How Many Watts is a Car Battery? [A Complete Guide!]

Most people know electricity involves three basic factors; volts, amps, and watts. Car batteries are no different and use these same three factors to provide power to our cars. The watt component is an important part of the equation since it will determine how much power can be supplied by the car battery before it goes flat or is damaged.

Batteries for small cars are usually a minimum of 1000 watts to provide starting power to the starter motor and engine-off systems. Large cars with big engines, more electronics, or cars used in cold climates require batteries of 4000 watts or more. Heavy duty vehicles may have 6000W batteries.

Understanding the watts in a car battery means exploring the other two components of a battery and how the combination affects the performance the battery can offer. Our complete guide for the watts in a car battery is easy to understand but will require some basic math for a complete picture.

What Is The Watt Rating Of A  Car Battery?

All electrical circuits will have a combination of volts, amps, and watts which will determine the power produced in the circuit and what devices the circuit can power.

An electrical system in a car works the same way, with the battery being the power storage system used to start the car and keep essential components running when the engine is turned off.

Car batteries will be rated differently depending on the size of the motor in the vehicle and the additional lights and accessories the battery must power when the engine is not running.

This means that all car batteries do not have the same watt rating, which makes choosing the right battery for your car important.

How To Determine A Car Battery’s Watts

The watt rating of a car battery is one part of a 3-part electrical equation, which means that if you have the other two components, you can calculate the watts.

The equation for the calculation is as follows.

Watts = Amps X Volts

If you have the amp and voltage components of the equation, you can work out the watts. Most car batteries are 12V, and the amps for the battery are usually marked on the battery on its specification plate.

As an example, a 12V battery with a 100 amp rating will have the following watt rating.

Watts = 100Amps X 12Volts

= 1200Watts

Why Are Watts Important In A Car Battery?

The watt rating of your car battery is important because it determines the amount of power the battery can generate.

The power generation of the battery is important since more power is needed for certain types of vehicles and running conditions.

  • Large engines. Large car engines need a large starter motor, which requires more power from the battery to power the starter motor.
  • Smaller engines. Smaller engines have less mass in the pistons, requiring less power from the starter motor. A smaller starter motor requires less power from the battery.
  • Cold weather. Cold weather makes car engines take longer to start than warm weather. More powerful batteries are needed in cold weather to provide more power for longer to start cars in cold climates.
  • Old ignition systems. Old cars with inefficient ignition systems need batteries with more power to start the car.
  • Accessories requiring power. The more accessories in a car that require power when the engine is not running, the higher the wattage needed on the battery. Components such as alarm systems, electronic locks, proximity sensors, lights, and sound systems all run directly from the battery when the engine is not running.

The more components a car battery must run when the engine is off, the larger the battery must be to prevent it from being completely drained overnight while it powers these components.

What Are Watt-Hours In A Car Battery?

The watt-hours of a car battery are sometimes referred to as the battery capacity. It is the measure in hours of how long the battery can sustainably supply its rated watts before becoming depleted and requiring a recharge.

The watt-hour rating is the measure of how many watts the battery can deliver over a period of time. This rating of your car battery will tell you how long the battery will be able to power the supplemental systems of the vehicle before the battery becomes flat.

You will need to know the amount of watts your car system draws from the battery while the engine is switched off. The amount can be measured at the car battery using a multimeter.

Car batteries are designed to provide large, short-duration power output rather than a sustained power draw over time. However, a car battery can provide low levels of power to certain components when the engine is off, but these must be small-draw systems.

Battery Capacity To Power Engine-Off Systems

Most of the power draw of a car when the engine is of will be small amounts. The current draw for these systems should be less than 50mA to prevent quickly depleting the battery charge. Most cars will have a current draw of 3mA to 5mA. Why are we talking about current when we should be talking about watts?

As mentioned previously, watts, amps, and voltage are all related. In our example of a 5mA current draw, we can calculate the watts used with the previous formula.

Watts = Amps X Volts

Watts = 0.05 X 12Volts

= 0.6Watts

This power will be drawn from the battery, giving us a reading of 0.6Wh or watt-hours. Car batteries usually have an amp-hour rating, which can be converted to a watt-hour rating with the following formula.

Watt-hours = Amp-hours X Volts

Thus, a 100AH battery will have a watt-hour rating as follows.

Watt-hours = 100Ah X 12Volts

= 1200Wh or Watt-hours

This result means that the battery can supply 1200Watts for a period of 1 hour. If your car requires 0.6Watts, the battery in our previous example with 1200Wh should be able to provide power to your car while the engine is off for about 83 days!

How Many Watts Do You Need For Your Car Battery?

Most smaller cars, such as hatchbacks and small sedans, typically require a battery of at least 1000W to provide enough engine starting power and power the engine-off systems.

Larger vehicles such as SUVs, pickup trucks, large sedans, and luxury vehicles will generally need a larger battery that delivers 4000W or more.

Large trucks, camper vans, and motorhomes require a large battery of 5000W to 6000W to cope with the added power demands on the battery.

Selecting a battery with a watt rating that is too low for your vehicle will result in frequent depletion of the battery, leaving you stranded with a flat battery. It also shortens the battery’s life and will require a replacement much sooner than an appropriately sized battery.

Car manufacturers usually stipulate the most suitable battery sizes for the various models, but if you modify the car’s electrical system in any way, the minimum battery size must be adjusted.


Car battery watts are an important battery metric to consider when purchasing a new battery for your car or making any modifications to the car that may draw more capacity from the battery.

The concept is not complex but must be considered together with the current (amps) and voltage specifications of the battery to match it to your car’s electrical requirements.

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