How Long to Charge a 100ah Battery with a 100W Solar Panel?

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Solar panels are sold in assorted wattages, but the 100W remains in demand among RV owners for its power and expandability. One question that’s often asked though, how long will it take a 100W solar panel to charge a 100ah 12V battery? There are many factors to consider, but the most important are the panel’s efficiency and the battery’s discharge capacity.

A 100 watt solar panel produces 8.33 amps an hour, so it is going to take 13 hours to charge a 100ah battery. If the battery is at 50% capacity, expect a 6 to 7 hour charging time.

How to Calculate 100W Solar Panel Battery Charge Time

The easiest way to calculate is to take the amps produced by the solar panel and multiply by the number of sun hours available. From there we can determine how many hors are needed to charge a 100 ah battery.

Solar panel amp output x number of sun hours available = battery charge time

If you have a quality monocrystalline solar panel like the Reongy 100W you can expect the output to be close to 100 watts or 8.33 amps an hour.

100 / 12 = 8.33

If the battery is completely discharged it would take 13 hours to replenish it.

8.33 x 13 = 108.2

A 100W solar panel can produce 108.2 amps in 13 hours, enough to recharge the battery. It would take two and half days assuming there were six sun hours available each day. If you have a fast charger like the Renogy AGM 100 expect the charge time to be a bit shorter.

You can use these same steps for other solar panel sizes. Just replace the watts and volts and you can estimate the runtime.

The 4 Factors That Affect Battery Charging Time

These assumptions are based on ideal weather conditions. If the skies are cloudy expect longer charge times. There are other factors to consider: the battery depth discharge, the solar panel output, weather and temperature. These three have a direct effect on how long a solar panel charges a battery.

Temperature. Contrary to popular myth, solar panels do not benefit from very hot temperatures. What solar panels need is the sun, not heat. Solar voltage rises with the temperature while amps drop, which means longer battery charge.

In our examples the solar panel is at 12 volts and produces 8.33 amps. But the voltage can reach 18V at midday during summer. if the solar panel voltage goes up to 18 volts.

100 / 18 = 5.5

Amp output drops to 5.5 amps.

A 100W solar panel charging at 12 volts needs 13 hours to recharge a 100ah battery. But if is an 18 volts the charge time goes up to 19 hours. So if your solar panel reaches 18V, the battery will take longer to charge.

Solar Panel Output. Solar panels cannot always produce their rated output. If the module is rated 100 watts that is its highest potential output. And this is achieved under ideal conditions. if the weather is cloudy then the output will drop, from 100 watts to 90 or lower.

Our calculations for a 13 hour charging time are based on the assumption that the solar panel generates 100 watts. If the panel does that for 13 hours the battery gets charged. But if the output drops , it could take 3 days to get the battery ready.

Depth of Discharge. AGM and gel batteries have a 50% depth discharge, meaning you have to recharge it when the energy capacity hits 50% remaining. You only get to use half the capacity per charge, but this does speed up charging. If it takes 13 hours to refill a depleted battery, it will only take 6 or 7 if you follow the 50% DOD rule.

Weather. You’ve probably heard this all before, but it’s worth repeating. The weather plays the most important part in all our calculations. The number of hours given in our calculations presuppose sunny weather. If the weather goes south it will take longer to top a battery. If you live in Arizona or another place with lots of sun, charge time will be cut down.

Before doing a charge, check the weather in your area. It will help you get an idea of how much time it’s going to take. The solar panel’s quality and that of the battery also plays a role. However it’s really the weather that can make or break a charge. You’re going to play weatherman yes, but that’s something solar power users do all the time.

How Many Solar Panels to Charge a 100ah Battery in 5 Hours?

it takes a 240 watt solar panel to charge a 100ah battery in five hours. This assumes the weather is ideal and the panel produecs 240 watts an hour.

To figure it out we have to make some assumptions. First we assume the battery is 12 volts, though you can change this to 24V in the following calculations.

Second there are 5 hours of sunlight available. Depending on your location this could 4 to 8 hours day. Again, just change it to fit your situation.

Third, the solar panel is positioned properly and is getting a high level of sunlight. There are no shades or obstructions blocking it. Fourth, the battery is completely discharged.

The calculation is:

100ah battery / 5 sun hours = 20A
20A x 12V = 240W

You won’t find a lot of 240W solar panels around unless you combine two 120W panels together. Better to buy a 250W or a 300W solar panel instead. A 300W solar panel is ideal because it produces more power in case the sky is overcast. Of course you can also connect 3 100W solar panels to do the job.

Any charge controller will do, though MPPT controllers are more efficient than PWM. MPPT controllers are best for high power charging and it will work well here. A PWM will also get the job done so it’s up to you. The difference is an MPPT controller keeps the battery charging at 14.4V at all times. If you don’t mind the energy wasted though, PWM is fine.

If your battery is of a different voltage, use that number in the calculations above. Do the same for the number of sun hours available. And if the battery is only half discharged, it will take just two and half hours to op it off with a 300W solar panel. Unless sunlight is severely limited in your area, you should be able to charge the battery in a day.

Why is My Solar Panel Not Producing Enough Power?

As we explained solar panels do not always reach stated output power. A 100W solar panel can probably reach about 85% efficiency on some days.

This efficiency level applies to all solar panels. Panels with a higher level of efficiency have more power, superior design (i.e, monocrystalline instead of polycrystalline) and larger. You could always install more solar panels and speed up charge times. But for purposes of discussion we will stick to one 100W solar panel. But if you can add more solar panels that will definitely speed up battery charge time.

We’re assuming your solar panel is also working properly. And solar panels need little maintenance anyway. As long as you don’t do anything risky (like directly loading onto the solar panel), then it should charge batteries fine.

How to Make Battery Charging Faster

If the thought of charging a 100ah battery for days is unappealing, there are a few ways to speed it up. The first is to use more solar panels (more panels, more power = faster battery charge). The second is to not let the battery discharge to zero.

We have a guide here on how to make batteries last, but recharging at the 50% mark is recommended. It’s good for two reasons, one it prolongs the battery’s life cycle, and two, charge time is cut in half.

Look back at the calculation we did earlier. it would take 14 hours to recharge a fully discharged 100ah battery. But what if it you recharged it at 50%? You only need 50ah to top it, or 7 hours. If it’s summer you can probably charge it in a single day. With lithium batteries you’ve got some more leeway, maybe recharge at 40% or even 35%.

if you have to use 100ah daily, it’s better to buy a higher capacity battery than let it drain completely. If your usage is 100ah every day, buy a 200ah battery. When you’ve used it up (i.e. drops to 100ah), recharge it. instead of 50ah now it needs 100ah, so it will take 14 hours again. If you pair the 100W panel with another, you can cut it down to 7 hours.


The key to successfully using solar panels is to know how much they’re capable of. Now that you know how long it takes for these panels to charge 12V batteries, you can be better prepared. If you’re new to solar energy, this information is going to go far in helping you maximize power output.