Charging A 100ah 12v Battery: Size Of Panels, How Many Panels, & How Long To Charge

If you are just getting into portable and off grid solar power, it can be amazing to learn how solar can help you in various activities.

From camping, to RV’ing, to your boat and marine needs – there are wide ranging applications for portable solar panels.

However, if you are new to it all, it can be very confusing to look at measurements like Ah, Watts, voltage etc. and know where to start.

This is an informational guide to help you get started – so you know rough requirements and costs for your system.


Charging A 100ah Battery: Size Of Panels, How Many Panels, & How Long To Charge

So, say you have a 12 volt 100Ah battery. The follow is a guide only of the size of solar panels, how many panels, and how long the charge will take…


What size solar panel to charge a 100ah battery?

If you refer to the calculation below, of ‘how many solar panels do I need’, we use the example of a 100Ah 12 volt battery.

With 6 hours of sunlight that we estimate we get in the day, and by overrating the solar system by 20%, we estimate we will need 240 Watts total power.

For 240 Watts, you might try some of the following sizes of solar panels and combinations:

2 x 120 Watt panels

3 x 100 Watt panels

5 x 50 Watt panels


You’d want to make sure the panels you get can be stacked together, and that your solar controller is rated for the total wattage and Amperage you have running through your system.

Most standard PWM solar controllers (the cheaper ones compared to the MPPT controllers) are rated for 400 Watts, but check the controller you get for it’s ratings.


How many solar panels to charge a 100Ah battery?

Number of panels =

Battery Ah x Battery Voltage = Watt Hours

Watt Hours / Hours Of Sunlight You Get A Day = Total Watts Of Solar Panels You Require – you may choose to have several panels that make up this Watts number, or 1 panel

Total Watts x 10 to 20% = Actual Watts Of Solar Panels You Require (due to sunlight being inconsistent, panels not functioning at stated capacity and other inefficiencies like solar controllers not functioning at capacity, it’s recommended you overcompensate your requirements by 10 to 20 %)



We have a 100 Ah 12 volt battery.

100Ah x 12 = 1200 Watt hours

1200/6 hours of sunlight in a day = 200 Watts

200 Watts x 20% = 240 Watts

So, if I got two 120 Watt panels, or 3 100 Watt panels, I’d have enough solar panels for this battery, as an estimation


How long does it take to charge a 12v 100ah battery?

Time of Charge =

Total Wattage of Your Panels / the voltage your system is running at (usually the voltage of your battery) = Amps delivered

Ah of your battery / Amps delivered = time it takes to charge



We have the same 100 Ah 12 volt battery, and we are using 2 x 120 Watt panels

240 / 12 = 20A

100Ah / 20A = 5 hours to charge the battery (roughly – actual time taken does depend on variables like sun on the day, and efficiencies of your system)



These are estimation calculations only, to help you get started.

You should always check with manufacturers, and installation professionals as to your final requirements, and certainly before you install a portable solar system

Charging may also be affected by other factors like if your battery is deep cycle or a starter battery, lead acid or lithium ion, number of batteries in the battery bank etc.


More Resources On Calculating Battery & Solar Panel Requirements

These resources give more calculations and information on battery and solar panel requirements:

How [To] Size Solar Battery Bank and Solar Panels – How Many Batteries? How Many Solar Panels?

Solar Panel Power Design & Matching With Batteries

4 thoughts on “Charging A 100ah 12v Battery: Size Of Panels, How Many Panels, & How Long To Charge”

  1. I have a 100 w solar panel 18 v , with a 500w inverter, what sixe of battery do i need to power my led tv 25 w, decoder 30 w and my phones + laptop

    • First of all the pannel is small but taking the size you’ve named let’s continue the way you wantin Simple terms pannel size diveded by two =battery size (100/2) =50AH
      For efficiency battery size x80 %=40AH
      So the battery size efficiently is 40AH
      But the inverter is big it won’t last long on that battery
      You would better use 150w inverter
      Coz inverter sise =battery AH x3 =120w or approximately 150w
      Thank you

  2. I have 4 batteries 100ah and how many panels do need to charge them and what type of connection is needed parallel or series connection?


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