How Many Watts From a Solar Panel Does It Take to Charge a 12-Volt Battery?

When it comes to solar panels and solar batteries, it’s more about how long your solar panels have been outside, how efficient they are, and how many of them you have. In this short guide, we’ll tell you how many watts it takes from solar panels to charge a 12-Volt battery.

The longer you have your solar panels outside, and the more solar panels you have, the more battery-life you’ll be left with at the end.

Many people are often confused about Wattage—it’s a measurement of energy over some time, but we’ll talk more about that in the article. Let’s jump into it!

THE MATH BEHIND IT

Amp-hours is the current supplied multiplied by time. The electricity that runs through a solar panel goes to the battery as current. This current is multiplied by the amount of time that a battery has been charging for. For example, 50 Amps being supplied to a battery for four hours equals 200 Amp-hours.

Check out the writing on the side of the battery to see how many Amp-hours it holds.

Now, to figure out how many Amps you need, you’ll have to multiply the Amp-hour capacity on your battery by about 30%, which is the general rule of thumb. For example:

500 Amp-hour battery x 30% = 150 Amps to charge up the battery

To get Wattage, you simply multiply the Amps needed by the Volts. If we still use our example of the 500 Amp-hour battery and the 12-Volt battery, we would get:

150 Amps x 12 Volts = 1800 Watts

That’s a lot of Wattage for one solar panel! Fortunately, since most conventional solar panels usually produce about 250 watts per panel, you can use about eight standard solar panels to charge a 12-Volt battery with varying levels of efficiency.

This is done just using examples for reference. Your solar battery might have a higher or lower Amp-hour limit, or it might have a higher Voltage amount. For comparison, a 12-Volt car battery has a capacity of 48 Amp-hours. That means that when the battery is fully charged, the battery can deliver one Amp for 48 hours, two Amps for 24 hours, three Amps for 16 hours, and so on.

WHAT TYPE OF SOLAR CHARGE CONTROLLER DO I NEED TO HANDLE THE VOLTAGE?

Solar charge controllers are devices used for overload protection, low-voltage disconnects, and block-reverse currents within solar systems. You will most likely need a solar charge controller for safety and give your solar battery a longer life.

There are two types of solar charge controllers: pulse width modulation (PWM) and maximum power point tracking (MPPT). PWM charge controllers are cheaper but less efficient, whereas MPPT charge controllers are highly efficient and expensive. Both controllers are used widely, do a great job protecting the battery, and last around 15 years.

Choosing the suitable solar charge controller is simple; if you have a 12-Volt battery, you select a charge controller that’s rated at 12 Volts. The same goes for 24-Volt and 48-Volt batteries.

Safety factors and current capacity impact your choices but not by much—some math may be involved. For example, a solar panel with a current rating of 8 Amps could produce up to an extra 25% electricity during a sunny day with packed snow everywhere since extra sunlight bounces off snow and reflects into your solar panels. That 8 Amps multiplied by 1.25 turns into 10 Amps of current running into your solar battery. In this case, a 10 Amp controller would be recommended.

It takes a little bit of playing around with your solar system to truly determine the proper size of solar charge controllers since the amount of power produced every day is different. There are many safety factors, efficiency factors, and other factors that throw off the math involved. Still, there are plenty of online calculators that will tell you approximately everything you’ll need.

WHAT TYPE OF SOLAR INVERTER DO I NEED FOR A 12-V BATTERY?

A solar inverter is another excellent device used within solar systems. Solar inverters need to fulfill two types of needs: peak or surge power and the typical or usual power.

Surge power is the maximum power that the inverter can supply for a short time (usually just a second). Some appliances don’t require a lot of power to run, but they need a lot more power to start the system up—some examples are motors, pumps, compressors, air conditioners, freezers, and refrigerators.

Usual power inverters are used to supply steady power for continuous use—their power is much lower than surge power inverters since they only need to keep their appliances running after starting up.

To determine what kind of a solar inverter you’ll need, let’s run through a quick example. Say that you have a couple of common appliances that need electricity to run.

