How Many Batteries Can a 100W Solar Panel Charge?

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100W solar panels are useful in many applications, and when combined with a battery provides you with greater flexibility. The question though is how many batteries can a 100W solar panel charge? The answer depends on several factors and that is what this article covers.

A 100 watt solar panel can charge a 35ah battery in 5-6 hours. The charge time will take longer if there is not nough sunlight available.

How to Calculate 100 Watt Solar Panel Battery Charging Power

The formula is sun hours x 100W / battery volt = battery charge capacity

If you live in Arizona you can get up to 7-8 hours of sunlight during the summer. While solar panels don’t always perform at peak levels , you can get 700W to 780W per day. You can expect similar numbers in other sunny states.

Now that you know the watts per day, it is easy to find out how many batteries the panel can charge.

Divide the total wattage by the battery volt. Assuming you have a good 12V battery, something like the Ampere Time 50ah 12V Lithium for example:

700 / 12 = 58.3 amps

So a 100W solar panel that produces 700W a day can provide 58.3 amps to a 12V battery. That number will change depending with a different voltage (example 24V), but the conversion procedure is the same.

If you have a 100W solar panel and a 12V 100ah battery, the panel can charge it up to 50% capacity. Lead acid batteries require recharging before it drops to 50%, so the panel can top it off in a day.

More about Battery Charging

The calculation formula above is based on a few important assumptions, in particular sunlight availability,.

In Arizona, Nevada and other nearby states, expect 5 to 7 sun hours during the summer. This will allow the module to produce the 500W to 700W total mentioned above.

But it is a different matter if you live in New York, Vermont, Seattle or other cold weather states. The same module may only generate 350W or so the entire day. You also have to factor in the season and climate.

In other words, the charging capacity of a powerful system like the Renogy 100W Monocrystalline Solar Panel will depend on the sun, the season and your location. However the formula will always be the same no matter the battery voltage or solar panel size.

The information here also shows why there are a lot of variations when it comes to solar power. It has nothing to do with solar panel inefficiency, though a high efficiency panel helps. It simply has to do with the way the modules convert the sun’s energy into electricity.

How Much Battery Capacity is Usable?

The usable amp hours of a battery are measured in 20 hours of use. Divide the amp hours by 20 and you get the amp hours you can draw for 20 hours.

If you have a 12V 100ah battery, divide 100 by 20, since 20 hours is the usable time that most solar batteries go by. If the number is other than 20 it will be indicated on the battery, but here we assume it is 20.

100 / 20 is 5

So a 100ah battery can deliver 5 amps for 20 hours. You can draw more amps for fewer hours, 10 amps for 10 hours for instance. But as the discharge rate increases, the battery capacity decreases, so a 100ah battery is only good for 40 minutes of use.

A 100W solar panel is equal to 8.33 amps (100 / 12 = 8.33), so an amp of current can charge the battery by 1 amp for 1 hour. You can use this formula for other types of batteries and solar panel sizes.

The charging capacity will be affected by the battery DOD (depth of discharge) rate. Lead acid batteries require recharging at 50%, while lithium ion batteries are good for up to 70% or more. While you can technically use a lead acid battery below 50%, it is not good for the battery’s long term health. It will shorten the life cycle and require a replacement sooner than later.

If you have a lithium battery you can use it for longer periods. You pay more for lithium, but you don’t have to worry about the 50% recharge limit, you can keep going until 70% or more.

What Can a 100W Solar Panel and Charged Battery Do?

We have listed what devices you can run with a 100W solar panel, but you can use it to run anything that does not exceed the available power. If your module is generating 90W, then you can use any number of devices totaling 90W or less.

If a 100W panel produces 500W a day you can run a 10W bulb for 50 hours straight, a 60W laptop for 7 to 8 hours or a 100W TV for five hours. You can also run each device for just a few hours so you can run several of them at the same time.

If you have a solar battery you have even more choices. As long as there is enough sun available to charge the battery you can keep going. You can use any battery type or size, but the bigger the capacity the more devices you can run. However the charge will be limited to the capacity of the solar panel.

You can increase the output by connecting multiple solar panels together. Two 100W panels gives you 200W, ideal if you have a high capacity deep cycle battery that needs a lot of amps. Just like with everything solar, it comes down to planning.

Remember that the battery you choose determines how many amps you can use. If you need 400W a day from your lead acid battery, the capacity has to be at least 66ah. The formulas are:

watts / volts = amps

amps x volts = watts

400W / 12 – 33.3, but since a lead acid battery is only 50% usable, you need 66ah at least. That is the minimum because if it is winter or you live in a cold weather state, you should probably get a 100ah battery instead.

Why is My Solar Panel Not Working?

Just as solar batteries cannot consistently produce at peak level, the same rule applies to solar panels. In sunny states you could receive peak sun for longer stretches, but even then it won’t last the entire day.

In ideal situations with no clouds, true south orientation and lots of sun, expect close to peak production for several hours. During cold weather expect the opposite so you have to make adjustments to its charging capacity.

Because there are a lot of variables involved, you have to account for them when you plan. Suppose you have a 100W solar panel and a 100ah battery. Calculate how many sun hours are available so you can estimate how many amps the panel can supply to the battery.

If it is summer and you live in the Midwest, there is probably more than enough sunlight available to keep the battery topped off. If it is winter you have to make allowances in your computation. The rule of thumb is to keep the battery charged so you can still use it if there is limited sunlight.

Tips For Charging Batteries with 100W Solar Panels

Sun hours. This is the number one factor. Your location and the season determines how many sunlight hours are available. You can avoid a lot of problems by taking those things into account. Do not use general information about solar panel power as your guide, you have to factor in your own location and season.
Battery usable capacity. Divide the amp hours by 20 and you have the usable amp hours in the battery. Again this will save you headaches when computing how much power is left.
Panel output and ratings. Solar panel production fluctuates as the sun’s angle and intensity changes, and this affects the power going into the module.
Your Usage. Determine how many devices you want to use. That will tell you how many watts are required and if a 100W panel is enough.
Make allowances. Just as you would want extra power for your inverter , you should do the same for solar panels and batteries.
Solar panel efficiency. Efficiency is the amount of sun energy the panel converts into electricity. 20% or higher efficiency is ideal.

The bottom lien is there is no single answer to how many batteries a 100W solar panel can charge, because production is affected by a lot of variables. During the summer and in sunny states, a solar panel can provide a higher charge. During the winter, solar production will go down. You can calculate how much solar power you will get on average though, and that will help.


A lot of the confusion that stems from solar panels is due to the math. Yes there is a lot of number crunching involved, but it is important to get the most out of your system. With 100W solar panels and charging batteries, it comes down to how much sunlight is available.