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Below is a complete Note on SOLAR PANELS AND BATTERIES CALCULATION, solar photovoltaic panel calculation , solar panel and battery calculation, battery rating / backup time, inverter / UPS rating, load and power required in watts. with Circuit, wiring diagrams and worked examples.


If you choose this article related to the installation of solar panels and calculation of solar panels, you will be able to:

✔ To calculate solar panel rating

✔ To calculate the rating of the batteries for the solar panel system

✔ To calculate the backup time of the batteries

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✔ To calculate the charging current of the batteries

✔ To calculate the charging time of the batteries

✔ To calculate charge controller rating

✔ How many watts of solar panel do we need?

✔ How to select the right solar panel for home

✔ UPS/Inverter rating for load requirements and much more…

Solar panel installation: step-by-step procedure with calculation and examples

Well now let’s start with the calculation of solar panels :

Suppose  we are going to install a solar power system in our house for a total load of 800W where the required battery backup time is 3 hours (you can use it as it is for sample calculation only)

Load = 800 Watts

Backup time required for batteries = 3 hours

what we need to know

1. Inverter/UPS classification 

2. No of batteries for backup power 

3. Hours of battery backup

4. parallel or series connection of batteries 

5. Charging current for batteries 

6. Charging time for batteries 

7. No solar panels required 

8.Charge controller classification 


Inverter / UPS Rating:

Inverter/UPS rating must be greater than 25% of full load (for future load as well as to account for losses)

800 x (25/100) =  200W

Our load + 25% extra power = 800 + 200 =  1000 watts

This is the rating of the UPS (Inverter), that is, we need a 1000W UPS / Inverter for the installation of the solar panel according to our needs (according to the calculations)

What is the number of batteries needed?

Now the required backup time of the batteries in hours = 3 hours

Suppose we are going to install  100 Ah, 12 V batteries  ,

12V x 100Ah = 1200Wh

Now for a battery (i.e. the backup time of a battery)

1200Wh / 800W = 1.5 hours

But our required backup time is 3 hours.

Therefore, 3 / 1.5 = 2 → that is, we will have to connect two (2) batteries each of 100 Ah, 12 V.

Battery Backup Hours

If the number of batteries is indicated and you want to know the backup time for these batteries, use this formula to calculate the battery backup hours.

1200Wh x 2 batteries =  2400Wh

2400Wh / 800W =  3 hours.

In the first scenario, we will use the 12V inverter system, therefore we will have to connect two (2) batteries (each 12V, 100 Ah) in Parallel. But a question posed next:

What is the battery charging time?

Series or parallel connection of batteries

Why batteries in parallel, not series?

Because this is a 12V inverter system, so if we connect these batteries in series instead of parallel, then the batteries evaluation becomes V1  +  V2  =  12V + 12V = 24V, while the Current rating would be same 100Ah.

Good to know  : In series circuits, the current is the same in each wire or section, while the voltage is different, i.e. the voltage is additive, eg V1 + V2 + V3….Vn.
This is why we will connect the batteries in parallel , because the batteries Voltage (12V) stays the same, while their Ah (Ampere Hour) rating will increase. i.e. the system would become = 12V and 100Ah + 100Ah = 200Ah
Good to know  : In parallel connection, the voltage will be the same on each cable or section, while the current will be different, i.e. the current is additive, for example I1 + I2 + I3 … + In

Now we will connect 2 batteries in parallel (each 100 Ah, 12 volts)

that is, 2 batteries of 12V, 100Ah will be connected in Parallel

= 12V, 100Ah + 100Ah = 12V, 200 Ah (parallel)

charging current for batteries

Now the  charge current required  for these two  batteries  .

(Charging current should be 1/10 Ah batteries)

200Ah x (1/10) =  20A

Required charging time for the battery

Here is the formula for Charge Time of a Lead Acid Battery.
Battery charging time = Battery Ah / Charging current
T = Ah / A

For example, for a single 12V, 100Ah battery, the charging time would be:

T = Ah / A = 100 Ah / 10 A = 10 hours (Ideal case)

due to some losses, (it has been observed that 40% of the losses occurred during battery charging), in this way, we take 10-12A of charging current instead of 10A, in this way, the time charge required for a 12V, 100 Ah battery would be:

100 Ah x (40/100) = 40 (100 Ah x 40% of losses)

the battery rating would be 100 Ah + 40 Ah = 140 Ah (100 Ah + losses)

Now the  charging current required for the battery  would be:

140Ah / 12A = 11.6 hours.

Solar panels calculation How much solar panels will we need?

Now we need the number of solar panels required for the above system as shown below.

Scenario 1: DC charging is not connected = battery charging only

We know the famous power formula (DC)

P = VI ………… (Power = Voltage x Current)

Putting the values ​​of the batteries and the charging current.

P = 12V x 20A

P =  240 watts

this is the required wattage of the solar panel (only to charge the battery, and then the battery will supply power, that is, the direct load is not connected to the solar panels)


240W / 60W = 4 solar panels

Therefore, we will connect 4 solar panels (each 60W, 12V, 5A) in parallel.

fig: Circuit diagram for above calculation for solar panel installation (solar panels for battery charging only)

The above calculations and system was just to charge the battery (and then the battery will supply power) to the AC electrical appliances, which will be powered via the inverter and DC loads via the charge controller (via charged batteries )

Scenario 2: DC charging is connected as well as battery charging

Now let’s say there is a 10A load connected directly to the panels via the inverter (or it can be a DC load via the charge controller).

During the day, the solar panel provides 10A to the directly connected load + 20A to the battery load, that is, the solar panels charge the battery and provide 10A to the load as well.

In this case, the total current required (20 A for battery charging and 10 A for directly connected load)

In this case above, the total required current in Amps,

20A + 10A = 30A

Now, I = 30 A, so Power is required

P = V x I = 12V x 30A = 360Watts

That is, we need a 360 W system for the system explained above (this is for direct charging and battery charging)

Now the number of solar panels we need

360 / 60W =  6

Therefore, we will connect    solar panels (each 60W, 12V, 5A)



fig: Circuit diagram for above calculation for solar panel installation (solar panels only for battery charging + direct connected load)

Charge Controller Rating

As we have calculated above, the charging current for 200 Ah battery is 20-22 Amps (22A for battery charging + 10A for DC direct charging), therefore we can use a load approximately  30-32 amps.

Note: The above calculation is based on the ideal case, so it is recommended to always choose a larger solar panel than we need, as some losses occur during battery charging through the solar panel, as well as the sunlight is not always ideal.

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