Updated September 2014

Highlights

• Rooftop solar power can meet up to 20% of an office building’s electricity requirements in India
• If your building consumes a lot of diesel for power generation, rooftop solar can abate up to 20% 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

Office buildings typically depends on electricity for most of their operations and have little need for heat energy. The consumption of power in an office building during the day time is much greater than that of the night time, which makes them ideally suited to solar power where electricity is generated only during the daytime. There are different loads in an office building depending on the nature of work done at the particular office; critical loads may be present that need to run continuously.

There may be several benefits from using solar power, such as reduction of grid and diesel consumption but there are few constraints in using solar power as well; both are discussed in detail 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 cost 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 covered parking areas, as long as the structure 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

• Rooftop space –The capacity of the solar plant that can be installed in an office building may be constrained by lack of sufficient shadow-free rooftop space. 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 on your roof may be sufficient to meet only part of your electrical load
  • In addition to the extent of the rooftop space available, the right to use that space may be a constraint especially in multi-tenant office buildings
• 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 loads 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
• 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. If the construction cannot support this weight the inverter may need to be placed on the ground floor, 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 facility 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 being used to support the light loads.

Typical electrical loads in an office include

• Lighting
• Fans
• Workstation (Desktops/Laptops)
• Security Systems
• Elevators
• Servers
• Air Conditioning

Air conditioning is usually the heaviest load, accounting for more than half the energy consumed. However most air-conditioning isn’t critical for business operations (air conditioning for server rooms being the usual exception) while the servers, workstations, security, and lighting are critical to the continuity of business. Some loads requiring high, frequent starting currents, such as elevators, may not run on solar or solar + batteries and may require another source of power such as grid or DG to function.

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 building make extensive use of natural lighting.

Our Recommendation

The need for uninterrupted, constant power and possibly limited roof space necessitates rooftop solar power being used in conjunction with other sources of power such as utility power, diesel generator, and/or batteries.

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

• Hybrid inverter – A rooftop solar PV system that utilises a hybrid inverterallows the solar plant to integrate with a diesel generator in addition to the utility grid. Here the rooftop 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 rooftop solar plant
  • 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 during power failure. Workstations, servers, and even lighting (if required) can form part of the critical load to be supported by solar + batteries
  • 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 limiting the battery bank to about an hour of backup
• Powering critical loads –When combined with a battery bank, a solar plant can be used to reliably support critical loads such as servers and workstations. Implementing such a solution would require that the critical loads be fed through a dedicated circuit

With this configuration, the office buildings can run only their air conditioners on the diesel generator, or even turn off the DGs during load shedding if the solar plant is large enough to support all other loads.

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.