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How to Size an Off-Grid Solar Power Plant?


An Off-Grid Solar PV System is not connected to the electricity grid and therefore requires batteries.

These Systems are recommended in regions with frequent power cuts and regions with no grid utility supply, the batteries have to be sized more carefully, to make sure that we are accounting for days with low sunlight. In such scenarios, it's always recommended to add some buffer to make sure the consumer is able to power all their loads through solar.


The main Components in an Off-Grid System are:

  1. Solar PV Panels

  2. Solar Inverter

  3. Charge Controller

  4. Batteries


1. First make a list of all the essential loads that need to be operated during a power outage.

2. Find out the wattage of each appliance on the list.

3. The Quantity of each equipment needs to be noted.

4. The hours of operation of each appliance.

5. Add up the wattage of all the appliances.

6. Multiply the wattage of each appliance with the hours of usage of that appliance to get the Total Energy.

While calculating power consumption for refrigerators, you could add the TOTAL ENERGY value directly from the BEE Ratings on the front of your refrigerator. Divide the value mentioned as "Electricity Consumption -Units per year" by 365, to calculate the energy consumed per day.

Once we have the above data, we can now calculate the Panel , Inverter and Battery Capacity Required for our backup needs.

 

Inverter Selection:

From the above data, we have a total wattage of 1270W. The inverter capacity should be designed with the assumption that all appliances might be operated simultaneously.


We can select a 1.5kW inverter for our application to make sure that any additions of similar loads should be taken care of by the inverter proposed.

Considering a 1.5kW inverter- The system voltage would be 24V.

 

Solar Array Sizing:

From the table, we have Total energy=3720 Wh/ 3.7 kWh.


Considering 4 units of generation/day/kW, we can now calculate the solar capacity required.


Array Size= Total Energy in Wh/ (Units /day).

= 3720/4

= 930W


We can consider 1000W/ 1kW of Solar Panels for our setup.

 

Battery Sizing


Total Energy = 3720Wh.


Considering a 50% DOD(Depth of Discharge) for the battery -

Energy to be stored in batteries- Total Energy/0.5= 7440 Wh.


Taking into account the system voltage of the inverter and the battery voltage of 12V.


= 7440Wh/12V= 620Ah


If we consider 150Ah batteries, then we would need 620/150= 4 nos.


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