Wh stands for watt-hours, a unit that measures how much total energy a battery can store. It tells you not just how much current a battery can deliver, but how much actual work it can do before it’s dead. You’ll find this rating on everything from laptop batteries to home solar storage systems, and it’s the single most useful number for comparing batteries of different types and voltages.
How Watt-Hours Work
A watt-hour represents one watt of power sustained for one hour. A battery rated at 100 Wh can, in theory, power a 100-watt device for one hour, a 50-watt device for two hours, or a 10-watt device for ten hours. The formula is straightforward:
Watt-hours = Volts × Amp-hours
So a 12-volt battery rated at 100 amp-hours (Ah) holds 1,200 Wh, or 1.2 kWh. A 24-volt battery with only 50 Ah holds the same 1,200 Wh. The voltage and amp-hour numbers changed, but the actual energy stored is identical. That’s exactly why Wh is the better comparison tool.
Why Wh Matters More Than Ah
You’ll often see batteries rated in amp-hours (Ah), especially car batteries and power tool packs. Amp-hours measure electrical charge, which is how much current a battery can push over time. The problem is that amp-hours ignore voltage entirely. A 3.7-volt phone battery rated at 5,000 mAh holds about 18.5 Wh. A 12-volt RV battery rated at 100 Ah holds 1,200 Wh. Comparing those two by amp-hours alone would be misleading because the voltage difference is enormous.
Watt-hours fold voltage into the equation, giving you a single number that represents usable energy. When you’re shopping for a portable power station, comparing solar battery systems, or figuring out if a backup battery can run your fridge overnight, Wh (or kWh for larger systems) is the number that actually answers the question.
Calculating Runtime From Wh
Once you know a battery’s Wh rating, estimating how long it will power a specific device is simple division:
Runtime (hours) = Battery Wh ÷ Device watts
A 500 Wh portable power station running a 60-watt mini fridge would last about 8.3 hours in a perfect world. In practice, no battery converts 100% of its stored energy into usable power. Inverters, heat loss, and the battery’s own electronics eat into that total. A more realistic formula multiplies the battery’s Wh by an efficiency factor, typically around 0.85 to 0.90 for lithium-ion systems:
Realistic runtime = (Battery Wh × 0.85) ÷ Device watts
Using that correction, the same 500 Wh station would run the 60-watt fridge for closer to 7 hours.
Common Wh Ranges by Device
Battery capacities span a huge range depending on what they’re powering:
- Smartphones: 15 to 20 Wh (typically listed as 4,000 to 5,000 mAh at 3.7 volts)
- Laptops: 50 to 100 Wh
- Portable power stations: 200 to 2,000 Wh
- Home solar batteries: 5,000 to 15,000 Wh (5 to 15 kWh). The National Renewable Energy Laboratory uses a representative residential system of 12.5 kWh as its baseline.
- Electric vehicles: 24,000 to over 100,000 Wh (24 to 100+ kWh), with the average EV battery sitting around 73 kWh
At the small end, milliwatt-hours (mWh) describe tiny cells in earbuds and smartwatches. At the large end, kilowatt-hours (kWh) are standard for EVs and home storage. They’re all the same unit, just scaled up or down.
Battery Chemistry and Energy Density
Not all batteries pack the same amount of energy into the same space or weight. This is measured as energy density, expressed in Wh per kilogram (Wh/kg) or Wh per liter (Wh/L). Lithium-ion batteries store roughly 150 Wh/kg, while old-school lead-acid batteries manage only about 25 Wh/kg. That six-to-one difference in energy density is why lithium-ion dominates in phones, laptops, and EVs, where keeping weight and size down matters enormously. Lead-acid still shows up in applications where weight is less important and cost is the priority, like backup power for server rooms.
Wh Limits for Air Travel
The TSA uses watt-hours as the cutoff for lithium batteries on flights, which makes this number directly relevant if you travel with power banks, camera gear, or portable electronics. Batteries rated at 100 Wh or less can go in either carry-on or checked bags when installed in a device. Spare batteries and power banks under 100 Wh must go in your carry-on only.
Batteries between 101 and 160 Wh require airline approval, and you’re limited to two spares in carry-on. Anything above 160 Wh is banned from passenger flights entirely. Most smartphones and laptops fall well under the 100 Wh limit, but larger portable power stations and some professional video equipment can exceed it. Check the label or calculate the Wh yourself (volts × amp-hours) before you pack.
Converting mAh to Wh
Phone and power bank manufacturers almost always list capacity in milliamp-hours (mAh) rather than watt-hours, which makes direct comparisons tricky. To convert, you need the battery’s voltage, which for most single-cell lithium-ion batteries is 3.7 volts nominal:
Wh = (mAh × Voltage) ÷ 1,000
A 10,000 mAh power bank at 3.7 volts holds 37 Wh. A 20,000 mAh power bank holds 74 Wh. Keep in mind that when a power bank charges your phone through a USB cable at 5 volts, conversion losses mean you won’t get the full rated capacity transferred to your device. Expect to lose 15 to 30 percent in the process, depending on the quality of the electronics inside.