How Long Will A 100W Solar Panel Take To Charge A 100Ah Battery?
Here is a list of things you need to know before answering the question:
 how much power can a 100Ah solar panel produce?
 what is the irradiance (sun’s energy) level in your location?
 what type of solar charge controller will you use?
 which type of battery will you use, lithium based or leadacid?
 what is the usual Depth of Discharge for your 100Ah battery?
The biggest factor is to determine how much energy needs to put back into the battery. Everything else flows from this.
A 100W rated solar panel using an MPPT solar charge controller will take approximately 12.5 hours to fully recharge a 50% discharged 100Ah leadacid deepcycle battery. 200 watts of solar panels is recommended to recharge the same 100Ah battery in one day, if the battery is used for home energy storage.
Video – 100 Watt Solar Panel and Battery Setup
How much power can a 100 watt solar panel produce?
Every solar panel comes with a label stating the electrical specifications, which can also be found on the panel’s specification PDF sheet online.
The image below shows the electrical specs. for a Centsys 100 watt panel.
The main values that interest us are the open circuit volts (Voc) 22V, Maximum Power Point voltage (Vmpp) 18 V and Maximum Power Point current (Impp) 5.56A.
RENOGY 100 Watt Solar Panels
The typical manufacturer’s spec sheet uses the STC (Standard Test Conditions), which are ideal test conditions.
The important thing to note is that these are ideal values under perfect laboratory test conditions.
It’s rare to find these conditions in the real world. As a general rule, expect to get only 75% of the stated power out of a standard quality 100 watt solar panel.
This mens that you should base any calculations on 75% of the stated maximum current, which for the above panel is a little over 4 amps.
Table – 100 solar panel annual energy output by irradiance per location
City 
New York, NY 
Birmingham, UK 
Lisbon, PT 
Brisbane, AU 
Nairobi, KN 
Irradiance (peaksunhours/day) 
4.086 
2.165 
5.163 
5.275 
3.955 
100W solar panel output per year (kWh) 
149.1kWh 
79kWh 
188.5kWh 
192.5kWh 
145.8kWh 
100W solar panel output per day (kWh) 
408.6 watts 
216.5 watts 
516.3 watts 
527.5 watts 
395.5 watts 
What are the two types of lead acid battery?
Leadacid batteries come in two basic types – auto, for cars and trucks, and deepcycle (leisure) for RV solar panel use. There is a hybrid battery for marine use which combines the two types.
Car batteries can deliver hundreds of amps to crank and start a cold car or truck engine. They shouldn’t be discharged regularly much more than 20% of their capacity.
How far can you drain a deep cycle battery?
On the other hand, deepcycle leadacid batteries can be discharged up to 80% of their capacity. However, 50% Depth of Discharge is recommended for maximum life.
As solar energy storage is essentially deepcycle, I’ll focus on this type for this article.
I own a 100Ah Varta marine battery, which I use as part of an emergency solar generator for home use.
It can run basic lights and appliances overnight if the power goes out, so it’s a useful backup power supply.
How many solar panels does it take to charge a 100ah battery?
Assuming a 100Ah solar panel current output of 4 amps minimum, then a 100Ah battery depleted 50% will need 12.5 hours to fully recharge.
If the battery was to be used as solar energy storage for use at nighttime, then 200 watts of solar panels is recommended to ensure that the battery is recharged fully each day.
Lithium iron phosphate vs lead acid
Lithium iron phosphate batteries (LiFeP04) are more expensive than lead acid but they have advantages. They are lighter, last much longer and are inherently deepcycle.
Discover your solar saving potential
You don’t have to purchase a special deepcycle battery, because deep depth of discharge characteristics are part of their chemistry.
I have a 14.4 volts 30Ah LiFeP04 model which can be discharged 95% regularly with no damage.
However, 80% discharge is recommended if you want the battery to last a long time – 2000 charge/discharge cycles for 95% discharge and a whopping 5000 cycles for 80%.
This makes a big difference to charging time, because there’s more energy to put back in the battery.
A 100W solar panel with an MPPT solar charger will take about 20 hours to fully recharge an 80% discharged 100Ah lithium iron phosphate battery. 250 watts of solar panels is recommended to fully recharge a 100Ah LiFeP04 battery in a day, if it is to be used for home energy storage.
