How Many Batteries Do I Need for a 100w Solar Panel

How Many Batteries Do I Need for a 100W Solar panel?

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100W solar panels are very popular because of their size, power and affordability. They are ideal for RVs and running small appliances and devices. To get the most out of this solar panel you have to connect it with the proper battery. In this post you will learn exactly what battery size and type is the best.

A 100W solar panel requires a 100ah 12V battery minimum. Solar panel output can range from 400-900 watts so the battery capacity must be at least 1000 watts. 100ah is equal to 1200 watts so it is sufficient.

To find out the right battery size, you have to know how much power your solar panel generates per day. Once you have this information it is easy to figure out how many batteries are needed.

How Many Watts Does a 100W Solar Panel Produce?

Solar panels need sunlight to produce power. The more sunlight or peak sun hours available, the more electricity the panel generates.

To find out how much power a 100w solar panel produces, multiply 100 x peak sun hours. If you get 5 hours of sun per day, your solar panel can produce 500 watts (100W x 5 hours = 500).

If you live in Arizona the average peak sun hours is 7-8 hours per day. Using the formula above a 100W solar panel can produce up to 700-800 watts a day.

On the other hand if you live in Illinois or New York, the peak sun hours are around 3.5-4 hours. In California and Oklahoma it is about 6 hours. So you have to know how many peak sun hours your location gets.

And bear in mind this is the maximum possible output. Under perfect conditions a 100W solar panel can produce 500 watts in 5 hours, or 800 watts in 8 hours and so on. In reality the output will probably be a bit lower than that.

This does not mean solar panels don’t work. Of course you should get a high quality system like the Newpowa 100W solar panel, but its output will still depend on the weather. And as we all know the weather is rarely, if ever perfect.

So if you get 5 hours of sunlight per day that will probably be closer to 450 than 500 watts. On some days it might be even lower depending on the prevailing condition.

The important thing is you know how to calculate how much power your 100W solaR panel can give you. The next step is to find out how many batteries you will need to store that energy.

What Battery Size Do I Need?

The number of batteries you need depends on how much power your solar panel produces. You also have to consider how much of that power you need to use per battery charge.

If you get 5-6 hours of sun per day, your solar panel gives you anywhere from 450-600 watts. So you need a battery that can store at least 600 watts. A 12V 100ah battery will be sufficient in this case.

Battery capacity is given in amp hours (ah). To find out its equivalent in watts, multiply by its voltage. So if you have a 100ah 12V battery then it would look like this:

100 x 12 = 1200

The battery has a maximum capacity of 1200 watts. Even if your solar panel output reaches 800 watts a day there is plenty of space available.

Bear in mind though that this is the maximum capacity, not how much you can use. Depending on the battery depth of discharge, this can be anywhere from 50-100%. So let us take a look at what this means.

Usable Battery Capacity

Usable capacity means exactly that, how much of the stored solar energy you can use per battery charge. This is known as depth of discharge (DOD). This applies to 100W, 400W solar and all deep cycle batteries.

For example: if you filled a 100ah battery with power from your 100W solar panel, it holds 1200 watts. But it does not necessarily mean you can use up all that power. If the battery has a 50% DOD you can only use 600 watts or 50ah before it has to be recharged.

The depth discharge rates varies, but in general:

  • Lithium batteries: 85-100%
  • AGM batteries: 75%
  • Gel batteries: 70-75%
  • Flooded lead acid batteries: 50%

Imagine you have a 100W solar panel and you receive 6 peak sun hours per day. The maximum output would be 600 watts in perfect conditions:

100 x 6 = 600

If you have a lithium battery, you can use 85 to 100ah of its capacity. Meaning you have access to the full 1200 watt capacity. High end lithium batteries with a 100% DOD allow you to use every last bit of its stored power without fear of damage.

But if you have an AGM or gel battery, you can only use up to 70-75ah or 840-900 watts. And if you have a flooded lead acid with a 50% DOD, only 600 watts is available for use. Once power drops to 50ah you have to recharge the battery. You can use this same method to figure out how long a 500ah battery lasts.

This goes to show how DOD and sun hour availability can change the performance of a 100W solar panel. So think carefully about what type of battery you should buy and how much of that power you need to use.

How Long Does a 100W Solar Panel Charge a 100ah Battery?

A 100W solar panel generates about 30 amps an hour. If there is 5 hours of sunlight per day, it will take 4 days to fully recharge a 100ah battery.

To find out how many amps your solar panel has, divide maximum power rating by maximum power voltage (VMP). For a 100W 12V solar panel that would look like this:

100 / 18V = 5.5 (5.5 amps or rounded off to 6)

A 100 watt solar panel has a maximum power rating of 100 watts, that is clear enough. But what about the voltage?

If you look at the spec sheets of 12V solar panels you will see that its VMP is 18 volts. This is because 12V solar panels can generate up to 18V when charging. This is why the calculation is 100 / 18 – 5.5.

Note that 18V is under ideal conditions. In certain cases the voltage might be lower. But assuming your solar panel gives you 6 amps an hour, it is easy to figure out how long it takes to charge a 100ah battery.

If you get 5 hours of sunlight per day, a 100W solar panel will need 4 days to recharge an empty 100ah battery.

6 amps x 5 hours = 30 amps a day
At 30ah a day you need four days to get to 100ah.

If your area gets 8 hours of sunlight, that solar panel will charge the battery in 3 days:

6 x 8 = 48
48 x 3 = 144

These calculations assume that the battery is fully discharged. If it is only 50% discharged the charging time will be sliced in half.

Which Battery Should You Buy?

In other words, a 100ah battery is the minimum size needed to store power from a 100W solar panel. However, the right battery size for you depends on how much energy you need.

To recap:

A 100W solar panel can give you 400-800 watts a day depending on how many hours of sun are available. The minimum battery size should be 100ah.

Batteries have different depth discharge levels ranging from 50-100%. If you need to use all 1200 watts in a day, get a lithium battery like the Weize 100ah which is built for heavy duty use.

So before you buy a solar panel or battery, calculate your power requirements and use that as a guide to find out the ideal solar panel and battery combination.