Can a 100-Watt solar panel charge a 100ah battery?

By Matthew Joseph Nandirio •  Updated: 05/31/22 •  7 min read

Yes, of course, you can, but you will need three units of 100W solar panels. You have to use 3 x 100W solar panels if you want to charge a 100ah battery. Only a single unit of 100W solar panel can’t charge a 100 amp-hour battery. The reason is, typically, it takes 240 watts of solar panels to charge.

As you know, without having the right solar panel and the appropriate battery combination, you can’t get the power you require.

Therefore, before you set up the solar system for your RVs, motorhomes, boats, caravans, etc., you need to determine how much solar power you need.

And to figure out the required power, you have to calculate the solar panel size and the battery capacity.

So, before going through the article, let’s look at some terminological calculations that will help you know how many solar panels you will require to charge the battery.

Terminological Calculation of Solar Panel and Battery Capacity

Solar panel size

The sizes of Solar panels are usually measured in Watts. This Watts is referred to as a unit of power. Here, you must remember that the watt is a consistent matter that won’t change.

However, this is not the case with current or voltage. The amount of current or voltage can be changed.

Well, to figure out the solar panel size or watts, here is the formula-

Watts = Amp-hour (ah) of the battery x battery voltage (V/volt)

Solar panel Watts = Watt-hours / available sunlight hours

Required solar panel Watt sizes = Solar panel Watts + extra solar panel power of about 20%

Battery capacity

The battery capacity is measured in Amp-hours. And the measurement of Amp-hours is usually based on the total availability of your battery usage of about 20 hours.

To measure Amps of the battery, the formula is-

Amps = Total size of solar panel watts/voltage of the battery

Required charging time = Battery capacity (Amp-hour)/ Amps

How many watts of solar panels do you need to use to charge a 100 amp-hour battery?

All right, now we will calculate how many watts of solar panels it takes to charge a 100ah battery by using the above equation.

So, before calculating, let’s assume that the available sunlight hours are 6 hours, and the battery voltage is 12V. Technically, 100 watts solar panels are designed for charging 12V batteries.

Moreover, around 20% of the energy from the total solar power gets lost during the daytime. Therefore, you should have to add extra 20% watts while calculating.

So, according to our formula-

  • Watts = Amp-hour (ah) of the battery x battery voltage (V/volt)

                          =100 ah x 12V

                          = 1200 watts

  • Solar panel Watts = Watt hours / available sunlight hours

                                 = 1200 W / 6 hours

                                 = 200W solar panel

  • Required solar panel Watt sizes = Solar panel Watts + extra solar panel power of about 20%

                                                          = 200W + 20% extra solar panel watts

                                                          = 240W

So, now you can see that charging a 100Ah battery takes 240W technically. However, you can also follow different combo sizes of the solar panel watts to charge your 100ah battery.

The combo sizes rating of the solar panel watts is provided below in a chart-

Battery capacity Total solar panel watts Rating of solar panel efficiency (no. of panel unit x solar panel watt)
  100ah 240 watts 1 x 240W solar panel
  100ah 240 watts 2 x 120W solar panel
  100ah 250 watts 5 x 50W solar panel
  100ah 300 watts 3 x 100W solar panel

How long does it take to charge a 100 amp-hour battery?

Here, the charging time depends on the availability of sunlight. Usually, it takes 5-6 hours to charge a 100 amp-hour battery. However, if sunlight is available for a few hours, the calculation of the charging time will be different.

Well, to calculate the charging time, I will use the formula mentioned above-

So, according to the formula, first, you need to figure out the amps of the battery. Let’s assume the solar panel watts are 240W and the battery’s voltage is 12V.

                          = 240W solar panel watts / 12V voltage

                          = 20 amps

  • Now, the required charging time = Battery capacity (Amp-hour)/ Amps

                                                                          = 100ah battery capacity / 20 amps

                                                                          = 5 hours

How many amps does a 100-Watt solar panel generate?

If you want to calculate the amps of your battery that a 100W solar panel generates, you need to divide the solar panel power in watts by the battery voltage in volts. So, the equation will be the same as above.

As you may know, a 100W solar panel usually charges the battery in 12V battery voltage. So, the amps will be-

  • Amps = Total size of solar panel watts/voltage of the battery

                          = 100W solar panel / 12V battery voltage

                          = 8.33 amps

So, with a 12V battery feeding power, your 100W solar panel will produce 8.33 amps per hour.

However, when measuring the output, the voltage of your battery will be 18V instead of 12V. So, in that case, the amps will be 5.5 amps (using the same equation- 100W/18V = 5.5 amps).


So, hopefully, after reading the article, you have now figured out whether a 100w solar panel can charge a 100ah battery or not.

Technically, the answer is no, as it requires 240 Watts to charge a 100ah battery. But, if you use the combo solar panel sizes, you will need a three-panel unit of 100 Watts. So, considering this fact, you can charge a 100ah battery using a 100W solar panel.

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Matthew Joseph Nandirio

Matthew Joseph Nandirio is the Founder of walkingsolar. After graduating from the University of Houston in 2002, matt started working as an electrical and electronic engineer for several multi-national solar energy companies. He has a wide range of experiences including solar system requirement analysis, planning, maintaining, debugging and even solar device development through research. He now shares his 20 years of expertise through his articles on the walkingsolar website. Further, he is also the author of two books on Solar Technology, “Solar Power for Villages” and “DIY Solar System for Dummies”.

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