For us to answer this question, we will need to make several assumptions or answer some questions. For instance, what is the capacity of the battery you are using? How many amp-hours have been discharged?

Knowing the answer to these questions allows you to establish the solar panel size needed to reduce the lost energy.

So, what size of solar panel do you need to charge a 100ah battery? **Generally, you will need a 180 watts solar panel to charge a 100Ah deep-cycle lead-acid battery from 50% Depth of Discharge (DOD). Assuming that solar energy is at its peak for 5 hours straight in a day, it will take 8 hours for the battery to charge fully. **

You can recharge two kinds of batteries: normal car batteries and deep-cycle batteries with different specifications.

In this discussion, we will use a lead-acid battery, and most of these batteries are never discharged 100% unless it is unavoidable. The lead-acid cells are often discharged between 20% to 80%. These batteries are either deep-cycle designs or normal car-type batteries.

This article will focus on lead-acid batteries because they are the most commonly used in homes and RVs.

Are you wondering whether solar panels are environmentally friendly or not? I wrote an article that I encourage you to read so you can decide if solar panels are a good choice for you.

**What Are The Differences Between Regular Car Batteries And Deep-Cycle Batteries?**

**Deep-cycle batteries are marine or leisure batteries that can be discharged deeper than the regular car battery. No wonder they are very expensive. Regular car battery cells and deep-cycle battery cells also deliver power differently.**

For instance, deep-cycle batteries are suited for applications that require steady medium-level current for many hours.

On the other hand, regular car batteries deliver hundreds of amps fast for moving heavy machines.

**Another notable difference is their discharge capacities. For example, deep-cycle batteries can be discharged to 50% or even 80% without damaging the battery cells.** However, car batteries should not be discharged more than 15 or 20% of their capacities.

**The Difference In Charging Auto And Deep-Cycle Batteries**

**Assuming that the 100Ah auto battery is discharged down to 20%, you will need 100Ah x 20% = 20Ah. We also assume that the 100Ah deep-cycle battery is discharged down to 50%, meaning you will need 100Ah x 50% = 50Ah**.

In the proceeding calculations, we will assume that the 100Ah auto battery can discharge to a maximum of 20% while deep-cycle discharges to 50%.

One terminology you should also know is Depth of Discharge (DOD), which shows how much battery power has been used. It tells you how much battery energy you need to replace using solar panels.

However, **how do you know the remaining battery capacity?**

You can use a multi-meter tester to check the terminal volts and determine the amp-hour capacity in your lead-acid battery.

**Ensure the battery is dormant for approximately 6 hours before checking the remaining capacity to avoid stimulating a chemical reaction. **

**Depth of Discharge Chart For a Lead-Acid Battery**

The table below shows the percentage of remaining battery capacity with their corresponding terminal voltage.

Battery Terminal Voltage | State of Charge% |

11.51 | 10 |

11.66 | 20 |

11.81 | 30 |

11.96 | 40 |

12.10 | 50 |

12.24 | 60 |

12.37 | 70 |

12.50 | 80 |

12.62 | 90 |

12.73 | 100 |

**Calculating Battery Capacity (Watt-hours)**

**Although some people use Amp-hours to refer to battery capacity, it oversimplifies how batteries work because amp and time do not have a straightforward relationship.**

For instance, a 100Ah battery can deliver 10 amps for a given period but not for 10 continuous hours. Several other factors weigh in, including temperature and rate of current draw.

Therefore, **watt hours are probably the best way to define battery capacity since the solar output is also measured in watt-hours. **

The regular 100Ah car battery will require 240 watt-hours (20% x 100Ah) x 12 volts to recharge, while a similar deep-cycle battery will need 600 watts-hours (50% x 100Ah) x 12 volts.

**What Does Solar Panel Size Mean?**

**Solar panel size could refer to its power output in watts or physical dimensions. You could consider the panel’s dimension and power when installing solar panels in your preferred space (in an RV, cabin, boat, or tinny house). **

Solar panels provide different wattages or power sizes. For instance, a small 1 wattage panel could be ideal for LEDs, while 1,000 watts can be used to power a house.

**Generally, a 100-watt solar panel would physically measure approximately 47 x 21.3 x 1.4 inches. We expect larger solar panels to provide higher wattage since they have more cells.**

However, solar panel power size can also depend on the efficiency and design of the panel itself.

