How long does a 100w solar panel take to charge a 100 amp-hour battery?

By Matt •  Updated: 05/29/22 •  11 min read

In short, a 100w solar panel usually takes 12.5 hours (approx.) to fully charge a 100 amp-hour battery if you use a solar charge controller such as an MPPT. 

As you are using a solar panel, the charging time will depend on the weather condition, the battery type, and the state of the battery. Therefore, the total battery charging time varies, and thus no one can exactly estimate the accurate charging time.

Solar Inverter
Solar Inverter

Yet, you can make some assumptions by assuming the charger’s efficiency in charging the battery, and thus by calculating, you can give a reasonable answer. Some solar panel users report that it takes around 14 hours to charge a 100 amp-hour battery based on the above facts.

In this guideline, let’s see how much time a 100w solar panel requires for charging a 100 amp-hour battery.

How long does it require to charge a 12v 100 amp-hour battery with one 100-watt panel?

To charge a 12-Volt 100 amp-hour battery with one 100-watt panel, the battery requires around 12.5 -14 hours depending on the sunlight availability and the battery condition. 

This is because the position of the sun and the availability of sunlight directly impact the charging speed of a solar panel. Besides, the depth of discharge, the charging efficiency, and the type of the battery also impact the total charging time.

So, suppose the voltage of your battery is 12-volt on average. In that case, the depth of discharge is 50%, the charging efficiency is assumed as 100%, and the battery type is a lead-acid deep cycle battery with an MTTP-solar charge controller; in this case, a 100w solar panel will take 12.5 hours to charge a 12-Volt 100 amp-hour battery.

Yet, if the charging efficiency is assumed as 85%, which indicates that the charger’s output power is 85W of a 100w solar panel, it will require 14 hours to charge a 12-Volt 100 amp-hour battery fully.

However, in the case of a 100 amp-hour lithium-iron-phosphate battery with an 80% depth of discharge rate, a 100w solar panel will take around 20 hours to recharge the battery while also using an MPPT-solar charger entirely.

How to calculate the charging time of a battery by a solar panel?

To calculate the charging time of the battery, here, I have used the following equation. 

  • Amps = Total solar panel size in watts(W) / voltage (V) of the battery
  • Battery charging time = Battery capacity in Amp-hour (Ah) / Amps (amp)

Well, so, as you see above, to find out the charging time of the battery; first, you have to calculate the battery amps.

So, to figure this out, let’s assume the average voltage power of the battery is 12 volts for a 100w solar panel size with a battery capacity of about 100 amp-hours. Here, also assume that the charging efficiency is 100%, so the charger’s output power is also 100W.

So, according to the equation-

  • Amps = Total solar panel size in watts(W) / voltage (V) of the battery

           = 100W-solar panel watts / 12V-voltage

                          = 8.33 amps

  • The required charging time of the battery = Battery capacity in Amp-hour (Ah) / Amps (amp)

                                                                                          = 100ah-battery capacity / 8.33 amps

                                                                                          = 12.5 hours

In the case of the 85% charger’s charging efficiency, the charger’s output power will be 85W for a 100w solar panel. So, in this case, the charging time will also be different.

  • Amps = Total solar panel size in watts(W) / voltage (V) of the battery

                         = 85W-solar panel watts / 12V-voltage

                          = 7.083 amps

  • The required charging time of the battery = Battery capacity in Amp-hour (Ah) / Amps (amp)

                                                                                          = 100ah-battery capacity / 7.083 amps

                                                                                          = 14.12 hours

How many 100w solar panels do you require to charge a 100 amp-hour battery in 5 hours?

Technically, to charge a 100 amp-hour battery in 5 hours, you will require 2 units of 100w solar panels, while you also need to add extra solar panel watts of about 20% if there is insufficient available sunlight for about 1 hour, which means that you will need 240w solar panel.

