How Long Will 20w Solar Panel Take To Charge 12v Battery?

in short, 20w solar panel will take anywhere from 9 to 44 peak sun hours to charge most of the 12v lead-acid batteries from 50% depth of discharge using an MPPT charge controller

How Long Will 20w Solar Panel Take To Charge 12v Battery?? “ ENTER THE BATTERY SPECS AND SOLAR PANEL SIZE INTO THE CALCULATOR ” to find out

12v Battery Charge Time Calculator With Solar Panels

Solar Panel Size
Battery Capacity (Ah)
Battery Volts (V)
Battery Type
Depth Of Discharge (DoD)
Charge Controller Type

Note! Use this solar panel size calculator if you want to find out what size solar panel you need to charge your battery in desired peak sun hours.


1- The estimated charge time is not the total number of sunlight hours but the number of peak sun hours ( keep reading I’ll explain in a bit )

Calculator Assumptions

  1. Lead-acid, AGM, & Gel Battery charge efficiency: 85% 
  2. Lithium (LiFePO4) charge efficiency: 98%
  3. MPPT Charge Controller: 98% efficient
  4. PWM Charge Controller: 80% efficient
  5. Solar Panel Efficiency: 80% (which is the most common figure)

How To Use The Calculator?

  1. Enter the total solar panel array size: if you’re using a 20-watt solar panel ” ENTER 20 ” and so on…
  2. Enter Batter capacity in amp-hours: is this a 20ah, 50ah, etc…
  3. Enter the battery voltage: is this a 12, 24, or 48v battery? ” Enter 12 for 12v battery ”
  4. Select Battery Type: Select “Lead-acid” if you’re using a flooded or sealed (AGM or gel) lead acid battery. Select “Lithium (LiFePO4)” if you’re using a lithium iron phosphate battery.
  5. Enter battery depth of discharge (DoD): DoD is the percentage of the battery that has been discharged relative to the total battery capacity. for half discharged battery you’d “ENTER 50”. If the battery is fully discharged ( which you can achieve on lithium battery ) “ENTER 100”. (Lead-acid batteries can be discharged 50%)
  6. Select the charge controller type: are you using a PWM or MPPT charge controller?

After entering the information, click “Calculate” button to find out how long it will take to charge a 100ah lithium battery.

if you’re a noob, i would suggest you watch this full video to get a complete understanding of electricity basics.

Note! If you manually want to calculate the battery time with solar panels scroll down to the bottom of this page or Click here.

What Are Solar Peak Sun Hours?

Before explaining peak sun hours and why to use them, first, let’s see how solar panels are rated.

“ Solar panels are designed to produce their rated wattage output under standard test conditions – STC ”

STC includes: 

  • 1000 watts per meter square (1kw/m2) of sunlight intensity hitting the solar panels directly
  • 25oC temperature 

So to calculate the sunlight intensity we use peak sun hour as a reference. 

  • 1 peak sun hour = Average of 1kw/meter2 of sunlight intensity for one hour.

This number will depend on the location to location. To calculate how many peak sun hours your city receives use this PVwatt solar calculator.

Step 1: Enter your full address of just a city name & Click GO

Step 2: Click on results

How many peak sun hours in florida?

Now you’d be able to see the total amount of solar radiation your location receives per day (from month to month)

Note! it would have to place my solar panels all day long under sunlight to get those total solar radiations per day. 

Read the below mentioned post to find out how much power output you can expect from your solar panels during peak sun hours. and also some tips to increase the solar panel’s output.

Must Read: Solar Panel Output Calculator – Average Solar Panel Output?

How Long Will 20w Solar Panel Take To Charge 12v Battery?

Here’s a chart about how long will a 20w solar panel take to charge a 12v battery.

Battery CapacityCharge Controller TypeEst. Charge Time With 20w solar panel
7ahPWM3.7 peak sun hours
20ahPWM10 peak sun hours
30ahPWM16 peak sun hours
50ahPWM26 peak sun hours
60ahPWM32 peak sun hours
100ahPWM54 peak sun hours
150ahPWM80 peak sun hours
7ahMPPT3 peak sun hours
20ahMPPT9 peak sun hours
30ahMPPT13 peak sun hours
50ahMPPT22 peak sun hours
60ahMPPT26 peak sun hours
100ahMPPT44 peak sun hours
150ahMPPT66 peak sun hours
Chart: how long to charge 12v battery with 20w solar panel


  • 20w solar panel will take about 10 to 54 peak sun hours to charge most of the 12v lead-acid batteries from 50% depth of discharge using a PWM charge controller

12v Battery Charge time: Manual Calculation

To calculate manually “how long will it take to charge the 12v battery with a 20-watt solar panel” follow these steps.


1- convert the battery capacity from amp-hours (Ah) to watt-hours (Wh) by multiplying the battery capacity (Ah) by battery volts. let’s say you have a 12v 20ah battery

Battery capacity in watt-hours = 20 * 12 = 240wh

2- Multiply the battery capacity watt-hours by the battery depth of discharge %. let’s suppose you have a lead-acid battery with a 50% depth of discharge limit (DoD).

Discharged battery capacity in watt-hours =  240 * 50% = 120wh

3- Multiply the discharged battery capacity (Wh) by 1.15 for lead-acid and 1.02 for lithium battery ( Battery charge efficiency rate, lead-acid; 85% efficient, lithium-ion; 98% efficient ).

Energy required to charge the battery = 120 * 1.15 = 138 watt-hours

4- Multiply the solar panel wattage by 80% to into account system losses. let’s suppose you have a 20 watt solar panel

Average solar panel output = 20 * 80% = 16 watts

5- Multiply the average solar panel output by the charge controller efficiency rate ( PWM; 80%, MPPT; 98% ). Let’s say you have an MPPT charge controller

Solar panel output after charge controller = 16 * 98% = 15.6 watts

6- Divide the Energy required to charge the battery by the solar panel output value after the charge controller.

Solar panel size required = 138 / 15.6 = 8.8 hours 

Note! Charge time (hours) will be the number of peak sun hours.

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I hope this guide was helpful to you. If you have any queries Contact Us, Thank You!

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Chris Tsitouris

Chris Tsitouris is a renewable energy professional with 10+ years of experience as Director of Engineering at Solar Spectrum, previously working as Project Manager at SunPower and Energy Analyst at the National Renewable Energy Laboratory. As a thought leader, Chris has authored numerous articles and research papers.

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