Charge Battery from Solar Panel: How to Calculate Battery Charging Time

In order to save electricity, solar energy system aims to go into every family.

Look, here are some positive views on solar system:

I have 8.1kW solar panels installed with a battery pack and, as long as there are bright sunshine, this fills up the battery and power my whole house with ample surplus power available. —from a net friend

Every hour of sun rays will bring you a full charge.”

The function of solar battery is to store extra electricity produced by solar energy and then work as a power source when there is no sunlight. Some people want to be mostly possibl e independent of grid power. Thus they are concerned about whether solar energy system can generate enough electricity for daily use and how long does the battery fully charge. Today, we are going to talk about charging time.

So what effect does it have on the overall charging time?

Here are some figures, let me explain something here:

Figure 1: The Efficiency of Solar Charge Controller

What is a solar charge controller?

Solar charge controller is a part of a solar installation and play a core role. Basically, it regulates the current and voltage coming from the solar panels going to the battery to keep batteries from overcharging. To put it in another way, it is a voltage and current regulator.

Why it will effect charging time?

It works like a water faucet switch. If we turn this water faucet on to maximum, we can fill our water cup quickly. So charge controller will effect charging time in the same way.

Figure 2: Battery State

Rechargeable batteries, including lithium-ion batteries, will age with time goes by. It will gradually lose capability, although the pace of deterioration will be slow. Battery capacity will decrease and battery internal resistance will increase with the process of battery aging. Charging speed also changes.

Figure 3: Battery Capacity

Battery power can change the charging time because of its different storage capacities. If you have a small battery and enough sunlight, you will get the expected power shortly. On the contrary, a large battery requires a longer charge than a smaller one. Smaller batteries store less power and take a short time to be charged.

Figure 4: Solar Panel Quality

The efficiency of the solar panel can affect the duration of charging. If you have solar panels with lower efficiency, it will take longer than the normal charging period. Photo-voltaic cells convert heat into electricity in a solar system.

So, the better cells you use, the better your result. It is suggested to use the standard cells with enough power production capacity.

Figure 5: The Amount of Sunlight Absorbed by the Solar Panels

Environment plays a vital part in solar energy production. It is strongly related to the weather and the sun. A place with bright sunlight will enhance productivity. But, winter and rainy seasons can affect the charging time. You can get the perfect calculation by analyzing where the panels are installed.

Solar panel size also very important in solar energy production. The larger the area of the solar panel, the more solar light the solar panel absorbs, and the more energy it converts into electricity, so the larger the area of the solar panel, the faster it charges under the same amount of light.

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How long does it take to charge a battery with a solar panel?     

How to calculate battery charging time?

In fact, the actual charging time is influenced by many factors. We can calculate it with some certain parameters. Considered energy losses, specific steps are as following:

Step1: Divide solar panel wattage by battery voltage to estimate maximum charge current output by solar charge controller

Input current: I=P/V

Step 2: Multiply current by rule-of-thumb system losses (20%) and charge controller efficiency(PWM: 75%; MPPT: 95%)

Actual current:    PWM —-I*(1-20%) *75%     MPPT —-I*(1-20%) *95%

Step 3: Multiply battery capacity by 1 divided by rule-of-thumb battery charge efficiency (lead acid: 85%; lithium: 95%)

Actual capacity needed: lead acid—-nominal capacity(ah)/85%   lithium—-nominal capacity(ah)/95%

Step 4: Divide battery capacity by current to estimate how long it’d take to charge the entire battery

Total time to charge the whole battery: Actual capacity needed/ Actual current    T1= C/I

Step 5: Multiply the charge time by the battery’s depth of discharge to estimate how long it’d take to charge the battery at its current level

Actual charging time:  T2=T1 * DOD

Step 6: Add 2 hours to account for the absorption charging stage of most charge controllers


Note: Our solar charging calculator, which takes into account more factors. estimates it’d take 6.6 hours for this setup.

For example:

here are some solar power systems, let’s calculate their charging time.

12V, 30Ah lead-acid battery DOD: 50%

100 W solar panel

PWM charge controller

Step 1:100W / 12V = 8.33A

Step 2:8.33A * (1-20%)*75% = 5A

Step 3:30Ah*(1/85%)= 35.29Ah

Step 4:35.29Ah / 5A = 7.06 hrs

Step 5:7.06hrs*50% DOD = 3.53 hrs

Step 6:3.53 hrs + 2 hrs = 5.53 hrs

It’d take 5.53 hours for this setup with continuous direct solar radiation.

