What is Watt-Hour(Wh)? (Chapter 2 Lesson 4)

In the previous chapter, we learned that a battery’s Ampere-hour (Ah) rating tells us how much electrical charge it can store.

But here’s an interesting question. Suppose you have two batteries, both rated at 2 Ah.

Does that mean they store the same amount of energy? Not necessarily.

Let’s find out why.

Why Ah alone isn’t enough

Imagine these two batteries:

• Battery A: 3.7 V, 2 Ah
• Battery B: 9.6 V, 2 Ah

Both batteries can ideally deliver 2 A for 1 hour.

So they store the same amount of electrical charge.

However, Battery B delivers that charge at a much higher voltage. Since electrical energy depends on both voltage and charge, the 9.6 V battery stores significantly more energy than the 3.7 V battery.

This is why Ampere-hours alone cannot be used to compare the total energy stored in batteries with different voltages.

For that, we use Watt-hours (Wh).

What is a Watt-hour?

A Watt-hour (Wh) is a unit of energy. It tells us how much electrical energy a battery can supply over time. To understand Watt-hours, let’s first recall what a watt is.

Electrical power is calculated using the formula:

Power (W) = Voltage (V) × Current (A)

Power tells us how quickly electrical energy is being used or delivered.

Example: Calculating power

Suppose a 100 Ω resistor is connected across a 9 V battery.

Using Ohm’s Law,

Current = Voltage ÷ Resistance

Current = 9 V ÷ 100 Ω = 0.09 A

Now calculate the power.

Power = Voltage × Current

Power = 9 × 0.09 = 0.81 W

This means the resistor converts electrical energy into heat at a rate of 0.81 watts.

Calculating battery capacity in Watt-hours

Battery energy can be calculated using a simple formula:

Battery Energy (Wh) = Battery Voltage (V) × Battery Capacity (Ah)

This allows us to compare batteries with different voltages.

Example 1

Battery:

• Voltage = 3.7 V
• Capacity = 2 Ah

Energy = 3.7 × 2 = 7.4 Wh

This battery stores approximately 7.4 watt-hours of energy.

Example 2

Battery:

• Voltage = 9.6 V
• Capacity = 2 Ah

Energy = 9.6 × 2 = 19.2 Wh

Although both batteries have the same 2 Ah capacity, the second battery stores much more energy because it operates at a higher voltage.

What does a Watt-hour mean?

A battery rated at 7.4 Wh can ideally supply:

• 7.4 W for 1 hour
• 3.7 W for 2 hours
• 1 W for approximately 7.4 hours

Similarly, a 19.2 Wh battery can ideally supply:

• 19.2 W for 1 hour
• 9.6 W for 2 hours
• 4.8 W for 4 hours

These are ideal values. In practice, the actual operating time depends on battery efficiency, temperature, discharge rate, and the connected load.

Ah vs Wh

Although both ratings describe battery capacity, they measure different things.

For batteries with different voltages, Watt-hours provide a much better comparison of how much energy each battery can store.