**To charge a 100Ah battery, you would need 240 watts, which means a single 100-watt solar panel is insufficient.** Three units of 100-watt solar panels are required for this task. The article provides a comprehensive formula for calculating the wattage needed based on the battery’s amp-hour (Ah) and voltage (V).

Different combinations of solar panel sizes can be used to meet the 240-watt requirement. **For instance, one could use two 120-watt panels or five 50-watt panels.** The article also includes a chart detailing various combinations and their efficiency ratings.

**Charging time for a 100Ah battery typically ranges between 5-6 hours, depending on sunlight availability.** The article uses a formula to calculate this, assuming an average of 6 hours of available sunlight and a 12V battery voltage.

**A 100-watt solar panel generates approximately 8.33 amps per hour when charging a 12V battery.** This calculation is based on dividing the solar panel’s wattage by the battery’s voltage. The proper battery size for 100w solar panel should be done based on these measurements.

## Terminological Calculation of Solar Panel and Battery Capacity

### Solar panel size

The sizes of Solar panels are usually measured in Watts. This Watts is referred to as a unit of power. Here, you must remember that the watt is a consistent matter that won’t change.

However, this is not the case with current or voltage. The amount of current or voltage can be changed.

Well, to figure out the solar panel size or watts, here is the formula-

**Watts = Amp-hour (ah) of the battery x battery voltage (V/volt)**

**Solar panel Watts = Watt-hours / available sunlight hours**

**Required solar panel Watt sizes = Solar panel Watts + extra solar panel power of about 20%**

### Battery capacity

The battery capacity is measured in Amp-hours. And the measurement of Amp-hours is usually based on the total availability of your battery usage of about 20 hours.

To measure Amps of the battery, the formula is-

**Amps = Total size of solar panel watts/voltage of the battery**

**Required charging time = Battery capacity (Amp-hour)/ Amps**

## How many watts of solar panels do you need to use to charge a 100 amp-hour battery?

All right, now we will calculate how many watts of solar panels it takes to charge a 100ah battery by using the above equation.

So, before calculating, let’s assume that the available sunlight hours are 6 hours, and the battery voltage is 12V. Technically, 100 watts solar panels are designed for charging 12V batteries.

Moreover, around 20% of the energy from the total solar power gets lost during the daytime. Therefore, you should have to add an extra 20% watts while calculating.

So, according to our formula-

- Watts = Amp-hour (ah) of the battery x battery voltage (V/volt)

=100 ah x 12V

= 1200 watts

- Solar panel Watts = Watt hours / available sunlight hours

= 1200 W / 6 hours

= 200W solar panel

- Required solar panel Watt sizes = Solar panel Watts + extra solar panel power of about 20%

= 200W + 20% extra solar panel watts

= 240W

So, now you can see that charging a 100Ah battery takes 240W technically. However, you can also follow different combo sizes of the solar panel watts to charge your 100ah battery.

The combo sizes rating of the solar panel watts is provided below in a chart-

Battery capacity | Total solar panel watts | Rating of solar panel efficiency (no. of panel unit x solar panel watt) |

100ah | 240 watts | 1 x 240W solar panel |

100ah | 240 watts | 2 x 120W solar panel |

100ah | 250 watts | 5 x 50W solar panel |

100ah | 300 watts | 3 x 100W solar panel |

## How long does it take to charge a 100 amp-hour battery?

Here, the charging time depends on the availability of sunlight. Usually, it takes 5-6 hours to charge a 100 amp-hour battery. However, if sunlight is available for a few hours, the calculation of the charging time will be different.

Well, to calculate the charging time, I will use the formula mentioned above-

So, according to the formula, first, you need to figure out the amps of the battery. Let’s assume the solar panel watts are 240W and the battery’s voltage is 12V.

- Amps = Total size of solar panel watts / voltage of the battery

= 240W solar panel watts / 12V voltage

= 20 amps

- Now, the required charging time = Battery capacity (Amp-hour)/ Amps

= 100ah battery capacity / 20 amps

= 5 hours

## How many amps does a 100-Watt solar panel generate?

If you want to calculate the amps of your battery that a 100W solar panel generates, you need to divide the solar panel power in watts by the battery voltage in volts. So, the equation will be the same as above.

As you may know, a 100W solar panel usually charges the battery in 12V battery voltage. So, the amps will be-

- Amps = Total size of solar panel watts/voltage of the battery

= 100W solar panel / 12V battery voltage

= 8.33 amps

So, with a 12V battery feeding power, your 100W solar panel will produce 8.33 amps per hour.

However, when measuring the output, the voltage of your battery will be 18V instead of 12V. So, in that case, the amps will be 5.5 amps (using the same equation- 100W/18V = 5.5 amps).

Eng. Matthew Joseph Nandirio is the Founder of walkingsolar.

After graduating from the University of Houston in 2002, matt started working as a Solar Electrical Engineer for several multi-national solar energy companies.

He has a wide range of experiences including solar system requirement analysis, planning, maintaining, debugging and even solar device development through research.

He now shares his 20 years of expertise through his articles on the walkingsolar website.

Further, he is also the author of two books on Solar Technology, “Solar Power for Villages” and “DIY Solar System for Dummies”.