What Size Solar Panel To Charge 120ah Battery? + Calculator

Selecting the right size solar panel, charge controller, and wire size will allow you to recharge your 120Ah battery in desired hours.

This is going to be a complete guide on charging a 120ah battery with solar panels.

You’ll learn:

Solar Panel Size Calculator For Battery

Battery Capacity (Amp-hours)
Battery Voltage
Battery Type
Battery Depth of Discharge (DoD)
Charge Controller Type
Desired Charge Time (Peak sun hours)

Note! Use this solar battery charge time calculator if you already have a solar panel in mind and want to know how long it will take to charge your battery.

Calculator Assumptions: 

  • Lead-acid Battery Charge efficiency rate: 85%
  • AGM Battery Charge efficiency rate: 85%
  • Lithium (LiFePO4) Charge efficiency rate: 99%
  • PWM charge controller: 80% efficient
  • MPPT charge controller: 98% efficient
  • Solar Panels Efficiency during peak sun hours: 80% (meaning a 100 watt solar panel will produce 80 watts during peak sun hours)
  • Charge controller is the only load connected with the battery

How To Use The Calculator?

Enter the following info into the calculator ( located at the top of this page )

  • Battery Capacity: Enter your battery capacity in amp-hours (Ah). in this case it’ll be 120
  • Battery Volts: is this a 12v, 24v, or 48v battery?
  • Battery Type: Select the battery type, is this a lead-acid, AGM, or lithium-ion (LiFePO4) battery?
  • Battery Depth of discharge DoD: is the percentage of the battery that has been discharged relative to the total battery capacity. for half discharged battery you’d enter 50, (if the battery is fully discharged which you can achieve on lithium battery, ENTER 100)
  • Charge Controller Type: Are you using a PWM or MPPT charge controller to charge the battery? 
  • Desired Charge time (in peak sun hours): How fast would you like to charge your battery or how many peak sun hours your location receives? (keep reading i’ll explain how you can calculate the number of peak sun hours)  

After entering the information, click “Calculate” button to find the required size for a 200ah battery. 

What Size Solar Panel To Charge 120Ah Battery?

Suppose you have a 12v 120ah lead-acid battery with 50% depth of discharge and an MPPT charge controller. You want to recharge your battery in one day (during summer days) and your location receives 7 hours of peak sunlight 

What Size Solar Panel To Charge 120Ah Battery?

You’d need about 140 watts of solar panels to fully charge a 12v 120ah lead-acid battery from 50% depth of discharge in 7 peak sun hours using an MPPT charge controller.

Read the below post to find out how fast you can charge your battery.

Related Post: Guide: Maximum Charging Current & Voltage For 12v Battery

if you’re a noob, i would suggest you watch this full video to get a complete understanding of electricity basics.

Peak Sun Hours

Before explaining peak sun hours and why to use them, first, let’s see how solar panels are rated.

“ Solar panels are designed to produce their rated wattage output under standard test conditions – STC ”

STC includes: 

  • 1000 watts per meter square (1kw/m2) of sunlight intensity hitting the solar panels directly
  • 25oC temperature 

So to calculate the sunlight intensity we use peak sun hour as a reference. 

  • 1 peak sun hour = Average of 1kw/meter2 of sunlight intensity for one hour.

This number will depend on the location to location. To calculate how many peak sun hours your city receives use this PVwatt solar calculator.

Step 1: Enter your full address of just a city name & Click GO

Step 2: Click on results

How many peak sun hours in florida?

Now you’d be able to see the total amount of solar radiation your location receives per day (from month to month)

Note! it would have to place my solar panels all day long under sunlight to get those total solar radiations per day. 

What Size Solar Panel To Charge 120Ah Battery? – Chart

Charge TimeBattery TypeRequired Solar Panel Size
5 peak sun hoursLead-acid200 watts
10 peak sun hoursLead-acid100 watts
15 peak sun hoursLead-acid65 watts
20 peak sun hoursLead-acid50 watts
25 peak sun hoursLead-acid40 watts
5 peak sun hoursLithium (LiFePO4)350 watts
10 peak sun hoursLithium (LiFePO4)170 watts
15 peak sun hoursLithium (LiFePO4)120 watts
20 peak sun hoursLithium (LiFePO4)90 watts
25 peak sun hoursLithium (LiFePO4)70 watts


  • You need about 200 watts of solar panel to charge a 12v 120ah lead-acid battery from 50% depth of discharge in 5 peak sun hours using an MPPT charge controller
  • You need about 350 watts of solar panel to charge a 12v 120ah lithium-ion battery from 100% depth of discharge in 5 peak sun hours using an MPPT charge controller

How To Calculate Number Of Solar Panels Required For 120Ah Battery?

This is the most accurate way to calculate the required number of solar panels to charge any size battery.


