Car battery wattage: 3 essential tips your car dealers might not tell you

(Last Updated On: October 20, 2021)

When you turn the key in your car’s ignition, nothing happens.

You know your car’s battery has been dying, and you now need to shop for a replacement.

This article will show you how to calculate the average wattage of a typical 12-volt car battery and why it is essential to know when shopping for a replacement battery for your car.

1. Watt, Amp, and Volt explained

Amps, volts, and watts are all vastly different. We don’t give a second thought to how electrical energy moves from our car’s battery to bring the engine to life. While this is a complex process, let’s begin at the top to understand Watts, Amps, and Volts better.

Amp and Volt

Imagine a garden hose with water flowing from its nozzle. The flow rate of water, the volume of water that passes through the nozzle, is the electrical energy we measure in amps. This can be represented by 5 amps, 10 amps, 50 amps, and so on.

Continuing with the water analogy, we can explain voltage too. Voltage is the pressure at which the water is pushed through the hose. Of course, a garden hose requires pressure to travel from the faucet through the nozzle. That electrical pressure is the voltage. This can be represented by 12 volts, 110 volts or 220 volts, or anything in between and higher.

Watt

What is wattage? Back to the water analogy, wattage is the power of the water shooting out of the hose’s nozzle. While the hose may be narrow in diameter, the water flows at a rate of 5 amps. You’ve only given the faucet handle a quarter-inch turn to open the water flow, so the pressure is a constant 12 volts. The power of the water spraying from the nozzle is 60 watts. Wattage is the power of that spraying water due to the pressure and the water’s flow rate. Wattage is the electrical energy’s power.

The majority fitted to family cars are of the lead-acid battery variety when it comes to vehicle batteries. These most commonly have a 12-volt rating. Different sized batteries have various capacities and outputs.

A battery’s primary function is to start a car’s engine. A healthy car battery has a voltage of 12.6 volts. This electrical pressure makes the electricity flow from the battery to the ignition system to start the engine.

The amp ratings of a battery tell you its storage capacity. The larger the size of the battery, the higher the amp rating. Most car batteries have a capacity of around 300 amps, and they can range from 300 amps to over 1000 amps.

However, that’s not the whole story. We still need to understand how and why a battery’s capacity is essential. A car battery’s capacity is measured in amp hours (represented by Ah). This is calculated by how long a battery can provide a constant supply of 1-amp per hour. The higher the amp-hour rating, the longer the battery can provide power to a device. An average family vehicle’s battery has a 48 amp-hour rating. This means that it can provide 1 amp of power for 48 hours before being fully discharged.

2. How to calculate wattage and watt hour

Wattage

To calculate wattage, multiply the number of amps by the number of volts. This formula can be represented by W = A x V. For example, if a car’s battery has 12 volts and 300 amps, that equates to a 3,600-wattage rating.

Why does one need to calculate wattage? A vehicle’s starter motor’s power is calculated in wattage. The average family sedan requires around 3,600 watts to start its engine. Electrical components are rated on how many watts of energy they use.

You might have noticed that vehicle batteries have a CCA rating. This is important to note. CCA stands for Cold Cranking Amps. This is the power that is required to crank over an engine. When temperatures plummet during winter times, more energy is needed to start an engine. CCA is the number of Amps that a 12-volt battery can deliver at 0 degrees Fahrenheit for 30 seconds. For example, a 12-volt car battery with a 300 CCA rating can provide 300 amps for 30 seconds at 0-degrees Fahrenheit.

Watt hour

A battery’s watt-hour rating describes how much electrical energy it can hold. A watt-hour (expressed as Wh) is the amount of energy used in one hour. The watt-hours of a battery depending on the battery size, temperature, condition of the unit, and other factors.

Let’s return to the example of the standard family sedan’s battery that is a 12-volt and 48-amp-hour lead-acid battery. To calculate the watt-hours (Wh) of this battery, multiply the volts (V) by the amp-hours (Ah). This standard car battery has a 576 watt-hour rating, meaning that it can deliver 1 watt of energy for 576 hours before being fully discharged.

3. How to pick the right battery with the right wattage

Battery size

Why is battery size important when picking out the right battery for your car? The Battery Council International (BCI) created specifications by which battery manufacturers can produce batteries of universal sizes. These specifications are known as group sizes listing batteries’ physical dimensions. These provide the length, width, and height of batteries in inches and millimeters. If you buy a battery that’s not the right size for your car’s battery tray, it won’t fit properly and may lead to damage.

The larger the group size, the more power it provides and stores. The most common group sizes are 24, 24F, 27, 34, 35, 48, 49, 65, and 78. These group sizes are based on your vehicle’s make, model, and engine size.

For instance, the latest Acura, Honda, Infiniti, Lexus, Nissan, and Toyota models are equipped with group size 35 batteries. Large SUVs and pickup trucks from Ford, Dodge, and the likes usually have a group size of 65 and larger.

Remember, a battery’s size determines the capacity of electrical energy it can hold. The larger a battery is, the more power it can provide for starting a given car’s engine.

How would you know what size battery you should get for your car? There are few ways you can check for this:

  • Check the group size on your old battery.
  • Consult your vehicle’s owner’s manual.
  • Check online resources for your car’s make, model, and engine specifications.

Battery brand

When shopping for a new vehicle battery, you’ll most likely be overwhelmed by the vast array of brands available. The best advice is to buy the same battery brand as specified in your car’s owner’s manual. However, suppose you stick to the biggest brand names from the most prominent car battery retailers. In that case, you’ll be buying a great battery that will likely provide between 3 to 5 years of dependable service in your vehicle.

Brand names to keep your eye on are:

  • AC Delco
  • Champion
  • Duralast
  • Everstart
  • Exide
  • Motorcraft
  • Optima
  • Odyssey

Those brands regularly top the reviews for the best battery.

Reserve capacity

Reserve Capacity (or RC) might be the last item in our checklist to consider when buying a new car battery, but it is far from the least important. The RC of a car battery helps power your vehicle’s electrical system for several minutes if the engine’s alternator fails. A battery’s Reserve Capacity is measured by how many minutes a fully charged battery operating at 80-degrees Fahrenheit can discharge 25 amps until the battery drops below 10.5 volts. In practical terms, the higher the RC number, the better for your vehicle.

You will find the RC rating on the battery’s label and the documentation for the specific battery online. RC ratings range from below 90 to well over 150. This means that the battery can reserve 90 to 150 minutes of energy to the engine’s alternator to keep the motor running.

Final thought

Shopping for a vehicle battery replacement isn’t as straightforward as simply picking one up at your local outlet. The size of your car’s battery tray determines the group size of battery you can purchase, your car’s engine size, the climate where you live, and the type of peace of mind you wish from your car’s battery in emergencies.

If your car’s battery still has legible labels and stickers on it, and the battery lasted its full expected 3 to 5 years, you can purchase the same make and model battery again. By consulting the vehicle owner’s manual, you’ll also learn about your car’s type and size battery rated. If you have none of those resources available to you, a quick check online will help to inform you of the stock battery that your vehicle originally came with. You can then start an online search for a replacement in that way.

Vehicle batteries might seem unimportant. That is until they stop working properly. Make sure your replacement battery is from a reputable brand you recognize.

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