Wh to Ah Calculator
This calculator converts energy in watt-hours (Wh) to battery capacity in amp-hours (Ah), using battery voltage. It's helpful for UPS sizing, solar battery planning, and electrical storage systems.
What Does Wh to Ah Conversion Mean?
Watt-hours (Wh) and ampere-hours (Ah) are two common ways of expressing battery capacity. Wh represents the total energy stored, while Ah measures the amount of electric charge. Since batteries operate at different voltages, it’s not enough to compare them using Ah alone — Wh gives a truer picture of actual stored energy. However, if you're designing a system or replacing batteries and you only have Wh available, you may need to convert it to Ah to determine how much current the battery can provide over time.
This conversion is essential for:
- Matching battery capacity to system current requirements
- Estimating how long a battery will last based on current draw
- Sizing backup power systems or solar energy storage
- Understanding product labels and specifications
Formula to Convert Wh to Ah
The formula is straightforward:
Ah = Wh / V
- Wh – energy in watt-hours
- V – battery/system voltage
- Ah – resulting battery capacity in amp-hours
Example:
Convert 240Wh at 12V:
Ah = 240 / 12 = 20Ah
Why Use This Conversion?
Many manufacturers list battery capacity in Wh for consumer clarity, while battery modules or UPS systems are rated in Ah. This conversion helps in:
- UPS Design: Matching energy demand to required battery bank capacity
- EV & e-bike battery planning: Estimating amp-hour needs from energy demands
- Solar & off-grid systems: Sizing charge controllers and battery storage banks
- Technical specs translation: Interpreting device datasheets that list energy in Wh
Voltage is the Bridge
Because 1 Wh = 1 Volt × 1 Ampere × 1 Hour, the voltage is the bridge between Wh and Ah. For example:
- 240 Wh at 12V = 20Ah
- 240 Wh at 24V = 10Ah
- 240 Wh at 48V = 5Ah
Same energy, but very different current availability depending on voltage.
Common Voltage Values
| Battery/System Type | Nominal Voltage |
|---|---|
| Smartphone / Lithium-Ion cell | 3.7 V |
| USB Power Bank (internal) | 3.7 V |
| Laptop Battery Pack | 11.1 – 14.8 V |
| Lead-Acid Battery | 12 V |
| Solar System (mid-size) | 24 V |
| Large UPS or EV System | 48 – 96 V |
Real-World Examples
- Example 1: UPS battery
1200Wh backup battery at 12V → 1200 / 12 = 100Ah - Example 2: e-Bike battery
504Wh battery at 36V → 504 / 36 = 14Ah - Example 3: Laptop power bank
74Wh pack at 14.8V → 74 / 14.8 = 5Ah
Runtime Calculation Using Ah
Once you've converted Wh to Ah, you can estimate how long the battery will last under a certain current load:
Runtime (hours) = Ah / Load (Amps)
For example, a 20Ah battery powering a 5A load will last:
20 Ah / 5 A = 4 hours
Remember, efficiency losses and temperature can impact this.
Efficiency Considerations
In real systems, 100% conversion doesn’t happen. There are always losses due to:
- Inverter efficiency (typically 85–95%)
- Battery self-discharge and internal resistance
- Environmental factors like temperature
When planning, always apply a safety factor. Example:
Usable Ah = Total Ah × 0.9 (90% efficiency)
Frequently Asked Questions (FAQ)
Can I use this conversion for all batteries?
Yes — lead-acid, lithium-ion, NiMH — as long as you use the correct voltage. For lithium batteries, use the nominal voltage (typically 3.6–3.7V per cell).
What is the difference between Wh and Ah?
Wh is total energy. Ah is charge. Wh = Ah × V. Ah alone doesn’t account for voltage, so it cannot describe usable energy across systems with different voltages.
How accurate is this conversion?
Mathematically it’s exact. But in real-world systems, battery health, discharge rate, and temperature will influence actual capacity.
Can I convert Ah to Wh instead?
Yes. Simply multiply Ah × V = Wh. That conversion is used when calculating energy capacity from current draw.
What happens if I use the wrong voltage?
Your calculation will be incorrect. Always use the voltage of the system or the rated voltage of the battery.
Conclusion
Converting watt-hours to amp-hours is a critical calculation for anyone working with batteries, from solar installers and UPS planners to EV hobbyists and engineers. It helps determine battery capacity in charge terms, aiding in current management, component matching, and system design. Always ensure you use the correct voltage and include efficiency considerations for best results.