1- Multiply the battery amp-hours (ah) by battery volts to convert the battery capacity into watt-hours (Wh). let’s suppose you have a 12v 120ah battery

Battery capacity in Wh = 120 * 12 = 1440wh

2- Multiply the battery watt-hours to the battery depth of discharge limit.

lead-acid, AGM, and gel batteries come with a 50% depth of discharge limit, and lithium batteries with 100% DoD. let’s suppose you have a lead-acid battery

Discharged Battery capacity in Wh = 1440 * 0.5 = 720wh

3- Multiply the battery capacity after DoD by 1.15 for lead-acid and 1.01 for lithium battery ( Battery charge efficiency rate, lithium: 99%; Lead-acid: 85%; )

Power required to charge the battery = 720 * 1.15 = 828wh

4- Divide the battery capacity value (after adding charge efficiency factor) by the desired number of charge peak sun hours. let’s suppose you want to recharge your battery in 7 peak sun hours

Solar power required in peak sun hour = 828 / 7 = 118 watts

5- Multiply the solar power required in peak sun hour by 1.2 if you’re using a PWM charge controller and 1.02 for an MPPT charge controller ( charge controller efficiency, PWM: 80%; MPPT 98%; ). let’s suppose you’re using an MPPT charge controller

Solar panel power required after charge controller = 118 * 1.02 = 120 watts

6- Add 20% to the solar power required after controller to cover up the solar panel inefficiency

Solar panel size required = 120 + 20% = 144 watts

What Size Charge Controller For 120ah Battery?

Connecting solar panels directly to the battery can damage the battery cells due to the fluctuations in the solar panel voltage output.

The charge controller or regulator is the device that regulates the current and voltage coming from the solar panels to charge the battery

The capacity of charge controllers is measured in Amps (A). Which determines how many amps or current it can candle

Formula to calculate the size of a charge controller

Charge Controller Size = (Solar panels capacity (watts)/Battery Volts) + 25%

An extra 25% is added for safety reasons because at some point your solar panel will give output beyond its full capacity but it will last for a few seconds so to handle those fluctuations 25% is mandatory to add

let’s suppose you have a 300-watt solar panel to charge your 12v 120ah battery

300 / 12 = 25
25 + 25% or 25 * 1.25 = 31 

You would need a 30A Charge controller with 300-watt solar panels to charge your 12v 120Ah battery in 5 hours

Victron 30A MPPT charge controller
Check Price (Amazon Link)

Renogy 30A MPPT charge controller
Check Price (Amazon Link)


This guy has explained soo well about the PWM vs MPPT charge controller and which one to use and when. ( if you’re a noob i suggest you watch this video! )

– PWM charge controller

will only decrease the voltage coming from the solar panels and will not increase the amps (current).

which as a result will cause power losses and which makes the PWM charge controllers 20-30% less efficient.

The formula to power is… ( Watts = amps * volts )

Remember: a 12v solar panel will produce about 18 volts under direct sunlight conditions… and the amps will be lower.

Note! If you’re using an PWM charge controller the voltage of solar panel and battery should be the same. ( eg. 12v solar panel for 12v battery and 24v solar panel to charge a 24v battery ). Otherwise you’ll experience a huge power loss. If you have different voltage solar panels and battery then use an MPPT charge controller.

– MPPT charge controller

An MPPT charge controller will not only decrease the voltage coming from the solar panel to charge the battery but will also increase the amps to cover up the losses.

which makes them 98-99% more efficient than a PWM charge controller.

Pro Tip: if your solar panel system is higher then 100 watts then i would suggest using an MPPT charge controller.

Related Post: How Many Watts Can A Charge Controller Handle?

What Size Wire To Charge 120ah Battery?

wire gauge chart
What size wire for solar panels chart

You would need the right size cable to handle the current coming from the solar panels to the charge controller and then from the charge controller to the battery

In the above chart, the size of the wire is mentioned as AWG size and how many amps or current it can handle is mentioned on the very right side

To make things more simple use this formula

Amps = Watts/Battery Volts 

For example:

  • 300 watts/12v = 25A
  • 200 watts/12v = 16.6A

You need about 8 AWG wire size to charge a 12v 120Ah battery with 300 watts of solar panels

120ah Battery Capacity In Watts

  • 12v 120Ah battery is equal to 1440 watts or 1.4kWh
  • 24v 120Ah battery is equal to 2880 watts or 2.8kWh
  • 48V 120Ah battery is equal to 5760 watts or 5.7kWh

Video – How To Built a Solar Power System To Charge a Battery

Watch this beginner-friendly video on how to build your first solar power system to change a battery and run AC appliances

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I hope this guide was helpful to you. still, have any confusion? LEAVE A COMMENT or contact us. Thank You!

I'm an experienced Sales Consultant with a demonstrated history of working in the renewables and environment industry. Skilled in Photography, Customer Service, and Sales.