Conversions Resource

Interactive Sizing & Electrical Reference

The Big Battery Marketing Illusion

If you search for a portable power bank for your phone, you will be bombarded with astronomical numbers: "Massive 30,000 mAh Capacity!" If you look at deep-cycle RV batteries, you'll see labels proudly proclaiming "100 Ah." But if you look at a high-end Jackery or EcoFlow portable power station, they are rated in "Watt-Hours (Wh)."

This inconsistency isn't just confusing; it allows marketers to disguise the true energy capacity of their products. To compare batteries fairly, you must understand that Amp-Hours (Ah) is a useless measurement unless you also know the battery's voltage. To find the true, absolute capacity of any energy storage device, you must convert it into Watt-Hours (Wh).

Key Takeaway: An Amp-Hour simply tells you how many amps a battery can deliver for one hour. A Watt-Hour tells you exactly how much absolute total energy is stored inside the chemical cells.

The Conversion Formula

Converting Amp-Hours to Watt-Hours is an application of Watt's Law (Power = Voltage × Current) extended over time.

Watt-Hours (Wh) = Amp-Hours (Ah) × Nominal Voltage (V)

If the battery is rated in milliamp-hours (mAh), you must first divide the mAh by 1,000 to convert it to Ah before multiplying by the voltage.

Wh = (mAh / 1000) × Volts

Why Voltage Changes Everything

Let's look at why buying a battery based solely on Ah is a terrible idea. Imagine you have two batteries in front of you. Both are labeled "100 Amp-Hours." Without knowing the voltage, you might assume they hold the same amount of power.

  • Battery A: A standard 12V lead-acid car battery.
    100 Ah × 12V = 1,200 Watt-Hours
  • Battery B: A 48V lithium solar backup battery.
    100 Ah × 48V = 4,800 Watt-Hours

Battery B holds four times as much energy as Battery A, despite them both having identical "100 Ah" labels. This is why comparing a 50Ah 24V battery to a 100Ah 12V battery can be confusing until you convert both to Watt-Hours (they both equal 1,200 Wh!).

The Power Bank Deception (3.7V vs 5V)

The most egregious use of misleading battery specs happens in the USB power bank industry. When you buy a "10,000 mAh" battery pack, you likely expect it to charge your phone (which has a 3,000 mAh battery) more than three times. But it rarely does. Why?

USB power banks list their mAh rating based on the internal lithium-ion cell voltage, which is 3.7 Volts. However, standard USB charging requires 5.0 Volts (and fast charging requires 9V, 12V, or 20V).

Let's do the math on a "10,000 mAh" pack:

  1. Convert to Ah: 10,000 mAh / 1000 = 10 Ah
  2. Calculate True Capacity (Wh) using the internal cell voltage: 10 Ah × 3.7V = 37 Watt-Hours
  3. Now, see how many Amp-Hours that actually provides at the 5.0V USB output requirement: 37 Wh / 5.0V = 7.4 Ah (or 7,400 mAh).

Before you account for any heat losses or circuit inefficiencies, that 10,000 mAh battery can only output a maximum of 7,400 mAh at standard USB voltage. If you look at the tiny print on the back of any reputable power bank, you will see a line that says "Rated Capacity: 37Wh." This is the only number that matters.

Airline Travel Limits: The FAA and TSA enforce a strict limit on lithium-ion batteries carried onto passenger aircraft: 100 Watt-Hours per battery. Airport security does not care about mAh; they check the Wh rating. A standard 26,800 mAh power bank (at 3.7V) equals 99.16 Wh, which is precisely why that specific size is so popular—it is the absolute largest battery you can legally fly with.

How to Use Watt-Hours for Real-World Sizing

Once you have converted your battery's capacity into Watt-Hours, determining how long it will run your devices becomes incredibly simple. You just divide the battery's Watt-Hour rating by the total Watts your devices consume.

Run Time (Hours) = Battery Wh / Device Load (Watts)

Worked Example: Off-Grid CPAP Machine

You are going camping and need to run a CPAP machine that draws 40 Watts continuously. You purchase an EcoFlow portable power station rated at 512 Watt-Hours. How long will the machine run?

  • Total Energy: 512 Wh
  • Continuous Load: 40 W
  • Calculation: 512 / 40 = 12.8 Hours

(Note: In reality, DC-to-AC inverter inefficiencies will drain about 10-15% of the power, so you should expect closer to 11 hours of functional run time.)

Conclusion

Amp-Hours are an outdated legacy metric tied to an era when almost all batteries were 12-volt lead-acid blocks. In today’s world of 3.7V cell phones, 18V power tools, 48V solar racks, and 400V electric vehicles, Ah is a meaningless number without context. By converting everything into Watt-Hours, you normalize the playing field, cut through misleading marketing, and empower yourself to calculate precise, reliable run times for your critical electronics.

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