Battery Capacity: Amp-Hours vs Watt-Hours Explained
Last updated: April 2026
Amp-hours (Ah) measure how much current a battery can deliver over time, while watt-hours (Wh) measure total energy stored. The two are related by a simple formula: Wh = Ah × Voltage. This means a 200Ah battery at 12V stores 2,400Wh of energy -- the same as a 100Ah battery at 24V. When comparing batteries or portable power stations, watt-hours are always the more useful metric because they account for voltage differences and tell you the actual energy available.
What Are Amp-Hours (Ah)?
Amp-hours measure electrical charge -- specifically, how many amps a battery can supply for how many hours. A 100Ah battery can theoretically deliver 1 amp for 100 hours, 10 amps for 10 hours, or 100 amps for 1 hour.
The problem with amp-hours as a standalone metric is that it tells you nothing about energy. Current (amps) without voltage context is incomplete information. A 100Ah battery at 12V stores a quarter of the energy of a 100Ah battery at 48V, but the amp-hour rating looks identical.
Amp-hours are still useful in one specific context: when you are working within a single, fixed-voltage system (like a 12V RV electrical system) and all your batteries operate at the same voltage. In that case, Ah directly tells you relative capacity.
What Are Watt-Hours (Wh)?
Watt-hours measure actual energy -- the total amount of work a battery can perform. One watt-hour equals one watt of power delivered for one hour. Your electric company bills you in kilowatt-hours (kWh), which is simply 1,000 watt-hours.
Watt-hours are calculated by multiplying amp-hours by voltage: Wh = Ah × V. This single number captures both the current capacity and the voltage, giving you a complete picture of how much energy is stored.
This is why every portable power station lists capacity in watt-hours. When the EcoFlow DELTA 3 Ultra says 4,096Wh, you know exactly how much energy it holds regardless of internal battery configuration. You can directly calculate how long it will run your appliances.
The Conversion Formula: Wh = Ah × V
The relationship between amp-hours, watt-hours, and voltage is straightforward:
Wh = Ah × V
Watt-Hours = Amp-Hours × Nominal Voltage
To find Wh: Multiply Ah × V
To find Ah: Divide Wh ÷ V
To find V: Divide Wh ÷ Ah
Always use the nominal voltage for conversions -- not the fully charged voltage. A "12V" LiFePO4 battery has a nominal voltage of 12.8V (but 12V is commonly used for rough calculations). A fully charged LiFePO4 cell sits at 14.6V, which would overstate capacity if used in the formula.
Ah to Wh Conversion Table
Quick reference for common battery configurations. Notice how the same Ah rating produces very different Wh values at different voltages.
| Amp-Hours (Ah) | Voltage (V) | Watt-Hours (Wh) |
|---|---|---|
| 50Ah | 12V | 600Wh |
| 100Ah | 12V | 1,200Wh |
| 200Ah | 12V | 2,400Wh |
| 100Ah | 24V | 2,400Wh |
| 200Ah | 24V | 4,800Wh |
| 50Ah | 48V | 2,400Wh |
| 100Ah | 48V | 4,800Wh |
| 200Ah | 48V | 9,600Wh |
When to Use Ah vs Wh
- Ah: Use amp-hours when working within a single fixed-voltage system. If you have a 12V RV setup and are comparing 12V batteries, Ah tells you everything you need. It is also the standard spec for standalone battery banks and deep-cycle batteries.
- Wh: Use watt-hours when comparing batteries at different voltages, comparing power stations, or calculating how long a battery will run specific appliances. Watt-hours are the universal energy metric and are always the better choice for cross-product comparisons.
Real-World Example: How Long Will It Run My Fridge?
Suppose your camping fridge draws 60 watts and you have a 200Ah 12V LiFePO4 battery.
Step 1: Convert to watt-hours: 200Ah × 12V = 2,400Wh
Step 2: Account for inverter losses (~85% efficiency): 2,400 × 0.85 = 2,040Wh usable
Step 3: Divide by appliance wattage: 2,040Wh ÷ 60W = 34 hours of runtime
If you tried this calculation in amp-hours alone, you would need to know the fridge draws 5A at 12V (60W ÷ 12V = 5A), then 200Ah ÷ 5A = 40 hours -- but this ignores inverter efficiency and only works because you happen to know the draw at 12V. Watt-hours make the calculation simpler and more universal.
Common Mistakes When Comparing Battery Capacity
| Mistake | Example | Reality |
|---|---|---|
| Comparing Ah across different voltages | Assuming a 200Ah 12V battery stores more energy than a 100Ah 24V battery | Both store exactly 2,400Wh -- identical total energy |
| Ignoring depth of discharge | Expecting 2,400Wh from a 200Ah 12V lead-acid battery | At 50% DoD, usable capacity is only 1,200Wh |
| Using fully charged voltage for conversions | Calculating 100Ah × 14.4V = 1,440Wh for a "12V" battery | Use nominal voltage (12.8V for LiFePO4, 12V for lead-acid) for accurate Wh |
| Forgetting inverter efficiency losses | Expecting 1,200Wh of AC power from a 1,200Wh battery | Inverters are 85-90% efficient. Expect ~1,020-1,080Wh of usable AC power |
Which Metric Matters Most for Off-Grid Systems?
For off-grid applications, watt-hours should be your primary comparison metric. Here is why:
- • Off-grid systems often mix voltages (12V batteries, 24V or 48V inverter banks, 120V AC output). Wh normalizes everything into one comparable number.
- • Your appliances are rated in watts, and your electric bill is in kWh. Using Wh keeps everything in the same family of units.
- • Solar panel output is rated in watts. Matching solar watts to battery watt-hours makes charge time calculations straightforward.
- • As you scale your system (adding batteries in series or parallel), Wh totals remain intuitive while Ah calculations become confusing.
That said, Ah still matters when sizing wire gauge and fuses within your 12V or 24V DC system. High amp draws require thicker wires to prevent voltage drop and heat buildup. So know both numbers -- but lead with watt-hours for capacity planning.