A complete solar panel installation guide with calculation of various terms

By Bharat Ribadiya -
In this article

  • How to calculate the number of solar panel required
  • How to calculate the rating of solar panel
  • How to calculate the ratings of batteries
  • How to calculate the backup time of batteries
  • How to calculate charging time & charging current of batteries
  • UPS rating for load requirements

Before reading this article it is recommend that to read our previous Article:

How to wire solar panel with AC and DC loads?
Series and Parallel connection of batteries with solar panel
Series And Parallel Connection of Solar Panel with Auto UPS System
General requirements for installation of solar panel
Wiring Connection of solar panel Junction box and function of diodes that used into it ?


Lets assume that the your requirements backup time is 3 hours and your load is 800W. Now you want to find the inverter rating, number of solar panel required and number of batteries are required.

Here,
Required backup time for batteries = 3 Hours
Load = 800 Watts
Inverters Rating =?
Required No of Solar Panel =?
No of batteries =?

→ As we know the Inverter should be greater then 25%  the total load.Now we calculate the total watts required so we have to consider the losses also.
So, 800 x (25/100) = 200
Total load = 800+200 = 1000 Watts
This is the rating of the Inverter

→The required backup time is 3 hours and if we install 12V, 100Ah batteries then the total watts-hour is
=12V * 100Ah
=1200Wh

Backup time for one battery
=1200Wh/800W
=1.5 hour

We required backup time is three hour. so, No. of batteries required=3/1.5=2
Our required batteries is 2 having ratings of 12V, 100Ah for 3 hour back-up. You must have to install both the batteries into parallel connection.

If you have batteries and wants to find the backup time then use the below formula:

Back-up Time = (Total watts-hour * No. of batteries) / Total Load
In our case, Back-up Time= (1200 Wh x 2 Batteries) / 800W = 3 hour

Now, you have a question in your mind, why batteries are in parallel,not in series?

In the parallel connection  voltage are same and current is different in each section. The total current can be found by by addition of all currents in the circuit. If we connect the two 100Ah batteries in parallel then total current will be 200Ah. So,connecting batteries in parallel we can get higher amp-hour for same voltage.

If we connect batteries into series connection then current is same and voltage can be  found by addition of all voltage into circuit or loop (Vn=V1+V2+V3+.....+Vx). So, if we connect the 2 batteries into the series then voltage will be 24V. Here, you can see the voltage are double but only voltage are double not the capacity of voltage.

Now, let's find how many  solar panels are required.

Here, we connected two batteries into parallel so the total amp-hour will be 200Ah and voltage will be 12V. As we know that the charging current should be 1/10 of batteries amp-hour. So, Charging current = 200Ah * (1/10) = 20A and P=VI=12*20=240W. This watts are required for solar panels to supply the our load.

Let's Suppose you have 60W solar panel then,
No. of solar panels required = 240/60 =4 

So, 4 solar panels are required having 60W, 12V and 5A ratings.

This calculation is based on first battery charge then it will supply on your AC load thorough invertet but in case if you wants to connect directly your DC loads that required 10A with solar panels then you have to calculate that load also. As per above calculation required amp for batteries is 20A and now we wants to connect load that required 10A directly to solar panels. So, the total current will be 30A and required power will be

Total power required (P) = VI = 12 * 30= 360W 

The number of 60W solar panels are required is, 360/60W = 6. Therefore you have to connect 6 solar panels are required having 60W, 12V and 5A ratings.
This is circuit diagram of above example and calculation
Here, the charging current for battery is 20A and 10A is required by direct DC load. So, we need to install charger controller having rating about 30A-32A.
[warning title="Warning message" icon="exclamation-triangle"] This calculation is based on ideal case, so we recommended that to you always choose panels bigger then you required. That is important because when solar panel charges the battery that time some power is waste during charing process.[/warning]

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