Milliamp-Hours (mAh) to Watt-Hours (Wh): The Clear, Correct Way to Convert
ZacharyWilliam1) Battery & Power Basics
- mAh (milliamp-hours): Charge capacity (current × time). 1,000 mAh = 1 Ah.
- Wh (watt-hours): Energy (power × time). Lets you compare across different voltages.
- Voltage (V): Electrical potential. Required to convert between mAh and Wh.
- Why Wh matters: Two batteries with the same mAh at different voltages do not store the same energy.
2) The Conversion Formula (Explained)
From W = V × A and energy over hours:
Ah = mAh ÷ 1000
Wh = Ah × V = (mAh × V) ÷ 1000
mAh = (Wh × 1000) ÷ VAlways use the nominal/rated voltage of the battery or pack when converting.
3) Step-by-Step: mAh → Wh
- Find the nominal voltage (e.g., 3.7V for many Li-ion cells, 3.2V for LiFePO₄ cells).
- Convert mAh → Ah: divide by 1000.
- Multiply: Wh = Ah × V.
- (Optional) For real-world estimates, apply usable DoD and efficiency factors.
4) Common Conversion Tables
Use these quick lookups for popular voltages. (Formula: Wh = (mAh × V) ÷ 1000)
Table A — mAh → Wh at Common Voltages
| mAh | 3.2V LiFePO₄ cell |
3.7V Li-ion cell |
5.0V USB output |
7.4V 2S Li-ion |
11.1V 3S Li-ion |
12.0V 12V system |
14.8V 4S Li-ion |
|---|---|---|---|---|---|---|---|
| 500 mAh | 1.6 Wh | 1.85 Wh | 2.5 Wh | 3.7 Wh | 5.55 Wh | 6.0 Wh | 7.4 Wh |
| 1,000 mAh | 3.2 Wh | 3.7 Wh | 5.0 Wh | 7.4 Wh | 11.1 Wh | 12.0 Wh | 14.8 Wh |
| 2,000 mAh | 6.4 Wh | 7.4 Wh | 10.0 Wh | 14.8 Wh | 22.2 Wh | 24.0 Wh | 29.6 Wh |
| 5,000 mAh | 16.0 Wh | 18.5 Wh | 25.0 Wh | 37.0 Wh | 55.5 Wh | 60.0 Wh | 74.0 Wh |
| 10,000 mAh | 32.0 Wh | 37.0 Wh | 50.0 Wh | 74.0 Wh | 111.0 Wh | 120.0 Wh | 148.0 Wh |
| 20,000 mAh | 64.0 Wh | 74.0 Wh | 100.0 Wh | 148.0 Wh | 222.0 Wh | 240.0 Wh | 296.0 Wh |
| 30,000 mAh | 96.0 Wh | 111.0 Wh | 150.0 Wh | 222.0 Wh | 333.0 Wh | 360.0 Wh | 444.0 Wh |
Table B — Wh → mAh (Most Requested)
Formula: mAh = (Wh × 1000) ÷ V. Shown for 3.7V (typical Li-ion cell) and 12V (common system voltage).
| Wh | mAh @ 3.7V | mAh @ 12V |
|---|---|---|
| 5 Wh | 1,351 mAh | 417 mAh |
| 10 Wh | 2,703 mAh | 833 mAh |
| 20 Wh | 5,405 mAh | 1,667 mAh |
| 50 Wh | 13,514 mAh | 4,167 mAh |
| 74 Wh | 20,000 mAh | 6,167 mAh |
| 100 Wh | 27,027 mAh | 8,333 mAh |
| 200 Wh | 54,054 mAh | 16,667 mAh |
Rounded to the nearest whole mAh for readability.
5) Worked Examples (Double-Checked)
- Phone cell: 3,000 mAh @ 3.7V → (3000 × 3.7) ÷ 1000 = 11.1 Wh.
- Power bank: 20,000 mAh @ 3.7V → 74 Wh (note: marketed mAh is at internal cell voltage, not 5V output).
- Laptop pack: 7,800 mAh @ 11.1V → (7800 × 11.1) ÷ 1000 = 86.6 Wh.
- 12V lead-acid: 7 Ah @ 12V → 7 × 12 = 84 Wh.
- Back-conversion: 100 Wh @ 3.7V → (100 × 1000) ÷ 3.7 ≈ 27,027 mAh.
6) Series vs. Parallel Packs (Avoid Common Errors)
- Series (S): Increases voltage. Example: 3S Li-ion → 3 × 3.7V ≈ 11.1V.
- Parallel (P): Increases capacity (Ah/mAh) at the same voltage.
- For Wh, use the pack’s nominal voltage, not a single cell’s voltage or a USB output voltage (unless you’re converting energy at that output).
7) Real-World Losses & Runtime
On paper, Wh is exact. In practice, usable energy depends on:
- Converter efficiency: USB buck/boost and AC inverters incur losses. AC inverters often ~80–90% efficient at moderate loads.
- Depth of discharge (DoD): Many systems leave a safety reserve; usable DoD might be 80–95%.
- Temperature & C-rate: Cold or high discharge rates reduce effective capacity.
Quick runtime estimator:
Runtime (hours) ≈ (Wh × DoD × efficiency) ÷ Load (W)Example: 74 Wh bank, 90% DoD, 85% efficiency powering 10W → (74 × 0.9 × 0.85) ÷ 10 ≈ 5.7 hours.
8) FAQs
Do I need voltage to convert mAh to Wh?
Yes. mAh is charge; Wh is energy. Voltage links them via Wh = (mAh × V) ÷ 1000.
Why is a 20,000 mAh power bank about 74 Wh, not 100 Wh?
Because the mAh rating is at the internal cell voltage (~3.7V). Wh = 20,000 × 3.7 ÷ 1000 ≈ 74 Wh.
What’s the difference between mWh and Wh?
1 Wh = 1,000 mWh. Multiply or divide by 1,000 as needed.
Does pure sine wave vs. modified sine change Wh?
No, stored Wh stays the same. However, usable runtime changes due to different conversion losses and device efficiency.
What nominal voltages should I expect?
LiFePO₄ cell ≈ 3.2V; Li-ion (NMC/NCA) cell ≈ 3.6–3.7V; 2S ≈ 7.4V; 3S ≈ 11.1V; 4S ≈ 14.8V; “12V systems” often operate ~12.0–12.8V nominal.
9) Conclusion & Tools
Remember: voltage is essential for correct conversion. Using Wh gives you an apples-to-apples view of energy across different battery types. Keep real-world losses in mind when estimating runtime.
Prefer a calculator? Try: Battery Unit Conversion Tools
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