Manufacturers tend to advertise battery life, but battery capacity is also important. Battery life is an estimate. It doesn’t tell you exactly what you’re getting.
Battery capacity is a more accurate measurement, and it helps you determine more about your machine.

Capacity helps determine energy efficiency
There are many ways to make a laptop with a battery that lasts more than 10 hours. One approach is to stuff as many battery modules into the laptop chassis as possible. Another way is to see how long you can use the smaller battery.
Laptops with 40W batteries use less than 4W per hour so the battery can last more than 10 hours. On the other hand, a laptop with an 80W battery can work for more than 10 hours, even if it uses 7W of power.
Information seen on the box or website that says the laptop lasts more than 10 hours doesn’t tell you how much power it uses. But if you also know the battery capacity of the laptop, then you can calculate it accurately.
Battery capacity affects electricity usage
Having a smaller battery means you use less power each time you charge your computer. If your laptop has a 30W battery, that’s how much power you consume whenever you charge it from 0% to full.
To see the impact on your electricity bill or battery bank, you may need to convert this to kW. There are 1,000 watts = 1 kilowatt, so 30W = 0.03kW. If you’re paying 12 cents per kilowatt, this adds up to 0.36 cents per charge.
0.36 cents doesn’t sound like much, and it really isn’t. Even if you run out of battery and recharge your laptop every day, your electricity bill only adds up to about 10 cents a month. But there are other factors to consider. For example, a laptop with a smaller battery may have a less power-hungry CPU. This means that your computer not only uses less power to charge, but also uses less power when operating.
Bigger battery means longer life

Nobody makes a powerful laptop with a small battery. It won’t last long enough to be considered portable. If you want a laptop that can play the latest games or compile videos, you might want one with a larger battery.
But if you are comparing different models, you may notice a correlation between processing power and battery capacity. Larger batteries can support longer usage time than small batteries.
If you’re comparing two machines that are identical, except for one with a larger battery, the decision is indisputable. But reality is not so simple. The small battery makes the laptop more compact and portable. Bigger batteries are more powerful. Which factor is more important to you depends on your preferences and needs.
(Refer to QTM)