Amp-hours vs watt-hours comes up every time someone compares a 12V battery to a 48V battery or to a portable power station. The short version: watt-hours tell you how much energy a battery holds. Amp-hours only mean something once you know the voltage. Here’s how to convert between them and compare batteries fairly.
Amp-Hours vs Watt-Hours: The Definitions
- Amp-hours (Ah): how much charge a battery can deliver. 100Ah means about 100A for one hour, or 5A for 20 hours, under the rated conditions.
- Watt-hours (Wh): how much energy it holds. Wh = Ah × nominal volts.
Why Watt-Hours Win
Two “100Ah” batteries can hold very different amounts of energy:
| Battery | Nominal voltage | Capacity | Energy |
|---|---|---|---|
| 12V LiFePO4 100Ah | 12.8V | 100Ah | 1,280Wh |
| 24V LiFePO4 100Ah | 25.6V | 100Ah | 2,560Wh |
| 48V LiFePO4 100Ah | 51.2V | 100Ah | 5,120Wh |
Nominal voltages are typical for 4-, 8- and 16-cell LiFePO4 packs (3.2V per cell).
Portable power stations and home batteries list Wh or kWh for this reason. Their internal voltage isn’t the same as their output.
Converting
- Ah to Wh: multiply by voltage. 200Ah × 12.8V = 2,560Wh.
- Wh to Ah: divide by voltage. 2,048Wh ÷ 51.2V = 40Ah.
- kWh: divide Wh by 1,000. 5,120Wh = 5.12kWh.
Usable vs Rated Capacity
Rated capacity isn’t all usable:
- LiFePO4 batteries are commonly used to 80–100% depth of discharge. Check your battery maker’s spec.
- Lead-acid batteries last much longer if you use only about 50%.
- Inverter losses take another slice when you run AC loads.
Example math: a 1,280Wh LiFePO4 used to 90%, running loads through a 90%-efficient inverter, delivers about 1,280 × 0.9 × 0.9 ≈ 1,037Wh to AC appliances.
The Lead-Acid “20-Hour Rate” Catch
Lead-acid batteries are usually rated at a 20-hour discharge (C/20). Pull current faster and you get noticeably fewer amp-hours, an effect called Peukert’s law. LiFePO4 holds its rated capacity much better at higher currents. So a “100Ah” lead-acid battery and a “100Ah” lithium battery don’t deliver the same usable energy, even at the same voltage. That’s one more reason to compare usable Wh.
Which Number to Use When
- Comparing batteries or power stations: Wh.
- Sizing a charger or wire on one battery: Ah and amps, at that battery’s voltage.
- Runtime: Wh ÷ load watts (after losses).
Worked Example: Comparing Three Batteries Fairly
Illustrative listings, converted to watt-hours so they can be compared directly:
| Battery (illustrative) | Rated | Nominal voltage | Watt-hours | Usable (assumed DoD) |
|---|---|---|---|---|
| 12V LiFePO4 | 100Ah | 12.8V | 1,280Wh | about 1,280Wh (100%) |
| 12V AGM lead-acid | 100Ah | 12V | 1,200Wh | about 600Wh (50%) |
| 48V LiFePO4 rack battery | 100Ah | 51.2V | 5,120Wh | about 4,600Wh (90%) |
All three say “100Ah,” yet the 48V battery stores four times the energy of the 12V lithium, and the AGM delivers about half its rating if you follow the common 50% depth-of-discharge guideline for lead-acid cycle life. See depth of discharge and cycle life and LiFePO4 vs lithium-ion.
Series vs Parallel: What Adds Up
| Wiring | Voltage | Amp-hours | Watt-hours |
|---|---|---|---|
| One 12.8V 100Ah battery | 12.8V | 100Ah | 1,280Wh |
| Two in series | 25.6V | 100Ah | 2,560Wh |
| Two in parallel | 12.8V | 200Ah | 2,560Wh |
| Four in 2S2P | 25.6V | 200Ah | 5,120Wh |
Watt-hours double either way, which is why Wh is the honest comparison number. Wiring details are in series vs parallel wiring.
Runtime in Three Steps
- Add up daily watt-hours for your loads (watts × hours).
- Divide usable battery Wh by that total.
- If loads run through an inverter, divide by the inverter efficiency from its datasheet.
Example math: 1,280Wh usable ÷ (400Wh a day ÷ 0.90) ≈ 2.9 days. A fuller walk-through is in how long will a 100Ah battery last.
Where Each Rating Appears
- Portable power stations list Wh because their internal pack voltage varies by model and the output is AC and USB.
- Home batteries list kWh, for example 13.5kWh usable on the Tesla Powerwall 3 per Tesla’s spec sheet.
- 12V and 48V DIY batteries list Ah at a nominal voltage, so multiply them yourself.
- Your utility bill uses kWh. 1kWh = 1,000Wh. See what is a kilowatt-hour.
Charging: Ah Helps Here
Amp-hours are handy for charger sizing. A 30A charger refills about 30Ah per hour in the bulk stage, so a 100Ah battery from empty takes a little over 3 hours, plus absorption time. Charge-rate limits are listed on every battery datasheet as a maximum charge current.
FAQ
Is 100Ah the same as 1,000Wh?
Only at 10V. At 12.8V, 100Ah is about 1,280Wh.
Why do power stations use Wh instead of Ah?
Because their internal battery voltage differs from their 12V, USB and 120V outputs. Wh is the one number you can compare across products.
Does wiring batteries in series add amp-hours?
No. Series adds voltage, and parallel adds amp-hours. Either way, total Wh adds up.
How do I convert mAh to Wh for a power bank?
Wh = mAh × volts ÷ 1,000. A 20,000mAh bank at a 3.6V cell voltage is 72Wh.
Which is better to compare, Ah or Wh?
Wh, because it already includes voltage.
Sources: U.S. Department of Energy, “How Does Electricity Work?” and “Estimating Appliance and Home Electronic Energy Use” (energy.gov); Tesla Powerwall 3 datasheet. Accessed Oct 2026.
Related: LiTime 12V 50Ah LiFePO4 Battery Review, EcoFlow Delta 3 Max Price per Wh, XZNY 12V 18Ah LiFePO4 Battery Review