Well, to better understand, let’s look over the following equation-

  • Watts = Amp-hour (ah) x battery voltage (V)

                          =100 ah x 12V

                          = 1200 watts

  • Solar panel Watts (W) size = Watt / available sunlight hours

                                 = 1200 W / 5 hours 

                                 = 240W solar panel 

Here, the solar efficiency rating will be 2 x 120W- solar panels.

However, if the available sunlight hours are 6 hours, then the size of the solar panel will be – 1200 W / 6 hours = 200W solar panel. And, you need to add an extra 20% of the solar panel watts.

So, the final solar panel Watt sizes = Solar panel Watts (W) size + extra 20% of the solar panel power 

                                                               = 200W + extra 20% of the solar panel watts 

                                                               = 240W

Which is the best solar panel size to charge a 100 amp-hour battery?

Usually, to charge a 12volt 100ah battery with 20 amps, the solar panel will require 5 hours; now multiply the battery amps with the battery voltage – 20 amps x 12V=  240 watts, and you will get the solar panel size.

Here, have a look at the following formula-

  • 100Ah-(amp-hour) / 5 hrs. = 20 amp
  • 20amp x 12-volts = 240-watts solar panel

The solar panel size depends on how large a solar panel you will require. Although from the calculation, you can see that a 240 watts solar panel is enough to charge a 100ah battery fully, however, the recommended solar panel size from the expert is a 300 watts size solar panel (3 x 100W).

Besides, you can also follow some of the following solar panel size in series-

  • 1 x 240W = 240-watts solar panel
  • 2 x 120W = 240-watts solar panels
  • 3 x 100W = 300-watts solar panels
  • 5 x 50W = 250-watts solar panels

How long does a 100 amp-hour battery continue an appliance requiring 1000-Watt?

Usually, a 100 amp-hour battery will take around 1.2 hours technically to continue an appliance requiring 1000-Watt, although the running time will vary because of the battery type.

However, if you have a 1000 watts appliance with a 12-volt 100 amp-hour battery, to figure out the running time, first, you need to calculate the amps of the battery that your appliance requires-

To calculate the amps, follow the given formula-

  • Amps of the battery =Watts / Battery voltage

                                      = 1000 watts / 12-volt

                                      =83.33 amps

Now, by dividing the battery capacity by the amps of the battery, you will get the running time.

  • The required running time of the battery = Battery capacity in Amp-hour (Ah) / Amps (amp)

                                                                          = 100ah-battery capacity / 83.33 amps

                                                                           = 1.2 hours

Here, it would help if you remembered that when using a 12-volt 100 amp-hour battery to run a house appliance such as AC, fridge, etc., or appliances like computer devices, try to use an absolute one. The reason is it will last longer and also will require less maintenance compared to a typical AG.

Besides, in the case of lead-acid batteries, you need to recharge them just when the charging levels of these batteries are halfway empty to preserve the battery’s lifespan for a long time.

Moreover, another thing you should know is that as the depth of discharge rate of a 100ah battery is 50%, it indicates that the availability of the usable battery capacity is only 50ah. Although the running time is 1.2 hours for a 1000-watt appliance, the battery will run it only for 0.60 hours.

Therefore, if you want to run the appliance for 1.2 hours, you should buy at least two 100 amp-hour batteries, or you can buy a 200 amp-hour battery.

Final verdict

So, I’m here to wrap up the article regarding how long does a 100w solar panel take to charge a 100 amp-hour battery?

I’ve mentioned in the article that the charging time depends on several factors. 

Besides, I’ve provided some of the related equations with the calculation. So, you can figure out the charging time and the running time of the 100ah battery easily on your own.

Matt

I’m Matt, a computer and electronic embedded system developer. Currently, working as an electronic engineer. In my leisure time, I do love to blog about technology stuff. Specially, new advance high tech devices. Portable solar systems are one of topic that I love to investigate. They are becoming more powerful day by day which keep me alert.