Wh = Ah x V. To convert amp hours to watt hours, multiply the amp hour (Ah) rating by the nominal voltage (V) of the battery.
In this formula, Wh stands for watt hours, Ah is amp hours, and V is the battery’s nominal voltage in volts. For milliamp hours (mAh), use Wh = mAh x V / 1000. Always check whether your battery specification lists capacity in Ah or mAh, and use the correct voltage for the chemistry and configuration.
| Capacity (Ah) | Voltage (V) | Energy (Wh) |
|---|---|---|
| 5 | 12.8 | 64 |
| 10 | 12.8 | 128 |
| 20 | 12.8 | 256 |
| 50 | 51.2 | 2,560 |
| 100 | 51.2 | 5,120 |
| 200 | 51.2 | 10,240 |
| 280 | 384 | 107,520 |
| 600 | 51.2 | 30,720 |
How do you use the amp hours to watt hours formula in practice?
To size a battery for a 48V telecom system with a 200Ah battery: Wh = 200 x 51.2 = 10,240Wh. For a 12V 100Ah battery: Wh = 100 x 12.8 = 1,280Wh. For a battery specified in mAh, such as 5,000mAh at 3.7V: Wh = 5,000 x 3.7 / 1,000 = 18.5Wh. Substitute your own figures to match your application.
What is the most common mistake in amp hour to watt hour conversion?
The most common mistake is using the wrong voltage—such as assuming 12V for all batteries, or using nominal instead of actual system voltage. This can misstate available energy by 10-20%, leading to either overspending on excess capacity or failing to meet backup requirements. Always confirm the rated voltage on the battery’s datasheet or label.
Why does amp hour to watt hour conversion matter in procurement?
Procurement decisions based on amp hours alone can mislead, since two batteries with the same Ah rating but different voltages deliver different energy. For example, a 100Ah 12V battery stores 1,280Wh, while a 100Ah 51.2V battery stores 5,120Wh. Comparing watt hours aligns your specification with actual usable energy, reducing the risk of under- or over-provisioning and making cost-per-kWh comparisons possible.
How do you avoid conversion errors in real-world battery sizing?
Always confirm whether the specification is in Ah or mAh, use the correct nominal voltage, and adjust for depth-of-discharge if sizing for usable energy. For critical applications, request the manufacturer’s datasheet and check that the watt hour figure matches your calculation. This prevents costly oversizing or shortfall in delivered capacity.
Frequently asked questions
Can I use amp hours to compare batteries with different voltages?
No, amp hours alone do not account for voltage. Always convert to watt hours (Wh = Ah x V) to compare total stored energy across batteries.
What voltage should I use for the amp hours to watt hours formula?
Use the battery’s nominal voltage as specified on the datasheet or label. Using the wrong voltage can misstate energy by 10-20%.
Does 100Ah always mean the same energy capacity?
No. 100Ah at 12V is 1,200Wh, but 100Ah at 48V is 4,800Wh. Always multiply by voltage to get watt hours for an accurate comparison.
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