How Long Does a 100W Solar Panel Take to Charge a Battery?

A 100 watt solar panel can power mobile devices and run small appliances in homes, RVs and other locations. But is it enough to charge a 12V battery? And how long will it take? We will answer those questions right now.

A 100 watt solar panel generates 5.5 amps an hour, so it takes 9 to 10 hours to charge a 12V battery. Divide the solar panel voltage by its wattage and you can determine how many battery amps per hour the solar panel produces.

Calculate 100W Solar Panel Battery Charging Time

There are three things we need to know: the battery size, voltage and how many hours of sunlight are available. Note: we use 12V solar panels and batteries, but you can try this calculation with 24V solar systems like the Newpowa 120W Solar Panel.

Example: you want to charge a 50ah 12V battery and there are 6 hours of sunlight.

Most 100 watt solar panels have a nominal rating of 12 volts, but this can go up to 18 volts when charging.

Divide solar panel voltage by its watts. Let us assume the solar panel produces 100 watts an hour.

100 / 18 = 5.5

A 100W solar panel that produces 5.5 amps an hour will fully recharge a 12V battery in 10 hours.

5.5 amps x 10 hours = 55 amps

There is another way to calculate the charge time:

Battery amps x volts = watts
Watts / solar panel output = estimated charge time

So let’s take that 50ah 12V battery again:

50 x 12 = 600

A 50ah battery has 600 watts. Divide this by the solar panel output, in our example 100 watts.

600 / 100 = 6

Why does this calculation show charge time is 6 hours instead of 10? Because it uses 12 volts instead of 18. If we used 18 volts the results will be closer to the previous calculation.

So which is the right one? With solar panels you should always be conservative with your estimates and assume it will take longer.

Either way both methods offer estimates only. There are many factors that influence solar panel production as we will explain.

Why is My 100 Watt Solar Panel Taking So Long to Charge Batteries?

The calculation formulas given are useful, but there are limitations. It assumes the solar panel produces maximum output and the skies are always clear. Plus we also assumed the battery was fully discharged, that is completely empty.

In real life you’ll encounter different scenarios that affect the battery charge time. While solar power has many benefits there are situations you need to know especially when using a 100 watt solar panel.

The following goes over some of these situations and what you can do about them. These are applicable to all solar panels regardless of size.

Solar Panel Output vs. Rating

A solar panel rated at 100 watts is not guaranteed to produce 100 watts. In ideal weather the panel might produce close to that, but rarely will it sustain that for 5 to 6 hours a day.

The solar panel rated output (100 watts in this instance) refers to its peak production. Solar panels are manufactured and tested in controlled (ideal) conditions where they achieve peak output.

Unless you are very lucky you won’t be able to charge a battery under perfect conditions. There will be some clouds, dirt, wind blowing leaves etc.

So instead of 100 watts an hour the panel might generate 90, 85, 80 watts etc. Let’s assume the panel generates 85 watts an hour.

85 / 18 = 4.7

The output drops to 4.7 amps an hour. Instead of 10 hours the system needs 11 hours to charge a 12V battery.

4.7 x 11 = 51.7

A 100 watt solar panel producing 85 watts an hour can charge a 12V battery in 2 days. This is provided there are 6 hours of sun each day. Charging time is going to take 3 days if there are only 5 or 4 sun hours a day.

These situations affect all solar panels, from 100 watts to large solar arrays . In our example we can see the effect sun hours and the environment have on battery charge time.

These same factors also influence rooftop solar systems that run entire households. Which is why most solar powered homes are grid tied or have a generator or battery bank that complement solar power.

Environment Factors

There are many factors we can cite, but the following are the most important. These will not stop solar panels from producing power, but it can slow or reduce output below 100 watts.

Clouds. Solar panels need a clear path to to the sun to covert its rays into current. A passing cloud serves as a hindrance. The longer the cloud blocks the sun, the longer the battery takes to charge. Your best bet is to power batteries during a clear day.

Dirt, dust, leaves and debris. These particles are carried by the wind and can scatter all over the solar panel. Even a small amount of dirt or leaves can affect performance. When you set the panel down to do a charge, make sure it is free of dirt.

If you have to clean the panel, use water or a mild soap. Large solar arrays may require professional assistance, but you can clean a 100 watt panel easily.

Orientation. A solar panel has to be oriented true south to receive maximum sunlight. This in addition to making sure the panel is free of dirt and debris.

Battery Depth Discharge

Throughout the examples we presume you were charging an empty battery. But if you use AGM or gel batteries, they need to be recharged before capacity drops to 50%.

These batteries are not designed to be fully discharged. You can try and empty the battery but it will probably stop around the 15% or 10% mark.

There are long, technical explanations why you should not fully discharge lead acid batteries. But basically a full discharge isn’t good for the battery life cycle.

So if you follow this rule and recharge a 12V battery at 50%, the charging time is cut in half. From 10 hours for a 50ah battery, a 100W solar panel can charge it in 5 hours.

Quality AGM units like the 2 Piece 100ah WindyNation AGM Batteries have a higher DOD (depth of discharge) and lithium batteries up to 90%. Some lithium battery manufacturers say you can even do a full discharge.

Charging at 50% capacity is faster, but the drawback is you will not be able to use the full capacity. You can either buy a lithium battery or or double the batteries so you can use as much power as needed.

How Many 12V Batteries Can a 100 Watt Solar Panel Charge?

The only limitation is the number of batteries you can connect to the solar panel. Ideally a 100 watt solar panel should charge one battery at a time.

The biggest reason is the output. Assuming there are 6 hours of sun and the panel produces 600 watts, that is equal to a 12V 50ah battery.

it will take 12 hours for a 100W solar panel to charge a 100ah battery. And that is possible if there are six hours of sunlight and the panel somehow produces 100 watts every hour.

You can see the limitations of this solar panel, but it is still useful for small appliances, batteries and devices. It has no problems charging a 25ah battery for instance, and you can charge several mobile devices too.

If you want to charge several batteries, connect two or more 100 watt solar panels in parallel. Four of these gives you 400 watts for example. Make sure you have a charge controller to prevent the solar panel from overcharging the battery.

If you want to charge a 24V battery, connect three 12V solar panels in a series. This increases the voltage to 36V and enough to power the battery.

You can also connect batteries in a series-parallel to get the best of both. As long as you know what this solar panel can do, you can use it to charge 12V batteries and power other devices.

Conclusion

Solar panels are now available in larger sizes, but 100 watt modules remain popular due to its portability and versatility. It does have limitations however, so if you want to charge a 12V battery with a 100 watt solar panel, make sure you understand how long it can take.