Let’s write down their Wattages and multiply the ones that require more power to start up by two:

  • Lights (200 Watts)
  • Television (150 Watts)
  • Heater (1,500 Watts x 2 = 3,000 Watts)
  • Refrigerator (1,000 Watts x 2 = 2,000 Watts)

The total Wattage equals around 5,350 Watts. For this example, you’ll need a power inverter capable of handling 5,350 Watts of power. Even though there are only 2,850 Watts of energy used for continuous supply, you’ll have to plan for the start-up power so that the inverter can handle the load.

Now, you must calculate how long you plan on running everything for. Using the same example, we have 2,850 Watts of continuous power, and let’s say that we plan on running these appliances for 4 hours. First, we must divide the continuous power by the Voltage of the battery:

2,850 Watts / 12 Volts = 237.5 Amps per hour

Now, we must multiply the current per time by the amount of time running:

237.5 Amps per hour x 4 hours = 950 Amps

If the batteries available are rated at 300 Amps in this example, you can divide the 950 Amps by the rated 300 Amps, so you’ll need about 3-4 batteries for your system.

We know that the math can be complicated, but many online calculators are available that’ll tell you the correct size and type of solar inverter that’s best for your solar system.

COMMON FAQS

Many questions are out there that mostly talk about making sense of the solar panel specifications on the owner’s manual or online on guides. We’re here to break those long, technical words down into meanings that are easy for people to understand.

What is a solar panel open-circuit Voltage (Voc)?

The open-circuit Voltage is the highest Voltage that a solar panel can output with no load on the panel. Generally, the open circuit Voltage is achieved between 10 AM and 2 PM since that’s when the sun is the highest in the sky, and the panel temperature is still relatively low.

Why is it important? The open-circuit Voltage amount multiplied by 1.035 needs to be less than the maximum solar Voltage that the solar charge controller is rated for. Otherwise, there might be some power loss or destruction on the battery.

What is a solar panel short circuit current (Isc)?

The short circuit current is the amount of current that comes out of the solar panels when the positive and negative terminals are shorted together. This current can be measured with a multimeter, but be careful to have the correct settings, or damage might occur to the multimeter.

Why is it important? The short circuit current multiplied by 1.2 should be the most that the solar charge controller can handle. This is generally achieved on a sunny day when the solar panels face the sun directly in the sky.

What is the optimum operating Voltage (Vmp)?

The optimum operating Voltage is the Voltage available when the panel is connected to a specific load and operates at its maximum capacity under standard testing conditions. Most solar panel manufacturers and engineers specify the optimum operating Voltage to be around 70-80% of the total open circuit Voltage.

What is the maximum series fuse rating?

The maximum series fuse rating is another indicator of the recommended size of a solar charge controller or other overcurrent protection devices that can be used within the solar system. Usually, this value has a rating of around 15-20 Amps.

Still, if calculations from above give you a slightly different value for what kind of a solar charge controller to have for your system, it’s entirely up to you on what type of rating you’d like to get for a solar charge controller.

KEY TAKEAWAYS

Here’s a TLDR (Too Long, Didn’t Read) version of this guide. It lists all of the essential takeaways that should be noted when looking at calculations, the owner’s manual, or other specifications:

  • Take the Amp-hour value of your 12-Volt battery and multiply it by 12 Volts and 0.3 to know how many Watts it will take to charge your 12-Volt solar battery.
  • Divide the Watts by the Wattage that the solar panel is rated for to get an estimate on how many solar panels it will take to charge your battery.
  • Get a 12-Volt solar charge controller for your 12-Volt battery, a 24-Volt charge controller for your 24-Volt battery, and so on.
  • Add up the total Wattage of your appliances (multiply the ones that need extra start-up power by 2) and divide it by the Voltage of your battery (usually 12 Volts). Take that number and multiply it by however many hours you plan on leaving the solar panels out—this is the Amperage that your battery should be rated at.
  • If the number in the previous bullet point is too high, you can distribute it amongst multiple batteries if needed.

WRAPPING UP

We hope this guide helped you figure out those complicated values on your solar panel and solar batteries. Wanting to know how many Watts it’ll take to charge a 12-Volt battery shouldn’t be hard, and it isn’t—it just looks pretty complicated.

Don’t let the math behind it scare you; it’s straightforward to calculate the correct values, but if you’re still having issues, you can always use an online calculator to figure out how many Watts it’ll take to charge your solar battery!

Kabeer Bhatia

Kabeer Bhatia has several years of experience camping and backpacking, and focuses more on the technical side of wilderness technology including solar energy.

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