Solar PV system Sizing: Step by step approach to design a roof top system and software analysis

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Solar photovoltaic system or Solar power system is one of renewable energy system which uses PV modules to convert sunlight into electricity. The electricity generated can be stored or used directly, fed back into grid line or combined with one or more other electricity generators or more renewable energy source. Solar PV system is very reliable and clean source of electricity that can suit a wide range of applications such as residence, industry, agriculture, livestock, etc.

Major system components                                                                                                                                                                                                        Solar PV system includes different components that should be selected according to your system type, site location and applications. The major components for solar PV system are solar charge controller, inverter, battery bank, auxiliary energy sources and loads (appliances).


PV Modules: Converts sunlight into DC electricity.

Solar Charge Controller:  Regulates the voltage and current coming from the PV panels going to battery and prevents battery overcharging and prolongs the battery life.

Inverter: Converts DC output of PV panels or wind turbine into a clean AC current for AC  appliances or fed back into grid line.

Battery:  Stores energy for supplying to electrical appliances when there is a demand.

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Example:  A house has one 18 Watt fluorescent lamp with electronic ballast used 4 hours per day,one 60 Watt fan used for 4 hours per day and one 75 Watt refrigerator that runs 24 hours per day with compressor run 12 hours and off 12 hours.The system will be powered by 12 Vdc, 100 Wp PV modules.

Solar PV system sizing

1.Determine power consumption demands:The first step in designing a solar PV system is to find out the total power and energy consumption of all loads that need to be supplied by the solar PV system as follows:

a. Add the Watt-hours of all appliances together to get  total Watt-hours per day which must be delivered to the appliances.        Total appliance use = (18 W x 4 hours) + (60 W x 4 hours) + (75 W x 24 x 0.5 hours) = 1212Wh/day.                                                 b. Multiply the total appliances Watt-hours per day times 1.3 (the energy lost in the system) to get the total watt-hours per the day which must be provided by the panels.Now the total PV panels energy required is 1212*1.3= 1575.6 Wh/day.[ezcol_1half]

2.Size the PV modules:Different size of PV modules will produce different amount of power. To find out the sizing of PV module, the total peak watt produced needs. The peak watt (Wp) produced depends on size of the PV module and climate of site location. We have to consider “panel generation factor” which is different in each site location. For India, the panel generation factor is around 4.32. To determine the sizing of PV modules, calculate as follows:                                                                                                                 a.Calculate the total Watt-peak rating needed for PV modules and the total Watt-hours per day needed from the PV modules  by 4.32 to get the total Watt-peak rating needed for the PV panels needed to operate the appliances.i.e 1575.6/4.32 =364.72 Wp.                                  b.Calculate the number of PV panels for the system and divide the answer obtained in before point by the rated output Watt-peak of the PV modules available   to you. Increase any fractional part of result to the next highest full number and that will be the   number of PV modules required.i.e 364.72/100 = 3.64 which is fraction so we will take as 4. It means that this system requires four 100Wp modules of 12 volts.[/ezcol_1half] [ezcol_1half_end]

Result of the calculation is the minimum number of PV panels. If more PV modules are installed, the system will perform better and battery life will be improved. If fewer PV modules are used, the system may not work at all during cloudy periods and battery life will be shortened.

3.Inverter sizing: An inverter is used in the system where AC power output is needed. The input rating of the inverter should never be lower than the total watt of appliances. The inverter must have the same nominal voltage as your battery.For stand-alone systems, the inverter must be large enough to handle the total amount of Watts you will be using at one time. The inverter size should be 25-30% bigger than total Watts of appliances. In case of appliance type is motor or compressor then inverter size should be minimum 3 times the capacity of those appliances and must be added to the inverter capacity to handle surge current during starting.For grid tie systems or grid connected systems, the input rating of the inverter should be same as PV array rating to allow for safe and efficient operation.i.e. Total watt of all appliances = 18+60+75 = 153W.For safety it shold be 25-30% more. So the value of inverter rating is 190W.[/ezcol_1half_end]

4.Battery sizing:The battery type recommended for using in solar PV system is deep cycle battery. Deep cycle battery is specifically designed for to be discharged to low energy level and rapid recharged or cycle charged and discharged day after day for years. The battery should be large enough to store sufficient energy to operate the appliances at night and cloudy days. To find out the size of battery, calculate as follows:

