How Long To Charge 12V Battery With 100-Watt Solar Panel? (+ Calculator)

Why not charge 12V batteries with 100-watt solar panels? Every house has some batteries, be it the small AAA and AA batteries, small accumulators, or even car batteries. The key question is this:

How long does it take to charge a 12V battery with 100-watt solar panels?

Here’s the short (and generalized) answer: It can take anywhere from 22.8 minutes to 76.8 hours. It’s useful to know when the batteries are fully charged to 100%. That’s how you know when to stop charging them.

22.8 minutes to 76.8 hours is quite a broad range. Luckily, there are only two factors that determine how long for a 100-watt solar panel to charge a 12V battery. These are:

  • Battery capacity (primary factor). Obviously, the most important question is what size is the 12V battery you are charging with the 100-watt panel. Battery capacity is measured in ampere-hours (Ah); small 1,000 mAh AAA takes about 22.8 minutes to charge and big 120 Ah batteries take about a good 2 days (46.08 hours, to be exact) to charge with a small 100-watt battery.
  • How sunny it is (secondary factor). The more sun you get, the quicker will the 100-watt solar panels be charged. We measure the amount of sunlight (solar irradiance) in ‘Peak Sun Hours’. Depending on where you live, you get anywhere from 4 (New York) to 6 (California) peak sun hours per day; this is an annual reported average.

Will a 100-watt solar panel charge a 12-volt battery?

It most definitely will. The great thing is that we have the specified voltage (12V). With that, we can calculate how long does it take to charge any 12V battery.

Here’s how we can do that:

  1. First, we need to express the battery capacity from ampere-hours (Ah) to watt-hours (Wh). We can do that because we know that the voltage is 12 volts. Example: 1,000 mAh 12V AAA battery contains 12 Wh of energy. Big 120 Ah contains 1,440 Wh of energy.
  2. Second, we need to determine the electrical power output of 100-watt solar panels in watt-hours (Wh). Example: On average, a 100W solar panel produces 0.375 kWh of electricity per day. That is equal to 375 Wh per day and, on average, 31.25 Wh per hour.

We will show how you yourself can determine how long to charge a 12V battery with a 100-watt solar panel. To help you out, we have also designed a calculator (insert battery size in Ah and get hours of charging to 100%) that makes the 12-volt battery charging time with a 100-watt solar panel much easier (located at the end of the article).

Here’s how the calculator looks like:

How Long To Charge 12V Battery With 100-Watt Solar Panel

You will also find a table with calculated charging times for different sizes of 12V batteries.

Note: Do keep in mind these are theoretical estimates that include averages and presume all-things-equal conditions.

Converting 12-Battery Capacity To Watt-Hours (Wh)

To figure out how long will it take for a 100-watt solar panel to charge your battery, we need to convert the size of the battery from Ah to Wh. We need Wh because the output of solar panels is also expressed in Wh; that’s how we will compare apples-to-apples.

The equation that converts Ah to Wh is this one:

Watt-hours (Wh) = Ampere-hours (Ah) × Volts (V)

We know that we have a 12-volt battery. That makes this conversion easy. In order to covert Ah to Wh, we just need to multiply Ah with 12 (because we have a 12V battery). Here is an example that illustrates how you can do that:

Example: Let’s say we have a 20 Ah 12V battery. Here’s how you calculate Wh:

Watt-hours (Wh) = 20 Ah × 12V = 240 Wh

Basically, a 20 Ah battery with 12-volt output contains 240 Wh of electrical energy.

Determining Average Output Of 100-Watt Solar Panels

100-watt solar panels are considered small solar panels. They are, however, rather useful when charging batteries. To determine how long does it take to charge 12V batteries, we need to calculate the output of 100W solar panels.

Output, obviously, changes depending on sunlight (solar irradiance). When the sun is shining, they can generate more than 100W of power and at night they generate about 0W of power. In the winter, they generate less power than in the summer. With so many variables, it makes sense that we take an annual average output of 100-watt solar panels.

Here’s how we can calculate how many Wh a single 100-watt generates per day (on average):

100W Solar Output (Wh/Day) = 100W × Peak Sun Hours × 0.75

We know we have 100W panels, the average system losses are about 25% (hence the 0.75 factor), and that on average we get 5 peak sun hours worth of sunlight. In colder climates, you might get 4 hours, in hotter areas, you might get 6; 5 hours is a good average here.

