How Many Solar Panels Does It Take To Charge A 100Ah Battery?
How many solar panels needed to recharge a 100Ah battery?
This is what you need to know to work how many solar panels you need to recharge your 100Ah battery:
- What type of battery are you using – lead-acid or lithium-based?
- What is the battery’s state of discharge?
- What kind of solar charger will you use – MPPT or PWM?
- What size solar panels will you use?
A fully discharged 12V 100Ah auto battery will need 3 solar panels each rated at 100 watts to fully recharge in one day, assuming average 4 peak-sun-hours. A 100Ah lead-acid deep-cycle battery discharged to 50% of it’s capacity will need 2 solar panels each rated at 100 watts to fully recharge in one day.
The starting point for any solar panel sizing begins with the load. How much energy is needed to fully recharge a 100Ah battery?
For convenience, I often convert amp-hours (Ah) into watt-hours (Ah) so panel watt-hours can be matched to the battery watt-hours needed to fully charge it.
The conversion is pretty simple:
watt-hours = amp-hours/battery voltage (12V)
Video – Setting up a 100W solar panel with a 100Ah battery
How to test battery capacity with multi-meter
You need to know how far your battery is discharged.
For lead acid it doesn’t matter if it’s a straight auto battery for starting cars and trucks, or a deep-cycle battery for leisure use in RVs – the procedure is the same.
An accepted method of estimating how much energy is left in a lead-acid battery is to measure the terminal voltage with a multi-meter.
Readings can be anywhere from over 11 volts to almost 13 volts. Note that a ’12 volt’ battery indicates nominal voltage.
Nominal 12 volts just means the battery is intended to supply devices designed for 12 volts supply for optimum operation i.e. such as the equipment found on cars and boats.
The terminal voltage gives a rough indication of the amount of energy left in the battery. The test must be done when the battery has been idle for several hours.
Table 1 – 12 volt battery discharge chart
|
Depth Of Charge % (12 V Pb) |
Battery volts |
|
100 |
12.73 |
|
90 |
12.62 |
|
80 |
12.50 |
|
70 |
12.37 |
|
60 |
12.24 |
|
50 |
12.10 |
|
40 |
11.96 |
|
30 |
11.81 |
|
20 |
11.66 |
|
10 |
11.51 |
How low can you discharge a lead acid battery?
This depends on the battery type. There are two broad kinds of lead acid battery – car or truck, and deep-cycle, sometimes called leisure batteries.
Car batteries shouldn’t be discharged more than about 15%. They are designed to deliver hundreds of amps to crank cold car or truck engines.
After the engine has started, the car’s alternator immediately gets to work charging the battery back up. It’s rare for a car battery to sit in any state of discharge.
How low can you discharge a deep-cycle battery?
Deep cycle lead-acid batteries can be discharged up to 80% without damage but only 50% is recommended for regular discharge.
This is because deep discharge affects the internal structure and shortens the battery life.
Leisure batteries are good for delivering medium currents for many hours and not for cranking engines, although a hybrid battery called a marines battery can fulfill both functions.
The different depth of discharge for both batteries is important, as it determines how much energy needs to be put back into the battery when sizing solar panels.
How Many Solar Panels Does It Take To Charge A 100Ah Car or Truck Battery?
How many solar panels are needed for a 100Ah car battery?
I can imagine two scenarios with a 100Ah auto battery – either it’s discharged to 15% as recommended, or it’s been left to totally discharge. Maybe it’s been used to power a home appliance through an inverter.
Auto battery at 15% discharge
Energy needed to fully recharge:
100Ah x 15% = 15Ah
15Ah in watt-hours = 15 x 12 volts = 180Wh
With an irradiance of 4 peak-sun-hours/day (an average value used by solar installers):
100 watts solar will generate 100 x 4 = 400 watt-hours/day
So a 100 watts solar panel will easily recharge this battery in a day. In fact, at a rate of about 5 amps solar output, the battery will be charged back up in 3 hours.
Auto battery 100% discharged
Energy needed:
100Ah x 12 volts = 1200 watt-hours
In this case at least 3 solar panels, each rated at 100 watts would be need to recharge a fully discharged 100Ah battery in one day.
Note: If an auto battery is deeply discharged regularly it will be ruined very quickly.
How many solar panels to charge a 12 volt 100Ah deep-cycle lead acid battery?
Assuming that 50% depth of discharge is respected, then:
Energy required = 50Ah x 12 volts = 600 watt-hours
In this case, 200 watts of solar panels would be adequate for recharging comfortably in one day. (About 5 hours in full sunshine.)
How long will a 100w solar panel take to charge a 100ah battery?
Everything is driven by the load, that is, how far is the battery discharged? You need to know this to work out how much energy to put back in.
Assuming a lead-acid battery, I can imagine 3 common situations:
- A 100 Ah car battery depleted 15% (normal operation)
- A 100Ah car battery or deep-cycle depleted 100% (left unused for a period of time)
- A 100Ah deep-cycle battery depleted 50% (normal recommended operation)
All calculations use a 100 watt solar panel in full sunshine i.e. 1000kWh/m2. Panel output current is 5 amps and an MPPT (Maximum Power Point Tracking) charge controller is used.