 Calculate total Watt-hours per day used by appliances which is 1212Wh/day and divide the total Watt-hours per day used by 0.85 for battery loss.Now divide the answer obtained  by 0.6 for depth of discharge. Divide this answer obtained  by the nominal battery voltage.(generally 12 volts. Multiply the answer obtained  with days of autonomy (the number of days that you need the system to operate when there is no power produced by PV panels) to get the required Ampere-hour capacity of deep-cycle battery. (Generally min 3 days).Overall formula can be written as below:

Battery Capacity (Ah) = (Total Watt-hours per day used by appliances x Days of autonomy)/(0.85 x 0.6 x nominal battery voltage)  i.e. (1212*3)/(0.85*0.6*12) = 594.11 Ah.So the battery should be rated as 12V, 600Ah, 3 days autonomy.

The calculation above was done manually and it can also be done in software (PVsyst).The procedure is as follows:

[ezcol_1half]1.Upon starting of PVsyst we will get page as below:1

In the above there are 3 options to choose and in that preliminary design is used when full data is not available with us and project design is used when data is available and tools for extracting data from software.

2.Now as data is available with us we will click on project design. Upon clicking that it will open next page as below:

2Now in project design we have four options as per the requirement we can click on the preferred option and now we are doing for standalone system.Clicking on standalone system will open the page as below:


This is the page where we will select required data and we can enter data which we have to input.

First we should click on project. It will open another page and in that first we should select New Project option.Now we should select where we are going to install the system.

3.Then it will ask for selecting site and meteo where we want to install the stand alone system.So we should select site and meteo then it will open another page as below.Here I selected for India, Delhi site.


Here we can select country, site area and meteo file and then click  next. It will open another page and in that page we should click on OK, then will open another page where it will ask for save option and we should click on save.Then it will open previous page and there we should click on back and it will open the home page for project parameters.[/ezcol_1half] [ezcol_1half_end]4.Now click on orientation it will ask for how much tilt we want to keep, It should be nearly equal to latitude angle of the site. Delhi latitude angle is 30 degrees that’s why I kept 30 degrees.And then click on OK then it will open again home page for project parameters.

5.Now click on systems and it will open a page like below:


In the above page we can select the daily consumption like I have selected 1 flouroscent lamp of 18 watts which will run for 4 hours per day and 1 fan(domestic appliance) of 60 watts which will run for 4 hours per day and fridge which consumes .9kwh/day. And then click on Next.

6.Then it will open the page as shown below:


In the above page we can select the battery as per requirement and as our requirement is 600Ah I selected 150 Ah of 12 volts  batteries of 4 which are connected in parallel.

Modules are also selected which are four 100Wp of 12 volts are selected which are connected in parallel.

And then click next then it will open another page and click on default regulator and click on OK. In this page we can also click what ever tracking system we want and then we should click on OK and it will open again home page for project parameters.

7.Now click on simulation it will open another page and there also click on simulation again then it will be simulated and opens another page where we should click on OK. Then it will show the results page as below.


In the above page we can get the production in whole year as 613kWh/year i.e. on an average 1.68kwh/day where as our normal requirement when calculate manually we got 1.6kWh/day is equal.So our data analysis is approximately equal to the software report.

And we can get report by clicking on report on the page above.[/ezcol_1half_end]

The report of this document can be downloaded from the below link:

-Naveen Yalla

Dr.Sanjay Vashishtha


5 Responses to "Solar PV system Sizing: Step by step approach to design a roof top system and software analysis"
  1. suresh says:

    thank u. very useful info

    kindly give info of how to use pvsyst for grid tie system especially where shadows are being cast on the area where panels are being installed

  2. Ganapatsa A says:

    How you are assuring zero leakage of rain water by using only the solar panels without using roofing sheets. Give detail sketch of panel dimensions.

  3. Manoj Nair says:

    Useful and impressive information.

  4. deepak says:

    Thanks dear for this wonderful work.please keep writing

  5. Thank you sharing the knowledge it is very good of you that you have given a detailed knowledge of setting up rooftop solar panels. Also I like the software analysis system as it is really a nice thing. We should all go rooftop solar systems to produce electricity which is pollution free rather than using coal.

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