With the growth of rooftop solar worldwide, both for residential and commercial sectors, a large number of prospective users are keen to know how to arrive at an approximate estimation for the capacity they require for their rooftop solar power plant.

The extent of rooftop solar capacity will of course be very different for different segments. Developed countries where per capital power consumption is much higher than for developing or underdeveloped countries will need high capacities from their rooftop solar panels. Even within a particular country, the extent of rooftop solar capacity will differ from region to region, from sector to sector etc.

 

Estimating the approximate capacity of the solar PV system you require and can install for your facility should be undertaken keeping in mind your requirements, your constraints, and the amount of sunlight available. We list a few steps that allow a methodical approach to sizing your system.

1. Scoping of the project

2. Calculating the amount of solar energy available

3. Surveying the site

4. Calculating the amount of energy needed

            5. Sizing the solar system

1. Scoping the project

Clearly laying out what you wish to achieve with your rooftop solar PV installation is critical to designing a plant that fits your needs. Examples of different kinds of needs we encounter in our work include

• Completely supports your daytime electrical needs
• Supports lighting loads
• Supports critical loads during power cuts
• Abates diesel consumption
• Provides power for night-time use

2. Calculating the amount of solar energy available

The amount of solar energy available to you is limited by the amount of sunlight that falls on a solar panel per day. This is expressed in kWh/m2/day. Solar insolation varies widely from one location to another worldwide.

At crystalline panel efficiencies (which are the kind used in rooftop systems due to their higher efficiency), we can generate 4 kWh of power per day from a 1 kWp panel. This is an average measure that can vary across different regions worldwide.

The approximate solar insolation at your location can be determined from the NASA website. To be absolutely certain of solar insolation at a particular site we would have to place sensors on-site that measure the actual insolation received over a period of time. This is both an expensive and time consuming process.

The site survey establishes the suitability of the roof for installing solar. Things to watch out for include

• Space available – 1 kW of panels would require 100 sq.ft (about 10m²) of shade-free roof area
• Orientation – A south-facing roof is ideal for those in the northern hemisphere. For those in the southern hemisphere (for instance, Australia), north-facing roof will be ideal.

The amount of energy needed is determined based on the load that needs to be supported. Since we have already determined the scope of the project in step 1 we know what equipment needs to be supported. The load represented by this equipment can be calculated as

Total energy requirement/day (Wh) = Wattage of appliance*No. of appliances*Hours of working

This should be divided by 1,000 to be converted into kWh/day. We can illustrate this formula by calculating the load for a sample home

 

Appliance	Number	Wattage	Working Hours	Energy (kWh/day)
Lights	8	30	8	1.92
Fans	5	50	8	2.00
TV	1	120	4	0.48
Computer	1	100	4	0.40
Refrigerator	1	300	12	3.60
Charging points	4	100	3	1.20
Total				9.60

This home would require 10 kWh of power per day to satisfy the load. At this point the plant designer might wish to identify large/variable loads that need not be supported by solar power or that can be operated through some other power source to reduce the investment in the solar system.

Let us assume that we have limited the load to be supported by the solar PV plant to this:

Appliance	Number	Wattage	Working Hrs	Energy (Kwh/day)
Lights	5	30	4	0.6
Fans	2	50	4	0.4
Computer	1	100	2	0.2
Charging points	2	100	3	0.6
Total				1.8

 

System size

This load requires 1.8 kWh/day.

Adding a 30% safety margin to this, and assuming the insolation to be 4 kWh/m²/day (this could be different for your region, so please check with an installer), we get

System size = (Energy Requirement*1.3) /insolation level

= 1.8*1.3/4 = 0.585 or 585 Wp.

Panel size

We calculate the panel requirement for this system size assuming we are using 130 kWp panels at 12V.

No. of panels = System size/Panel Rating

= 585/130 = 4.5

Therefore the system requires 5 panels of 130 Wp at 12V.

At this point the system designer may wish to verify if there is sufficient roof space available for installing five 130 Wp panels. Typically, a 1 kWp system requires 100-130 sq.ft so a 585 Wp (0.585 kWp) system would occupy about 59-76 SF of shade-free roof area.

If sufficient roof space is not available, the system designer could revisit the loads that need to be supported to determine which critical loads can be supported based on the amount of energy generation that the available roof area permits.

We use a 45% safety margin when calculating the inverter size.

Required Inverter size = Total Wattage of all appliances*(1+45%)

Total wattage of appliances is calculated in this table:

 

Appliance	Number	Wattage	Total Wattage
Lights	5	30	150
Fans	2	50	100
Computer	1	100	100
Charging points	2	100	200
Total			550

Therefore, required inverter size = 550 * (1+45%) = 798 W

The inverter size is greater than the required solar panel capacity (585 Wp), eliminating the risk of the inverter throttling the panel’s output.

The solar PV system required to power this load would need 5 x 130 Wp 12V panels and an inverter of at least 800 W.

If sufficient solar energy is not available

If we find from the above steps that the rooftop system will not be able to generate sufficient energy to support the entire load (often due to lack of sufficient rooftop space), we have several options before us:

Solar power for critical loads

In this system the critical loads are identified and solar power with battery backup is used to ensure that the critical loads receive power even during a power cut. More details of this system are provided here.

In this system the rooftop solar system is used to support non-critical loads that are not power hungry, such as lighting. Such a system requires the light points to be wired through a separate circuit that can be powered only through solar. The solar system can be coupled with batteries to provide lighting at night as well.

This system is favoured by consumers who consume a lot of diesel due to load shedding. Here the rooftop solar PV system works along with the diesel generator to support the load, and helps reduce diesel consumption.

Due to the complexity in matching the load that can be powered with the power generating potential of the rooftop we recommend that the final decision on sizing of your rooftop system be taken after consulting with an experienced rooftop solar installer.

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