12V, 30Ah lead-acid battery DOD: 50%

100 W solar panel

MPPT charge controller

Step 1:100W / 12V = 8.33A

Step 2:8.33A * (1 -20%)*95% = 6.33A

Step3:30Ah*(1/85%)= 35.29Ah

Step4:35.29Ah/6.33A= 5.57 hrs

Step 5:5.57hrs*50%DOD=2.79hrs

Step 6:2.79 hrs + 2 hrs = 4.79 hrs

It’d take 4.79 hours for this setup with continuous direct solar radiation

24V, 50Ah lithium battery DOD: 80%

150 W solar panel

MPPT charge controller

Step 1:150W / 24V = 6.25A

Step 2: 6.25A *(1-20%) * 95% = 4.75A

Step 3:50Ah * (1 / 95%) = 52.63Ah

Step 4:52.63Ah / 4.75A = 11.08hrs

Step 5:11.08hrs*80% DOD = 8.86 hrs

Step 6:8.86 hrs + 2 hrs = 10.86 hrs

It’d take 10.86 hours for this setup with continuous direct solar radiation

24V, 50Ah lithium battery DOD: 80%

150 W solar panel

PWM charge controller

Step 1:150W / 24V = 6.25A

Step 2: 6.25A *(1-20%) * 75% =3.75A

Step 3:50Ah * (1 / 95%) = 52.63Ah

Step 4:52.63Ah / 3.75A = 14.034hrs

Step 5:14.034hrs*80% DOD = 11.28 hrs

Step 6:11.28 hrs + 2 hrs = 13.28 hrs

It’d take 13.28 hours for this setup with continuous direct solar radiation

Things become much easier if we do not take into account losses:

Charging time of battery = Battery Ah / Charging Current


T : Time in hours

C: Ampere Hour rating of battery

I : Current in Amperes

If we don’t know “I”, but we know solar panel wattage. We can use     I=P/V to figure I out.

Here are some questions asked by people on the internet:

Q1: How Long Will It Take For a 24V 120Ah Battery to be Charged with 300W Panel?

I=P/V=300W/24V=12.5 A


In ideal condition, it takes 9.6 hours to fully charge this setup with continuous direct solar radiation

Q2: How Long Will It Take For” pair of 230 amp hr 6v batteries in series for 12v “to be Charged with 250W Panel?

According to above formula and  knowned parameters, we can calculate

I=P/V=250W/12V=20.8 A


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Q&A: Whats the function about solar charge controller?

1.Overcharge protection
2.Over-discharge protection
3,Load over-current and short circuit protection
4, Over-voltage protection
5,Anti-reverse charging function
6, Lightning protection function
7, Solar cell reverse connection protection
8, Battery reverse connection protection
9, Battery open circuit protection
10,Temperature compensation function

Q&A: How many kinds of solar charge controller are there?

3 types

First generation technology:   Solar Charge Controller ( being phased out because of inefficiency)

Second generation technology: PWM Charge Controller

Third generation technology:  MPPT Charge Controller

Q&A: Whats the Difference between PWM and MPPT?

Both of them have the same function to control battery charging. However, differences exist:

Following is a chart to illustrate their differences clearly

PWM Charge Controller MPPT Charge Controller
Structure simpler module structure just like a high-speed switch more complex
Cost cheaper more expensive -- several or even more ten times price of PWM

Charging Method
strong charge →balanced charge →floating charge current limiting charge →constant voltage equalization charge → constant voltage floating charge
Temperature influence Yes, it will show best work efficiency with temperature rang from 45 to 75℃. No, it work with high efficiency all the time

Use space
Matched voltage
( E.g: 12V system panel can only support the use of 12V controller and battery)
can't be over-paneled
more flexible:
the solar panel voltage between 12V~170V, battery voltage 12~96V adjustable
tolerant of being over-paneled
Applicability small off-grid solar system less than 2 KW large off-grid system over 2KW
User operation very easy difficult
others 1. solve the problem of unsatisfactory battery charge and ensure the service life of the battery
2. work best when the nominal voltage of the solar array matches that of the battery pack.
1. adjust input voltage to gain maximum power from solar battery panel.
2. change voltage into required battery charging voltage
3. enable the high-voltage solar panel to charge the low-voltage battery.
Compared with PWM controller,
MPPT controller has the maximum power tracking function.That is, before the battery has not reached the saturation state, it can ensure that the solar panel always at the maximum power output during the charging period.
PWM controllers have less own consumption than MPPT controllers, on designing low-power solar powered systems, it may matter.

As for what kind of charge controller you need to choose should be based on the actual situation.

Q&A: To Charge A 100Ah Battery, How Many Solar Panels Are Required?

To obtain amps, we divide power in watts by voltage in volts using the same formula. A 100 amp hour battery will take five hours to charge when charged at 12 volts and 20 amps.

You’ll need 240 watts of solar power if you multiply 20 amps by 12 volts, thus we propose a 300-watt solar panel or three 100 watt solar panels.

Q&A: Is It Possible To Charge A Dead Battery Using A Solar Panel?

Nope. The charging process will stop if the battery is dead because charge controller will automatically detect battery voltage. We propose using an Individual Battery Charge to fix and restore the dead battery before reconnecting it to the solar system.

Q&A: Where solar systems can be used?

Wide rang of use. Vehicle mounted system, cabin or tiny home, shed or shipping container mounted systems, solar water heating….

Want More Details: Download our battery design ebook.

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