How to size solar panel for 12V/200Ah battery?

A 450-watt solar panel can charge 12V/200 Ah battery to complete in a day.

Solar panels and batteries make a great energy combination when you want clean energy as a backup.

Solar panels work during the day by capturing sunlight and producing current.

This current goes inside the battery through the charge controller.

The charge controller regulates its charging and prevents the backflow of current from the battery toward the solar panels.

The battery stores it in the form of chemical energy.

It is potential energy that can be used when required.

Let us understand some terms before sizing the solar panel for the battery.

What is the P(max) of a Solar Panel?

P(max) is the maximum power any solar panel can produce under Standard Test Conditions (STC).

These conditions are:

• When the sunlight intensity is 1000 W/m²
• Air mass index of 1.5
• Wind velocity of 1 m/s at 10 meters above the ground
• Cell temperature 25ᴼC

For example, a solar panel with P(max) of 100 watts will produce this amount of power when the above set conditions are met.

However, these values keep changing.

Which means you do not get 100 watts always.

You get less than 100 watts.

These changing environmental conditions make the solar panel to lose some power in the form of:

• Temperature related losses
• Shading loss
• Dirt loss
• Conversion loss
• Also, Age-related losses

When I add all these losses, they can eat up to 30% of P(max).

Phew!

I know it is a good amount of loss.

Still, a solar power system is financially attractive.

After compensating for the losses, a 100-watt solar panel will give 70 watts {(100 – 30% of 100) or (70% of 100 watts)}.

In this case, the multiplying factor is 0.7

The Peak Sun Hours (PSH)

The value of Peak Sun Hours (PSH) in your area decides the amount of energy a solar panel will produce.

A good amount of sunlight makes solar panels produce more energy.

The sun and the earth are in relative motion since their inception.

Therefore, the amount of sunlight throughout the day keeps changing. On a clear sunny day, the sunlight intensity gradually increases from sunrise and reaches a maximum around noon.

Thereafter, it starts decreasing till sunset.

Whenever we see a variation in values, we work on averages.

We talk about the average PSH in a day.

1 PSH = 1000 W/m²

It could be 4 PSH, 5 PSH, 6 PSH, or more depending on the location of the region, its climate, and the weather.

Let me share with you a simple mathematical formula for finding the size of the solar panel which can charge 12V/200 Ah battery.

Solar Panel Size (watts) = {(Battery volatge x capacity) / (PSH x loss factor)}

Before, finding the panel size let us understand some terms related to the battery.

Battery Capacity and its DOD

The amount of charge any battery can store is defined by its capacity.

It is represented in Ampere-hours.

A 12 volts/200 Ah battery has a capacity of 200 Ampere-hours.

The battery capacity becomes the base for sizing the right solar panel.

Another aspect that we should know is the depth of discharge of the battery.

It is the amount of charge that you can withdraw safely from the battery without impacting its useful life.

In the case of lead-acid batteries, the DOD is 50% to 80%.

I am taking 50% DOD in this example for deciding the size of the solar panel.

It means I can deplete my battery at 50% of its capacity.

And I need a solar panel of that size which can again fill it by 50%.

I need to replenish 100 Ah (50% of 200 Ah) every day.

My effective battery capacity after considering DOD becomes 100 Ah.

Let us recall the solar panel size formula.

Solar Panel Size (watts) = {(Battery volatge x capacity) / (PSH x loss factor)}

Let us see how many variables we have found:

• Battery capacity = 100 Ah (after DOD)
• Loss factor = 0.7
• Battery voltage = 12 volts.
• PSH =?

We are left with PSH value.

As discussed, it depends on your location, season, climate, and weather conditions.

Sizing the solar panel for 12V/200 Ah battery

Let us assume this setup is in New Delhi (India) with an average PSH of 5.35 per day.

You see that PSH values are different in different months.

Even Day 1 of say month of April will have a different PSH from Day 2.

However, too much variation will lead to confusion.

When I take 12 months average, the daily PSH is 5.35.

Putting this value in the formula, we get

Solar Panel Size (watts) = 12 volts x 100 A-h/ (5.35 x 0.7)

= 321 watts

Therefore, you need 321 watts panel to charge your battery full which is 50% depleted.

Let us size based on monthly daily averages of PSH. Let me show you in the form of a table:

You see that January is the month with the lowest daily PSH value of 3.93.

A 321-watt solar panel would not be able to charge your battery in a day.

Therefore, we should size the solar panel based on the lowest PSH value of the region.

In this case, it is 3.93 PSH.

And based on this value the solar panel size is 436 watts.

This can charge your 12 volts/200 A-h battery even in the lowest PSH value in New Delhi.

A 436-watt is a high-power solar panel.

We usually do not find this power rating in 12 volts.

These solar panels come with a nominal voltage of 24 volts or more.

So, there is a large voltage difference between the solar panel voltage and battery voltage (12 volts).

This voltage difference creates a high probability of power loss.

To overcome this, the MPPT solar charge controller is used between the two.

It detects this voltage gap and captures the maximum power from the solar panel in real time.

And feed to the battery.

In this way, it avoids power loss and the battery gets the maximum power which charges it in less time.

Conclusion

A 450-watt solar panel (round off 436 watts to 450 watts, as you do not get a 436-watt solar panel in the market) can charge the 12V/200 Ah battery with 50% DOD in a day.

You need a right-sized MPPT solar charge controller in between.

You can use PWM solar charge controller when there is no voltage difference between the solar panel’s nominal voltage and battery voltage.

Obviously, a right tilt is necessary for a solar panel to capture the maximum sunlight and charge the battery effectively.

Thank you

Yash

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