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How Long Will a 100-Watt Solar Panel Take to Charge a 12V Battery?

Charging a battery with a solar panel has nothing to do with how many watts the solar panel produces. It has to do with how many amps your solar panel can produce and how much energy can be stored in your battery.

For example, if you have a 12V 10 watt-hour battery, charging it with a 100-watt solar panel will take about 6 minutes. However, if you have a 12V 1000 watt-hour battery, charging will take about 10 hours.

Because a 1000 watt-hour battery can store 100 times more energy than a battery with just 10 watt-hours of storage capacity.

So the question of how long it takes to charge a 12V battery with a 100 Watt solar panel doesn’t provide sufficient data for an accurate answer.

But you may not always find watt-hours listed on a battery. Instead battery capacity will be listed in amp-hours (Ah). To find watt-hours from amp-hours, divide the watt-hours by the volts. So a 12V battery with 100Ah of capacity will have 1200 watt-hours.

So to answer the question, how long it takes to charge a 12V battery with a 100-watt solar panel, will depend on how many amp-hours the battery has and how many amps the solar panel can produce.

The above calculations are done considering a perfect situation: the solar panel is constantly perpendicular to the sun, the battery is new, there is no shading, and the battery and solar panel is working at their optimum temperatures.

In the real world, all of these factors will reduce the charging time. Because a solar panel will never likely reach its peak power rating.

Now let’s take a look at some of these factors that will affect the charging time of a battery:

What does affect the charging time of the battery?

There are several factors that will affect how long it takes to charge a battery with a solar panel. Here are the most important ones:

The angle of the sun to the solar panel

The angle of the sun to the solar panel affects the charging time by changing the amps the solar panel produces. The better the angle, the more amps the solar panel will produce.

If the sun is perpendicular to the solar panel, it will produce the most amps and charge the battery the quickest. As the sun moves later in the day, the amps produced by the solar panel may decrease, and takes longer to charge the battery.

Availability of a Charge Controller

A charge controller is a device that regulates the charging of a battery. It helps to prevent overcharging and also allows you to optimize the charging based on the type of battery and solar panel you are using.

If you do not have a charge controller, the solar panel will try to put as much energy into the battery as possible.

This can overcharge and damage the battery. So if you are using a solar panel and battery that are not specifically matched, it is recommended to use a charge controller to prevent damage to the battery.

The amount of cloud coverage

The amount of cloud coverage and atmospheric conditions also affect the charging of the battery. If it is overcast, the battery will not charge as quickly as if it was a sunny day.

The temperature of the battery and solar panel

The temperature of the solar panel also affects the charging of the battery. Solar panels work best when they are between 25 and 30 degrees Celsius. If the solar panel is cooler or hotter than this, it will not produce as many amps.

Batteries also do well typically when they are between 0 and 30 degrees Celsius. If the battery is cooler or hotter than this, it may get difficult to charge.

Condition of the solar panel

As solar panel ages, they will not produce as many amps as when it was new. So an older solar panel may take longer to charge a battery than a newer solar panel.

Thus, you shouldn’t expect the calculation to be the same for a brand new solar panel and a 15-year-old solar panel.

Condition of the battery

As a battery age, it will not hold a charge as long as when it was new. This also affects how long it takes to charge the battery with a solar panel.

For example, lithium-ion batteries typically lose 12.4%–24.1% of their capacity after 500 cycles. This translates to 0.025–0.048% capacity loss per cycle.

Wiring and connectors on the solar panel and battery

Wiring and connectors on the solar panel and battery also affect charging time. If they are not in good condition or improperly connected, it may take longer to charge the battery.