Case 1 Calculation:
Energy required = 15Ah
Charging time = 15Ah/5A = 3 hours
Case 2 calculation:
Energy required = 100Ah
Charging time = 100Ah/5A = 20 hours
(300 watts of solar recommended to charge the battery fully in one day)
Case 2 calculation:
Energy required = 50Ah
Charging time = 50Ah/5A = 10 hours
(200 watts solar recommended to charge the battery fully in one day)
How long will a 300W solar panel take to charge a 100ah battery?
Assume a lead-acid battery again, in these situations:
- A 100 Ah car battery depleted 15% (normal operation)
- A 100Ah car battery or deep-cycle depleted 100% (left unused for a period of time)
- A 100Ah deep-cycle battery depleted 50% (normal recommended operation)
A 300 watt solar panel in full sunshine is proposed i.e. 1000kWh/m2. Panel output current is 15 amps and an MPPT (Maximum Power Point Tracking) charge controller is used.
Case 1 Calculation:
Energy required = 15Ah
Charging time = 15Ah/15A = 1 hour
Case 2 calculation:
Energy required = 100Ah
Charging time = 100Ah/15A = 6.7 hours
(400 watts of solar recommended to charge the battery fully in one day)
Case 2 calculation:
Energy required = 50Ah
Charging time = 50Ah/15A = 3.4 hours
How long will a 100w solar panel take to charge a battery?
This depends on the rated capacity of the battery and how far it has been discharged. Use the table below to find the right values for your situation:
Table 2 – Battery charge times with 100 watt solar
|
12 volt battery capacity in Ah |
Time to fully charge with 100 watt solar (hours) |
|
|
Car battery DoD 20% |
Deep-cycle DoD 50% |
|
|
40 |
1.4 hours |
4.2 hours |
|
60 |
2.5 |
6.3 |
|
80 |
3.4 |
8.4 |
|
100 |
4.2 |
10.5 |
|
120 |
5 |
12.6 |
How long will a 100 Ah battery last?
Although there are other minor factors, 100Ah battery run-time will be driven by the load wattage.
However, the relationship isn’t strictly linear, because heavy current draw reduces the overall available Ah capacity.
How long will a 100ah battery run an appliance that requires 1000W?
The table below gives estimates of 100Ah run time in hours for various DC and AC loads:
Table 3 – 100Ah battery run times with variable loads
|
Hours Run Time 12 volts 100Ah deep-cycle lead-acid battery (50%DoD) |
||
|
Load in watts
|
Run time in hours |
|
|
DC Load |
AC Load |
|
|
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 |
How long will a 100Ah battery run a fridge?
How many hours would a 100Ah run an average fridge?
Fridges are a special case as far as home appliances go. Together with heat pumps, freezers and AC units, they have compressor motors on board which complicate calculations.
A 400 watt rated fridge doesn’t really tell us how much power a fridge will draw at any given time, because the motor has three states:
- Compressor motor stopped – current is almost zero
- Motor starts – current is several times bigger than normal running
- Motor runs – current reduces to steady state
For this kind of appliance the annual energy usage is used to assess battery needs.
The table below shows estimated run times in hours for various fridges running on both lead-acid deep-cycle and lithium iron phosphate batteries:
Table 4 – 100Ah battery run times for various sized fridges
|
|
How long will a 12V 100Ah run fridges – annual energy consumption 200kWh to 1000kWh |
|
|
Annual Fridge Energy Consumption (kWh) |
Lead-acid deep-cycle (discharge to 80% of capacity) – hours run time |
Lithium iron phosphate (discharge to 95%) – hours run time |
|
200kWh/yr |
42 hours |
50 hours |
|
300 |
28 |
33.3 |
|
400 |
21 |
25 |
|
500 |
17 |
20 |
|
600 |
14.5 |
17.2 |
|
700 |
12 |
14.3 |
|
800 |
10.5 |
12.5 |
|
900 |
9.4 |
11.2 |
|
1000 |
8.4 |
10 |
How many solar panels do I need to charge a 100ah battery in 5 hours?
Let’s imagine 3 scenarios:
- 100Ah auto battery discharged 20% (normal regular operation)
- 100Ah deep-cycle lead-acid battery discharged 50% (normal regular operation)
- Any 100Ah lead-acid battery fully discharged
The following calculations assume an irradiance of 4 peak-sun-hours/day, so 1 solar watt can generate 4 watt-hours of energy in a day,
Case 1: 100Ah discharged 20% of rated capacity
Energy required to recharge = 20Ah x 12 volts = 240 watt-hours
Solar panel power required = 240 watt-hours/4 = 60 watts
Case 2: 100Ah discharged 50% of rated capacity
Energy required to recharge = 50Ah x 12 volts = 600 watt-hours
Solar panel power required = 600 watt-hours/4 = 150 watts
Case 3: 100Ah discharged 20% of rated capacity
Energy required to recharge = 100Ah x 12 volts = 1200 watt-hours
Solar panel power required = 1200 watt-hours/4 = 300 watts
Notes: In practice, a solar panel doesn’t generate its rated power. It’s always less, due to the natural efficiency of the panel and also due to system losses.
I would recommend adding 25% to all the solar ratings calculated above.
