Wh = mAh x V / 1000 is the formula for converting amp hours to watt hours in battery sizing.
In this formula, Wh means watt hours, mAh means milliamp hours, and V means the battery’s nominal voltage in volts. If your battery is rated in amp hours (Ah), multiply Ah by 1000 to get mAh before using the formula. For example, 2 Ah = 2000 mAh.
What is the conversion table for amp hours to watt hours?
Use this table to quickly convert common battery ratings to watt hours, assuming a nominal voltage of 12.8V (typical for LiFePO4 packs):
| Capacity (Ah) | Capacity (mAh) | Voltage (V) | Energy (Wh) |
|---|---|---|---|
| 1 | 1000 | 12.8 | 12.8 |
| 5 | 5000 | 12.8 | 64 |
| 10 | 10000 | 12.8 | 128 |
| 20 | 20000 | 12.8 | 256 |
| 50 | 50000 | 12.8 | 640 |
| 100 | 100000 | 12.8 | 1280 |
| 150 | 150000 | 12.8 | 1920 |
| 200 | 200000 | 12.8 | 2560 |
Scan down the left column to find your battery’s amp hour rating, then read across to get the watt hour value.
How do you work out amp hours to watt hours for real batteries?
Example 1: A 100Ah 12.8V LiFePO4 battery. Wh = 100,000 x 12.8 / 1000 = 1280Wh.
Example 2: A 24V 210Ah forklift battery. Wh = 210,000 x 24 / 1000 = 5040Wh.
Example 3: A 51.2V 200Ah stackable home battery. Wh = 200,000 x 51.2 / 1000 = 10,240Wh.
What mistakes do people make when converting amp hours to watt hours?
The most common mistake is confusing mAh and Ah. If you use 100Ah as 100mAh, your result will be off by a factor of 1000. Another issue is using the charging voltage instead of the nominal voltage—always use the nominal voltage printed on the battery’s spec sheet. Finally, remember that the calculated Wh is total energy, not usable energy. Usable Wh is lower due to depth-of-discharge limits (often 80% for LiFePO4) and inverter or system losses.
Why does amp hours to watt hours conversion matter in procurement?
Procurement decisions depend on matching the battery’s watt hour rating to the application’s energy needs. Quoting only in amp hours without specifying voltage leads to mismatches, especially when comparing batteries with different chemistries or voltages. Always request both Ah and Wh figures, and check that the voltage used in the calculation matches the battery’s nominal voltage.
What should you check before finalizing your battery sizing?
Before ordering, confirm the battery’s nominal voltage, rated amp hours, and the manufacturer’s stated watt hour value. If possible, ask for usable watt hours at your target depth-of-discharge. Double-check cable sizing, clearance for installation, and ambient temperature limits, as these affect both safety and actual usable capacity on site.
Run the calculation on your own battery specs before placing an order or approving a design.
Frequently asked questions
Can I use amp hours to watt hours conversion for any battery type?
Yes, the formula Wh = mAh x V / 1000 applies to all battery types, but always use the correct nominal voltage for the specific chemistry.
What is the difference between amp hour (Ah) and milliamp hour (mAh)?
1 Ah equals 1000 mAh. Always check your units before converting, as confusing them will give a result off by a factor of 1000.
Does the calculated watt hour value represent usable energy?
No, the calculated watt hours is total theoretical energy. Usable energy is lower due to depth-of-discharge limits and system losses.
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