If you are buying batteries based on "Ah" alone, you are likely calculating your range wrong. In this deep dive, we explain why Watt-Hours is the only universal metric for energy, debunk the "Powerbank" marketing scams, and explain Peukert's Law comparisons between Lead Acid and Lithium.
The Water Tank Analogy
In the world of portable power, two units dominate the conversation: Amp-Hours (Ah) and Watt-Hours (Wh). While they both measure "capacity," they are not interchangeable. Confusing them can lead to dangerous system mismatches or disappointing runtimes.
To simplify, let's use the water analogy:
- Amp-Hours (Ah): This is the Volume of water in the tank. A 100Ah tank holds "100 units" of electrons.
- Voltage (V): This is the Height of the tank (Pressure).
- Watt-Hours (Wh): This is the Potential Energy (Work) that the water can perform when it hits the bottom.
A bucket of water (10Ah) dropped from 1 foot (1V) does very little work. That same bucket dropped from 1000 feet (1000V) can crush a car. This is why looking at Ah without knowing the Voltage is meaningless.
1. The Formula for Truth
The relationship is governed by the Power formula:
Watt-Hours (Wh) = Amp-Hours (Ah) × Voltage (V)
This formula allows you to compare apples to oranges. Let's look at a common confusing scenario.
Case Study: The "20,000mAh" Power Bank
You see a USB power bank on Amazon advertised as "20,000mAh" (20Ah).
You also have a small 12V 7Ah motorcycle battery.
A novice might think: "The Power Bank has 20Ah, the motorcycle battery has 7Ah. The Power Bank is 3x bigger!"
Let's do the Math (Wh):
- USB Bank: The cells inside are 3.7V.
$20Ah imes 3.7V = 74 Wh$. - Motorcycle Battery: The cells are 12V.
$7Ah imes 12V = 84 Wh$.
The motorcycle battery actually contains more energy, despite having a much lower Ah number. The marketing department uses the cell voltage (3.7V) to inflate the Ah number, but the work is done by the total Watt-Hours.
2. Why Airlines Care About Wh
Have you ever noticed the TSA limit for batteries is 100Wh? They don't list an Amp-Hour limit.
This is because a 100Ah battery at 1.2V (NiMH) is fairly safe. A 100Ah battery at 50V (Lithium) is a bomb. Watt-Hours represents the total chemical energy stored that could be released in a fire.
To check if your drone battery is legal to fly:
$14.8V (4S) imes 5Ah = 74Wh$. (Safe to fly).
3. Comparing Voltage Systems (12V vs 24V vs 48V)
When designing a solar bank, you have a choice of voltage.
Let's say you need to store 5000 Watt-Hours (5kWh) of energy.
- At 12V: You need 416 Amp-Hours. (Requires massive 4/0 cables).
- At 48V: You need 104 Amp-Hours. (Requires standard 4 AWG cables).
Both banks store the same energy. Both will run your TV for the same amount of time. But the 48V system is more efficient because it moves that energy at higher pressure and lower flow (Amps), reducing heat loss in the wires. See our Ohm's Law Guide for more on this.
4. The Lead Acid Lie: Peukert's Law
Ah ratings on Lead Acid batteries are often misleading due to a phenomenon called Peukert's Effect.
Lead Acid capacity is rated at a "20-hour rate" (C/20). A 100Ah lead battery will give you 100Ah only if you drain it slowly over 20 hours (5 Amps).
If you drain that same battery in 1 hour (100 Amps), you won't get 100Ah. You might only get 55Ah. The internal resistance wastes nearly half the energy as heat.
The Lithium Advantage:
Lithium has a Peukert constant very close to 1.0.
A 100Ah Lithium battery will give you ~100Ah whether you drain it in 20 hours or 1 hour.
This is why a "100Ah" Lithium battery effectively replaces a "200Ah" Lead Acid battery in high-power applications. You get the usable energy you paid for.
5. Calculating Your Requirements
When sizing a battery bank, ignore Ah initially. Calculate your daily load in Watt-Hours.
- Refrigerator: 50 Watts x 24 hours = 1200 Wh
- Laptop: 60 Watts x 4 hours = 240 Wh
- Lights: 10 Watts x 5 hours = 50 Wh
- Total Daily Energy: 1490 Wh
Now you can shop for batteries. You need a battery with >1490Wh usable capacity.
If buying a 12V battery: $1490Wh / 12V = 124Ah$.
If buying a 24V battery: $1490Wh / 24V = 62Ah$.
Summary
Amp-Hours are useful for matching cells within a specific voltage platform. But Watt-Hours are the truth. Whenever you are comparing batteries of different voltages, chemistries, or form factors, always convert to Watt-Hours to see the real value.
