Highlights

• Rooftop solar power can meet up to 20% of a warehouse’s electricity requirements
• If your warehouse consumes a lot of diesel for power generation, rooftop solar can abate up to 15% of your diesel bills, subject to timing of load shedding
• 100 SF of shade-free rooftop space can provide 4 kWh of solar power per day, on average

Overview

Warehouses typically have large areas of unutilised rooftop space and feature extensive lighting and refrigeration installations. Warehouses may also be in off-grid or grid-scare locations, making them depend heavily on diesel power. Rooftop solar PV holds several benefits for warehouses but a few constraints should be considered as well before deciding on the configuration of your rooftop solar system; both are discussed below.

Advantages

• Energy security – Rooftop solar plants can deliver power during load-shedding, ensuring that critical loads are always running
  • Not all solar plant configurations can deliver power during load-shedding. More details here
• Cost-effective – Rooftop solar power has a levelised cost of Rs. 4.5-5/kWh (or less), considerably lower than diesel power at Rs. 18/kWh (or more). Additionally, your energy cost is now fixed for the next 25 years, unlike diesel power which keeps increasing
• Reliable – A solar power plant has no moving parts, ensuring reliable power over 25 years
• Minimal maintenance – A solar plant requires very little maintenance from the energy consumer
• Flexible configurations – Solar panels can be installed on different kinds of roofs, including metal roofed warehouses, as long as the structure/mounting points can bear the weight of the panels. They are also highly scalable, with rooftop plants ranging in capacity from less than 1 kW to more than 1 MW

Constraints

• Roof type and Rooftop space – While warehouses usually have large roof spaces, the nature of the roof may make them unsuitable for solar installations e.g., some warehouse roofs have portions with transparent roofing to enable natural lighting. Solar panels cannot be installed on the transparent portion of the roof as the panels will block the light. Weight of the panels on the transparent roofing is also an area of concern
  • Roof requirements are discussed in detail here; a rule of thumb is that you will need about 100 SF of shade-free roof area for 1 kW of solar panels. Insufficient roof area will mean that the capacity of the solar plant that can be installed on your roof may be sufficient to meet only part of your electrical load
  • When estimating the available roof area any alternate uses of the roof, including seasonal usage, should be considered
• Infirm power – Solar power is dependent on the sun shining, and output varies depending on meteorological conditions e.g., passing clouds can temporarily reduce the solar plant’s output. Therefore solar power for critical equipment should be used in conjunction with another source of power
• Daylight power – Solar power is only available when the sun shines. Therefore night time applications will require other sources of power, or power from batteries charged through solar
• Load-shedding timings – If your warehouse experiences load shedding primarily at night, solar power may not help in reducing your diesel consumption as it is available only during the day
• Inverter weight – The DC power output from the solar panels needs to be converted to AC via an inverter which can be very heavy: a 100 kW inverter will weigh about 1,000 Kgs but occupy only a few square feet of space. As the inverter will need to be placed indoors it will need to be located on the ground floor instead of on the roof, with appropriate cables chosen to compensate for energy loss

Typical Load

As the rooftop space may not be sufficient to support the entire electrical load of your warehouse with solar, it becomes necessary to estimate the different kinds of loads to identify loads that can be/need to be supported by solar.

Electrical loads are estimated by calculating the wattage or amperage of electrical equipment in use (as shown here), which can be further classified as light loads and heavier loads, with solar typically being used to support the light loads.

Typical continuous electrical loads in a warehouse include

• Lighting
• Refrigeration

When estimating the load to be supported by solar it is important to consider the extent to which that particular load is used during the daytime when solar power is generated. For e.g., lighting load during daytime may be very low especially if the warehouse makes extensive use of natural lighting. There is the risk of solar power going to waste if the plant is sized for peak load at night rather than typical load during the daytime.

Our Recommendation

Based on the unique needs and constraints faced by warehouses, Solar Mango recommends

Alternative 1 – Grid-interactive configuration where solar power is supplied in conjunction with EB or diesel power. This is suitable for warehouses that feature a heavy refrigeration/air conditioning load during the daytime, as the solar plant may not be sufficient to support the entire load by itself

• Hybrid inverter – A rooftop solar PV system that utilises a hybrid inverter allows the solar plant to integrate with a diesel generator in addition to the utility grid. Here the solar plant serves to reduce diesel bills by supporting part of the overall load
  • Integrating a rooftop solar plant with a diesel generator involves several challenges (discussed here) that need to be overcome with careful design and sizing of the solar plant
• Load sizing – If the warehouse’s load is seasonal, we recommend careful sizing of the solar plant to ensure that solar power does not go to waste as the solar plant will generate power whenever there is sunlight even if the power is not required – and unlike running a diesel generator, costs are not saved by turning off a solar PV plant as the bulk of the investment is made at the time of installation

Alternative 2 – A battery-backed grid-interactive solar plant that only supplies specific loads such as lighting, but can support these loads without interruption even at night. This is suitable for warehouses that feature a great deal of lighting load, especially at night

• Hybrid inverter – In addition to the other functionality described above, hybrid inverters also incorporate charge controllers to regulate battery charging
• Battery backup – A battery bank can be used which will be charged by solar power, and will support critical loads for short durations during the daytime when solar output may be reduced; and at night time during power failure. Lighting can be supported by solar+batteries this way, as can other light loads such as fans and computers
  • Battery sizing decides the duration of battery backup available. Batteries add significantly to the cost of the project, need to be replaced every few years, require maintenance, and impose weight and space requirements. Therefore we recommend considering both current and future cost and space requirements before deciding on the size of the battery bank
• Powering critical loads – When combined with a battery bank, a solar plant can be used to reliably support critical loads such as lighting. Implementing such a solution would require that the critical loads be fed through a dedicated circuit

Cost of a Rooftop Solar Plant

The cost of a rooftop solar plant is discussed in detail here (including incentives and subsidies) and returns from substituting diesel with solar are discussed here. As a rule of thumb, a 1 kW solar plant that generates 4 kWh of solar power per day (on average) will cost Rs. 1 lakh (without considering subsidies, including installation charges but excluding batteries).

Batteries can add about 30% or more to the cost of the plant, depending on the extent of battery backup required.