Let’s plug the ‘5’ in the equation and we get:

100W Solar Output (Wh/Day) = 100W × 5h × 0.75 = 375 Wh/Day

That means that, on average, a 100-watt solar panel produces 375 Wh of electricity per day. That’s 31.25 Wh per hour.

How Long Does It Take To Charge 12V Battery With 100-Watt Solar Panel?

Now that we know that an average 100-watt solar panel will generate 31.25 Wh every hour, we can calculate how long it will take to charge any 12V battery.

Let’s solve 2 examples. After those, you will find a table with calculated charging times for 12V batteries with sizes ranging from 1000 mAh (1 Ah) to 200 Ah.

Example 1: How long to charge a 50 Ah 12V battery with a 100-watt solar panel?

First, we convert 50 Ah to Wh. That means that we have to calculate this 50 Ah × 12V = 600 Wh.

Alright, we know that 100-watt panels generate, on average, 31.25 Wh every hour. Here’s how we calculate how many hours does it take for a 100-watt solar panel to charge a 50 Ah 12V battery:

Charging time (50 Ah) = 600 Wh / 31.25 Wh per hour = 19.2 hours

It takes 19.2 hours to change the 50 Ah 12V battery with 100-watt solar panels.

Example 2: How long to charge a 120 Ah 12V battery with a 100-watt solar panel?

This is a big battery. 120 Ah battery with a 12-volt output contains 1440 Wh of electrical energy.

Let’s calculate the charging time:

Charging time (120 Ah) = 1440 Wh / 31.25 Wh per hour = 46.08 hours

There you have it; it takes 46 hours and about 5 minutes (a bit more than 2 days) to charge a big 120 Ah 12V battery with 100-watt panels.

Table With Calculated Charging Times For 12V Batteries With 100-Watt Solar Panel

Battery Capacity (in Ah) Charging Time (in hours)
1000 mAh (1 Ah) 0.38 hours
2 Ah 0.77 hours
3 Ah 1.15 hours
4 Ah 1.54 hours
5 Ah 1.92 hours
6 Ah 2.30 hours
7 Ah 2.69 hours
8 Ah 3.07 hours
9 Ah 3.46 hours
10 Ah 3.84 hours
15 Ah 5.76 hours
20 Ah 7.68 hours
25 Ah 9.60 hours
30 Ah 11.52 hours
35 Ah 13.44 hours
40 Ah 15.36 hours
45 Ah 17.28 hours
50 Ah 19.20 hours
55 Ah 21.12 hours
60 Ah 23.04 hours
65 Ah 24.96 hours
70 Ah 26.88 hours
75 Ah 28.80 hours
80 Ah 30.72 hours
90 Ah 34.56 hours
100 Ah 38.40 hours
110 Ah 42.24 hours
120 Ah 46.08 hours
130 Ah 49.92 hours
140 Ah 53.76 hours
150 Ah 57.60 hours
160 Ah 61.44 hours
170 Ah 65.28 hours
180 Ah 69.12 hours
190 Ah 72.96 hours
200 Ah 76.80 hours

12V Battery Charging Time Calculator (With 100-Watt Solar Panels)

Here is an easy-to-use calculator that helps you determine the charging time. You simply insert the 12V battery capacity in Ah, and you will get an estimate of how many hours does it take for a 100-watt solar panel to charge the battery to 100%:

0.00 hours

Battery Charging Time

Hopefully you can use both the table and calculator to adequately estimate how long will it take for 100-watt solar panels to charge your 12V battery.

3 thoughts on “How Long To Charge 12V Battery With 100-Watt Solar Panel? (+ Calculator)”

  1. I have 20 ( 120ah 12V).
    4 connected in series.
    120ah X 48V

    The series connected in parallel
    120ah X 48V X 5 = 28,800Wh

    If I’m to charge this in 6 hours,
    How many 400W solar panels will I need.

    • Hi Frank, alright, you are looking for a solar system that can charge 28.8 kWh batteries in 6 hours. Let’s say that these 6 hours are 6 peak sun hours, and that the solar system has 25% losses (quite realistic). Here is how you can calculate the solar system size: 28.8 kWh/(0.75 * 6 hours) = 6.4 kW. So, you would need a 6,400W system.

      If you want to construct it with only 400W solar panels, you will need 16 400W for a 6,400W system. Hope this helps.


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