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  Jun 28, 2021   

Off-Grid Solar System: Design Guide

Off-Grid Solar System: Design Guide

Solar energy is ample enough to cater to all our ever-increasing energy demand, But the only solution to attain our energy needs is to design an efficient and reliable solar energy system. 

“Are these solar PV modules enough to replace the traditional grid-connected system? 

              If yes, how are these solar energy systems designed?”

I know as a consumer these queries might strike your mind when thinking of installing a solar energy system.

This article is going to be the answer to your dilemma, and we hope we give you useful insights into the design of a solar energy system.

If you are interested in learning more about solar in detail please check our blog where we have added a series of articles on solar energy systems right from basics to enhance your knowledge. 

Rooftop Off-Grid Solar Energy System: Design

Before you plan to install a solar energy system on your rooftop, would advise you to check these points:

  1. Shadow free rooftop space available for installing the required sets of solar panels. (In general 100 sq feet area is required for 1 kWp solar plant installation)
  2. Check the roof handling capacity: Rooftop must be able to withstand the weight of this solar panel and handle natural calamities such as cyclones, storms etc.

Solar Energy System: Components

Solar energy system consists of four major components:

  1. Solar Panel
  2. Inverter
  3. Energy Storage: Charge Controller and Battery
  4. Loads

Design Steps:

Design Steps

Let us step-wise design a rooftop standalone energy system

Step 1: Load Calculation

               Identify all the AC and DC loads that you want to target. 

In this example, we are concentrating on AC loads. 

(For DC loads we don’t need an inverter although a charge controller is required, as the solar-generated DC power can be directly fed to the DC loads)

Once you have targeted the loads, calculate the energy rating for each load as follows:

  1. Note the power rating specified on the loads (devices connected such as TV, fans etc)  in Watts
  2. Note the running time of each load in hours
  3. Calculate the energy consumption as per the below formula(consider approximately 25% as energy loss factor)

                    Energy(watt-hour)= Power(Watt) x Duration(hours)

  1. Summation of daily consumed energy by all the loads

Note down all the target appliance rating and energy consumption as described below:

Target Appliance Rating And Energy Consumption

One can also check for the previous electricity bills and can consider the highest of all as the energy consumption required for the design of a solar energy system.

By following the above steps for all the AC loads we have calculated: 

 Power = 380 watts 

Calculated Energy = 2170 watt-hour

 Total Energy(add 25% as energy loss factor) = 2170 *1.25 

                                                                                 =2712.5 W-h

Will design the solar energy system by keeping in mind the above ratings.

Step 2: Sizing of Inverter

Once we have estimated the energy requirement, the next task is to calculate the inverter rating for the same. 

Inverter selection plays an important part in our solar energy design, as it is responsible for converting the direct current generated from the solar panel into alternating current(as the loads connected at our home mostly run on AC supply) as well as perform other protection measures.

  1. Consider an inverter with fair efficiency, we have considered an inverter with 85% efficiency
  2. The total power wattage consumed by the loads is considered as an output of the inverter (i.e. 380W)
    Will add 25% as a safety factor in the required power wattage.
    380 * 0.25= 95
    Total power wattage required = 380+95= 475 W
  3. Calculate the inverter input capacity rating

      Input(VA)  = Output(watt) / efficiency X 100
= 475(watt) / 85 X 100
= 559 VA = 560VA

  1. The required input power for the inverter is estimated as 559 VA, now we need to estimate the energy input required by the inverter.

Input Energy(Watt-hour) = Output (watt-hout) / Efficiency x 100
= 2712.585 X 100
= 3191.1 Watt-hour 

Now, once we have determined the inverter capacity, the next task is to check the inverter available in the market. The typical inverter available comes with 12V, 24V, 48V system voltage.
As per our estimated energy rating of 560VA, we can select a 1 kW system inverter. Generally, a 1 kW inverter has a 24V system voltage. (Generally 1kW and 2kW – 24V, 3kW to 5kW – 48V, 6kW to 10 kW – 120V) It is always necessary to see the inverter specification datasheet to determine the system voltage.  
Now, the next step is to calculate the battery capacity, for further calculations, we will consider a 24V system voltage.

Step 3: Battery Selection

            For a standalone rooftop solar system, a battery is required so as to store energy during a sunny day and retrieve it during the evening or cloudy weather.

