system voltage.

Practical Guidance for Solar Designers & Installers

1. Design your BOS early

BOS significantly affects cost, layout, safety, and constructability. Begin defining BOS during early design phases using Solar Designing.

2. Size wiring and protection correctly

This requires accurate calculations of current, voltage drop, and breaker ratings—tools like the Voltage Drop Calculator help prevent design errors.

3. Use accurate shading analysis

Poor BOS placement can create shading issues. Validate design with Shadow Analysis.

4. Ensure NEC-compliant disconnects & rapid-shutdown devices

Residential and commercial systems require specific safety hardware.

5. Optimize BOS placement for efficient installation

Shorter conduit runs and efficient inverter placement reduce cost and simplify construction.

6. Match structural BOS to site conditions

Especially important in high wind or snow regions.

7. Verify BOS compatibility

Ensure all hardware is compatible with module size, inverter type, and racking manufacturer guidelines.

Real-World Examples

1. Residential Rooftop System

A 7.5 kW PV system includes:

Roof attachments
Rails & clamps
Microinverters
Wiring, junction boxes, and AC disconnects

BOS makes up ~50% of total installed cost.

2. Commercial Flat Roof

A 200 kW ballasted system uses:

Heavy-duty racking
Multiple string inverters
Combiner boxes
Long AC/DC conduit runs

BOS is optimized for walkway spacing and fire code compliance.

3. Utility-Scale Ground-Mount System

A 20 MW site includes:

Tracker systems
Central inverters
MV transformers
SCADA monitoring

BOS is engineered to reduce wiring loss and maximize energy yield.