**How Long Will a 100w Solar Panel Charge a 100Ah Battery?**

**A 100-watt solar panel is relatively smaller in the solar market, making it easy to transport and install. **Therefore, they are popular at campsites, RVs, and off-grid activities.

However, since **a single 100w panel can only produce 30Ah of power daily, it will take 5-6 hours of peak sunlight to charge the battery.** If you are camping, your power needs may be 80Ah, which means one 100w panel will not be enough to charge the battery faster under average weather conditions.

Two or more 100w solar panels may do a better job, or you can buy a 300-watt solar panel. **Intensive energy consumption activities such as air conditioners or extended days of off-grid camping may require as much as 1800w of solar panel power.**

**Factors Affecting The Amount Of Solar Energy From The Panels**

Besides the size of the solar panel, several factors affect the amount of energy by the panels to charge the battery.

**1- Solar Irradiance**

**Irradiance is the amount of sun’s energy falling on the panels, which is dictated by the weather, season, and location.**

For example, the amount of sun in Texas is 3 times more than in London, meaning solar panels with similar specifications will produce more power in Texas than in London.

**2- Type Of Solar Panels**

**Polycrystalline solar panels are considered more inefficient than monocrystalline panels**. Therefore, we expect the latter to recharge the batteries faster than polycrystalline panels.

If you are interested in exploring more about Monocrystalline And Polycrystalline, how do they differ? I wrote an article that I encourage you to read.

**3- Solar Charger**

Solar chargers are categorized into two- MPPT and PWM. PWM stands for Pulse With Modulation solar charger, while MPPT stands for Maximum Power Point Tracking charger. **PWM is a less efficient solar charger since it lowers the panel’s output voltage to a figure equal to or above battery volts. **

On the other hand, **MPPT solar charge uses all the generated voltage and current to charge the battery, thus resulting in most power generated. **

MPPT also performs better because it adjusts its internal resistance to match your panel’s resistance, thus contributing to maximum power generation.

**The Size Of Solar Panel Needed To Charge a 12V 100Ah Lead Acid Battery.**

The table below shows the amount of time estimated solar panel size will take to charge a 12V 100Ah battery when using an MPPT charge controller.

Charge Controller Type | Peak Charge Time | Required Solar Panel Size |

MPPT | 5 sun hours | 290 watts |

MPPT | 10 sun hours | 110 watts |

MPPT | 15 sun ours | 70 watts |

MPPT | 20 sun hours | 50 watts |

MPPT | 25 sun hours | 40 watts |

Now consider the figures for PMW.

The table below shows the amount of time estimated solar panel size will take to charge a 12V 100Ah battery when using a PMW charge controller.

Charge Controller Type | Peak Charge Hours | Required Solar Panel Size |

PWM | 5 sun hours | 360 watts |

PWM | 10 sun hours | 140 watts |

PWM | 15 sun hours | 90 watts |

PWM | 20 sun hours | 70 watts |

PWM | 25 sun hours | 50 watts |

**The above tables indicate that you need approximately 290 watts of solar panels with an MPPT charge controller to fully charge a 12V 100Ah battery from 50% DOD if the solar is at peak for 5 hours.**

On the other hand, you will need approximately 360 watts of solar panel with a PWM charge controller to fully charge a 12V 100Ah battery from 50% DOD in 5 peak sun hours.

**If your battery voltage is 24, you will need larger solar panels with higher watts to charge it.** For instance, with 5 peak sun hours and an MPPT charge controller, you will need 570 watts of solar panels to fully charge the batter from 50% of the depth of discharge.

But, **if you have a PWM charge controller, you will need 5 peak sun hours and approximately 720 watts of solar panels to fully charge a 24 V 100Ah from 50% DOD**.

**Final Thoughts**

Different solar panel sizes can charge a 100Ah battery depending on what you want to use the power for and for how long. Some off-grid activities require limited power, and a 100-watt solar panel producing 30Ah would be enough.

On the other hand, some activities such as running an air conditioner would need large solar panels to produce power. You may need between 300 watts and 1800 watts of solar energy to charge your 100Ah battery faster under average weather conditions.