For selecting a battery system one needs to define the below criteria:

  1. Type of battery: Batteries are of two types i.e. rechargeable and non-chargeable. For renewable energy rechargeable batteries are recommended. 
    Rechargeable batteries are again further classified as lead or lithium-ion batteries. 
    For small residential buildings, lead-acid batteries are preferred due to their low cost, for industrial applications one can go for lithium-ion batteries or industrial grade lead-acid batteries.
    Here for our solar rooftop system, we have selected a 12V lead-acid battery 
  1. System voltage ampere-hour rating

    We have chosen a 1 kW inverter with 24V system voltage 

Ampere-hour capacity = Energy output (watt-hour) / System Volt (V)
=2170 Wh / 24 V
= 90 Ampere-hour (Ah)

  1. Depth of discharge(DOD)– It indicates the charging capacity of the battery. For lead-acid batteries, DoD is 50% and for lithium-ion batteries, DoD is 80%.
    For small residential areas, lead-acid batteries are preferred because of their low cost.
    Battery loss factor= 0.85 and DoD = 0.5

Actual battery capacity = Estimated capacity(Ah) / DoD* battery loss
= 90(Ah) / 0.5 X 0.85
= 212 Ah

  1. Days of autonomy i.e number of days battery can furnish power in case of non-sunny days. 
    The above-calculated energy is the actual battery capacity i.e the capacity which the battery can supply, now considering the autonomy days we will calculate the battery capacity required for our system.

    We have considered autonomy days as 2

Required battery capacity = Actual battery capacity(Ah) X No. of autonomy days
= 212Ah x 2
= 424 Ah

  1. The final steps evolve the calculation of the number of batteries required to supply the amount of energy estimated.
    Available lead-acid batteries in the market are 40Ah, 100Ah, 150Ah, 200Ah and the voltage level of the battery is 12V. Since we have arrived at 424 Ah battery, we can either go with a 400Ah battery bank or 500Ah battery bank. 400Ah will reduce the estimated backup hours (only in minutes) and 500Ah will increase the estimated backup hour. (Can be chosen according to the client’s need). We must ensure that the battery bank voltage must be equal to the inverter system voltage. Batteries can be connected in a series connection to attain the voltage level and parallel connection to attain the required Ah to form a battery bank.

    For Example, if the inverter selected is 1 kW, 24V and the required battery capacity is 424 Ah
    Case 1: Considering 12V, 200Ah battery
    The minimum battery required in series is 2 (Ensures 12V+12V = 24V)
    Minimum battery required in parallel- 2 sets of 2 batteries in series (Ensures 200Ah+200Ah = 400Ah 24V)

    Therefore we require 4 numbers of 200Ah, 12V batteries to get the required output energy of 400Ah, 24V.

    Case 2: Considering 12V 150Ah battery
    Minimum battery required in series – 2 (Ensures 12V+12V = 24V)
    Minimum battery required in parallel- 3 sets of 2 batteries in series (Ensures 150Ah+150Ah+150Ah = 450Ah 24V)
    Therefore we require 6 numbers of 150Ah 12V batteries – 450Ah 24V

As per the client requirement and cost factor, we can select the battery rating as per the above-described conditions.

Don’t worry, I know it’s difficult to select a proper battery rating, but don’t worry our design team ensures to provide the client with the best cost-effective battery solution as per your requirement.

Step 4: Size of PV module

Solar panel/module is the base of our solar energy system. To meet our energy needs solar panels should be installed and designed with utmost care.


Note: While designing one should keep in mind that the energy feed from the battery should be higher than the energy requirement by the inverter as battery efficiency is not 100%, it is usually between 80 to 95% depending on the choice of the battery.

Solar panel output depends on various factors other than solar module rating such as location, climatic conditions, solar radiation intensity, shadow prone area etc.

According to the thumb rule, 1kW panel generates 4kWh, so to generate the required amount of energy i.e. to generate 2712.5 watt-hour,
2712.5/4 = 678.12 W

Now, the final step is to calculate the required number of solar panels to generate the required energy

The available module ratings are 40,60,75, 100, 120,200,330 watt peak etc.

Let us consider 330Wp solar module rating,

Total solar panels required = 678.12/330
= 2.05 = 3 modules

 Note: Solar module counts are non-prime integers.

Depending on the above system design, fuse wire, junction box and other components are also to be chosen wisely keeping in mind the maximum current and voltage rating.

Tips: Before installing a solar panel

  1. Check for the government subsidies to avail maximum out of the solar installation.
  2. Depending on the grid availability and location, decide the type of solar energy system suitable for your energy requirement 
  3. If going for rooftop solar installation check rooftop capacity to install the required number of solar panels.
  4. To get optimum result shading analysis must be done in order to ensure that solar panels installed are not covered with shadow from neighbouring trees/buildings or other factors.

I hope this article has provided you with some insights into the design of the solar energy system. 

If you are planning to install a solar panel system at your location and still have some doubts, don’t worry our technical team will guide you with the best possible solution.

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