Understanding Battery Voltage, Amps, Wattage, and Capacity and Why It All Matters

Batteries may look simple on the outside, but several key measurements determine how they perform, how long they last, and what devices they can safely power. The main terms you’ll see are voltage (V), current or amps (A), capacity (Ah or mAh), and wattage (W). Each plays a different role, and understanding them helps you choose the right battery for the job.

1. Voltage (V): The “Pressure” of the Battery

Voltage is the electrical pressure that pushes power out of the battery.

Higher voltage = more electrical force available.
Devices are built to run at specific voltages (e.g., 3.7V for lithium-ion cells, 12V for car batteries).

Why it matters:
Using the wrong voltage can damage electronics or cause them not to run at all. Voltage must match what your device requires.

2. Amps (A): The Flow of Electricity

Amps measure the current, or how fast electricity is flowing from the battery into the device.

A common misconception is that a battery always outputs its full amperage. It does not.

Devices pull (or draw) the amps they need.

A battery can supply up to a certain maximum amperage.
The device decides how much it actually uses.

Example:
If a device needs 2A, a battery capable of supplying 10A will still only deliver 2A.

Why it matters:
If the device needs more amps than a battery can safely provide, the battery overheats or shuts down.

3. Capacity (Ah or mAh): How Much Energy the Battery Stores

This is one of the most important measurements for runtime.

Ah (amp-hours) = how many amps the battery can deliver continuously for one hour.
mAh is the same thing but smaller (1,000 mAh = 1 Ah).

Example:
A 3,000 mAh battery can supply

3A for 1 hour
1.5A for 2 hours
0.5A for 6 hours

Why capacity matters:
More Ah = longer battery life, assuming voltage and usage remain the same.

4. Wattage (W): The Total Power Used or Delivered

Wattage ties everything together:

W = V × A

This tells you how much real power the device consumes.

Example:
A 3.7V battery delivering 2A is providing:
3.7V × 2A = 7.4 watts

Wattage is especially important for comparing batteries of different voltages, because it measures power in a way that’s voltage-independent.

5. Watt-Hours (Wh): The Most Important Measurement for Battery Life

While Ah tells you capacity, watt-hours (Wh) gives the true measure of how long a battery lasts across different voltages.

Wh = Voltage × Ah

This is the most reliable way to compare batteries.

Example:
Two batteries:

Battery A: 3.7V, 3,000 mAh
- 3.7 × 3 Ah = 11.1 Wh
Battery B: 7.4V, 2,000 mAh
- 7.4 × 2 Ah = 14.8 Wh

Even though Battery A has “more mAh,” Battery B actually holds more energy because of its higher voltage.

Why Wh matters most:

It tells you exactly how much total energy the battery stores.
It predicts runtime better than any other measurement.
It lets you compare any batteries, no matter the voltage.

Summary

Voltage = electrical pressure; must match the device.
Amps = the flow of current; the device draws what it needs.
Ah/mAh = capacity; more means longer life.
Wattage (W) = total power being used at any moment.
Watt-hours (Wh) = the true measure of how long a battery will last.

🔋 Most important measurement for battery lifespan?

➡️ Watt-hours (Wh) — because it accounts for both voltage and capacity.