🔋 Battery Basics: Watt-Hours vs. Amp-Hours for Tool Performance

When shopping for cordless power equipment, the battery is the engine. You’re constantly bombarded with two main specifications: Amp-Hours (Ah) and Watt-Hours (Wh). Which number really matters, and how do they relate to the runtime and power of your tools?

The short answer: Watt-Hours (Wh) is the single most important number for measuring the total energy stored in a battery, but you need to understand both to make smart purchasing decisions.

⏳ Amp-Hours (Ah): The Capacity Rating

Amp-Hours (Ah) measures the battery’s capacity—specifically, how long a battery can deliver a certain amount of electrical current (Amps) before running out of power.²

• Definition: If a battery is rated at 4.0 Ah, it means it can theoretically sustain a current draw of 4 Amps for one hour, or 1 Amp for four hours.
• The Problem: Ah is useful for comparing batteries within the same voltage line. For example, a 40V 4Ah battery has twice the capacity and runtime of a 40V 2Ah battery.
• The Trap: Ah is not useful for comparing batteries across different voltages. A 60V 4Ah battery is much more powerful and has far more total energy than a 40V 4Ah battery, even though the Amp-Hour rating is the same.

Think of Ah as the size of the fuel tank.

⚡ Watt-Hours (Wh): The True Energy Score

Watt-Hours (Wh) measures the total amount of energy stored in the battery pack, incorporating both the capacity (Ah) and the system’s electrical pressure (Voltage, V).³ This is the only standardized measurement for stored energy across all platforms.

The relationship is simple:

$$\text{Watt-Hours (Wh)} = \text{Voltage (V)} \times \text{Amp-Hours (Ah)}$$

• Definition: Wh tells you exactly how much total work the battery can perform.⁴
• The Advantage: This calculation lets you compare batteries across different voltage lines fairly.

Real-World Comparison

Let’s compare three different battery packs you might find on the market:

Conclusion: Packs A and B both store the exact same amount of total energy (240 Wh), meaning they will provide similar runtime when used in tools that demand the same overall Watts. However, Pack B (60V) will deliver that power more efficiently and with less heat than Pack A (40V), as we discussed in the previous article on voltage.

Think of Wh as the total amount of fuel (BTUs) in the tank.

💡 Matching Specifications to Performance

For serious DIYers and professionals, don’t just look for the highest Ah number—that can be deceiving. Instead, focus on the system voltage (V) needed for the power level your task requires, and then compare batteries based on the total Watt-Hours (Wh) to determine your necessary runtime.