How Many Watt-Hours in a Car Battery?

There is a lot of jargon relating to batteries, talking about measurements, capability, power, etc. There is voltage, ampere, amp-hours, Watt, and watt-hour. It is essential to know these terms to choose or identify a faulty battery. Do not worry; they are quite easy to understand, and this article will break down what they mean- with a focus on watt-hours. 

Voltage – this is the difference in electric potential between two points. It is also defined by words such as pressure or tension. It is measured using a voltmeter. 

Ampere – measurement of the amount of electrical current moving through an electrical conductor. 

Amp Hours – this is the amount of current (amperes) multiplied by time.

Watt – this is the standard measure of electrical power.

Watt-Hours – the amount of power generated per time

What are Watt-Hours?

Firstly, Watt is the measure of how much power a device consumes. It is the voltage multiplied by the number of amperes. Watt-Hours is a measurement of a battery’s capability. So when choosing a battery, you have to look out for how many Watt-Hours the battery can hold.

The equation for Watt-Hours = total number of watts x total number of hours. 

How Many Watt-Hours in a Car Battery

Are there Instruments that Measure Watt-Hours?

  • The watt-hour meter is a device consisting of a small electric motor (also known as a transducer) and a counter that measures and records the electric power moving through a circuit. The different types of watt-hour meters are Mercury type, commutator type, induction type, and specialized type. Mercury and Commutator meters measure in direct current circuits. Induction-type circuits measure alternating current circuits and are mostly found outside of buildings. Specialized power meters consist of two types- totalizing meters and portable meters. Totalizing meters record the power used in more than one circuit, while portable meters are highly accurate for testing installed watt-hour meters. 
  • Fluke Power Quality Analyzers require a step of calculation to determine watt-hours. The device can be used to measure average wattage, which is then multiplied by the amount of time over which the average wattage was measured. 

How Many Watt-Hours in a Car Battery?

The watt-hour is not directly measured on a car battery, so to get this, you need to multiply the number of amperes with the battery voltage. Amperes measure how much electricity flows in a battery per second, while voltage is the amount of energy an electric circuit gets, which is usually fixed (6V, 12V, 36V, etc.). The battery used for a device should have a corresponding voltage to the device.  

The amp and watt-hours are important factors to consider in choosing a car battery. 

For example, if a battery is at 200 watts, producing 5amps in 10 hours. The amp-hours here will be 50 amp-hours, multiplied by the 200 watts, which are 1kW/hrs.

How Many Watt-Hours can a Car Battery Supply? 

Typically, a car battery’s watt-hours are, on average, 720 watt-hours. This is because a car battery usually has an amp-hour of 60AH and a voltage of 12V. 

The duration also depends on how much of the power is used to supply electrical accessories such as headlights, stereo, and so on. 

How Many Watts to Start a Car?

The wattage of a car depends on various factors such as the model, how recent it is, its service and maintenance history, and the make of the vehicle. Typically, a lead-acid battery is about 12V, which will have around 60 watts of power to start an engine before requiring more to keep up the capacity. 

Electric Cars and Wattage

Electric vehicle power output depends on the motor volume and the wattage of incoming current. In electric vehicles, the power supplied by the electric grid or charger is not used up completely. The actual useful energy is mechanical energy. The kWh determines an EV’s efficiency. For example, a 50kWh battery can cover up to 400 km in range. How the car is used also determines its electricity consumption. For example, the higher the speed, the more energy required, and if high speed is maintained for a long period, it could wear out the battery. One of the best ways to increase the range of an electric car is to eco-drive, that is, drive relaxedly at a steady pace. 

How Many Watt-Hours to Charge a Car Battery?

To fully charge a 48 amp, the flat battery would take about 24 hours for a basic charger that charges at 2 amps. The higher the charge output, the quicker a car battery charges. For example, for a charger that charges at 10 amps, it would take about 5 hours to fully charge to 48 amps. A regular car battery being charged with the usual charge amp of 4-8 will take between 10-24 hours to fully charge. Ensure to charge your car battery slowly to maintain its life cycle. Higher amps equal a faster charge, but it comes with some risks, such as overheating. 


  • How many watt-hours is in a 12V battery?

To calculate this, you need to determine the amp-hours. If the amp-hours 1Ah, then the watt-hours is the product of 12V and 1Ah, which is 12Wh

  • Why do amps matter?

In building an electrical system, it is important to know the amps to guide your choice of wire sizes to prevent hazards such as fires. Higher amps need thicker, bigger wires. 

  • What is reserve capacity, and how does it apply?

Reserve capacity is a measurement of how long a battery can maintain a useful voltage under a 25 amps discharge. It determines how long a battery can power before needing a recharge. This plays a role in the battery life cycle and choice of battery. Look out for batteries with high reserve capacity that suits your engine’s needs. 

  • What does battery life cycle mean?

This is the number of cycles a battery can go before complete failure. One cycle is the battery going from full charge to full discharge and then back to full charge again. 

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