Why do you need a solar charge controller?
Typical open circuit voltage for a 100 watt solar panel is about 22 volts, while it delivers maximum current at 18 volts.
If you connect the panel directly to a 12 volt battery, there is no way of knowing what the charge current will be. In fact, in the early stages of charging, there would probably be no problem.
However, eventually the battery would be full but the panel may still be pushing current into the battery resulting in permanent damage to the internal structure.
The charging voltage needs to be regulated so that the charging current is controlled throughout the charging cycle.
The difference between MPPT and PWM solar charge controllers
PWM (Pulse Width Modulation) chargers are cheap and give a much coarser control than their more expensive MPPT cousins.
They work by splitting up the charging current into pulses, which are then widthmodulated to give an average voltage. The average value of this modulated voltage determines the current.
PWM chargers are not very efficient but they are really cheap. There are many brands available for less than $10.
MPPT solar charges work in a different way. The Maximum Power Point of a solar panel is when the volts and amps are at the values necessary to deliver maximum power.
An MPPT controller tracks this point, which is dynamic according to changes in irradiance and other factors – the ‘T’ stands for ‘Tracking‘.
The Maximum Power Point happens when the internal resistance of the load (battery) equals the internal resistance of the solar panel.
The MPPT controller matches the panel internal resistance so that maximum power is drawn from the panel. These controllers are up to 30% more efficient than the cheaper PWM type.
How many solar panels do I need to charge a 100Ah battery in 5 hours?
If the battery is completely discharged (and it shouldn’t be!) then it will need:
100Ah/5 = 20 amps/hour
However, solar charge controllers have an efficiency of about 90%, so we need to add 10% to this figure, making it 22 amps/hour.
A 100 watt solar panels delivers about 5 amps in full midday sunshine, so the solar panel can be calculated this way:
22 amps/5 amps = 4.4 solar panels of 100 watt rating
The above assumes irradiance of 1000/m2 for the 5 hours period.
5 solar panels rated at 100 watts would be needed to charge a 100Ah battery in 5 hours.
How long does it take a 100 watt solar panel to charge a battery?
For charging batteries with solar you should gather the following information:
 What is battery capacity when fully charged?
 Depth of discharge in regular operation
 Irradiance in your location? (If not known, use 4 peak sun hours as an average value.)
 The type solar controller to be used (MPPT is more efficient and reommended.)
As a general rule, a typical size 12v 50Ah auto battery at 20% discharge will need 2 hours to fully recharge with a 100 watt solar panel.
A leadacid deepcycle 12v 50Ah battery at 50% discharge will take about 4 hours to fully recharge using a 100 watt solar panel.
Both examples above assume a solar panel current output of 5.75 amps using an MPPT controller.
How long will a 100Ah battery run a fridge?
Fridges don’t operate in quite the same way as many home appliances.
Like freezers and airconditioners, they use compressor motors to drive the cooling cycle, which start and stop as demanded by the device internal temperature.
When a motor starts up it pulls an inrush current which can be several times bigger than the normal running current. The motor has 3 states – not running, starting and continuous running.
A 100Ah deepcycle leadacid battery will power a refrigerator with energy consumption of 630kWh/annum for 13.3 hours. 80% DoD (Depth of Discharge) is assumed. At the recommended discharge of 50% a 100Ah battery can power the same fridge for 8.3 hrs. A lithium (LiFeP04) 100Ah battery will power the same fridge for 15.8 hours with 95% discharge.
How long will a 100Ah battery run an appliance that requires 1000w?
Use the table below to find 100Ah runtime with various loads:
Hours Run Time 12 volts 100Ah deepcycle leadacid battery (50% recommended discharge) 

Load supplied in watts 
Run time in hours 

DC Load 
AC Load (inverter losses subtracted) 

20 
30 
28.5 
40 
15 
14.25 
100 
6 
5.7 
300 
2 
1.9 
500 
1.2 
1.14 
600 
1 
0.95 
1000 
0.6 
0.57 
1500 
0.45 
0.427 
3000 
0.2 
0.19 