Understanding "One Kilowatt-Hour" (1 kWh) vs "Ampere-Hour" (Ah)

The terms "One kilowatt-hour" (一度电) and "Ampere-hour" (Ah) belong to different physical dimensions. They cannot be converted directly without knowing the operational Voltage (V) of the system.

  • Kilowatt-hour (kWh): This is a unit of Energy (Electrical Work). It represents the total amount of electricity consumed by a 1000-watt appliance running for exactly 1 hour. (Note: 1 kWh = 1000 Wh).
  • Ampere-hour (Ah): This is a unit of Battery Capacity (Electric Charge). It indicates how much current a battery can deliver continuously for 1 hour.

The formula linking these two units is defined by Ohm's Law and electrical power equations:

Capacity (Ah) = Energy (Wh) / Voltage (V)

1. Calculation for Your 20S LFP Battery Pack

Since you are assembling a 20-Series (20S) Lithium Iron Phosphate (LFP) battery pack, we can calculate exactly how many Ampere-hours equal "one kilowatt-hour" of energy for your specific setup:

  • Nominal Cell Voltage: 3.2 V
  • Total Pack Voltage (20S): 3.2 V * 20 = 64 V
  • Conversion Calculation: Capacity = 1000 Wh / 64 V = 15.63 Ah

Conclusion: For your custom 64V battery pack, 1 kWh (一度电) is equivalent to approximately 15.63 Ah.

2. Reference Table Across Different Voltage Systems

Because voltage acts as the multiplier, 1 kWh of energy translates to vastly different Ah ratings depending on the device or vehicle platform:

Device / Vehicle Type Nominal Voltage (V) Equivalent Ah for 1 kWh (一度电)
Smartphone Power Bank 3.7 V (汇单一锂电池) 1000 / 3.7 = 270.27 Ah (or 270,270 mAh)
Car Starter Battery 12 V (标准铅酸电池) 1000 / 12 = 83.33 Ah
Electric Motorcycle 72 V (高功率电摩) 1000 / 72 = 13.89 Ah
Electric Vehicle (EV) 400 V (标准电车平台) 1000 / 400 = 2.5 Ah

Summary for Your Project

To find out how many Ampere-hours you need for any targeted energy storage level, simply divide your target Watt-hours (Wh) by your operating voltage (64 V).

Since you are configuring a 20S2P array, what is the rated Ah capacity of the individual 3.2V LFP cells you plan to use?

battery test

voltage cutoff voltage min.VOLT ad. capacity time current
12V 9V 7.5V 20000 mAH 10h 2A
3.7V 3V 2.5V

2. Example for a Typical Li-ion 26650 (5000 mAh)

  • Discharge Current: 0.5 A (500 mA)
  • Expected Capacity: 5000 mAh Time = 5000 mAh ÷ 500 mA = 10 hours

3. Practical Notes

  • Cutoff Voltage:
    • Li-ion NMC/NCA: ~2.5–3.0 V
    • LiFePO₄: ~2.0–2.5 V
  • Temperature: Test at room temp (~25 °C) for rated results.
  • CC Test: Your tester should log voltage & time; capacity is the area under the discharge curve.

Car Sedan Lead-Acid battery

Calculating Energy (Wh) = Voltage (V) × Capacity (Ah)

  • Minimum Energy: 12 V × 40 Ah = 480 Wh
  • Maximum Energy: 12 V × 70 Ah = 840 Wh

So, the energy stored in a typical car lead-acid battery is usually between 480 Wh and 840 Wh.

20000 mAh * 3.7V

Energy (Wh) = 20 Ah × 3.7 V = 74 Wh

2.6Ah * 12V

Energy (Wh) = 2.6 Ah × 12 V = 31.2 Wh

1000 Wh

1000 watt-hours (Wh) == 1 度

Runtime = 1000 Wh / 5V * 1A = 1000 Wh / 5W = 200 hours

quick calculation

2000 mAh = 2 Ah Runtime ≈ (2 Ah * 3.7 V * 0.85) / (1 A * 5 V) ≈ 1.26 hours

for 20000 mAh, == 12.6 hours

Calculating Runtime for a 2000mAh Power Bank Supplying a 1A @ 5V Device

Here's a breakdown of how to estimate the runtime:

1. Power Bank Energy

  • Capacity: 2000 mAh (milliampere-hours) = 2 Ah (ampere-hours)
  • Nominal Voltage: 3.7 V (typical for lithium-ion/polymer batteries)
  • Total Energy (Watt-hours, Wh): Capacity (Ah) × Voltage (V)
    • 2 Ah * 3.7 V = 7.4 Wh

2. Device Power Consumption

  • Current: 1 A (ampere)
  • Voltage: 5 V (standard USB output)
  • Power Needed (Watts, W): Current (A) × Voltage (V)
    • 1 A * 5 V = 5 W

3. Efficiency Consideration

Power banks are not 100% efficient when converting their internal battery voltage (3.7V) to the required 5V output. Energy is lost, primarily as heat, during this conversion.

  • Estimated Efficiency: Let's assume an average efficiency of 85% (or 0.85). This can vary between 80% and 95% depending on the quality of the power bank circuitry.

4. Effective Energy Available

This is the amount of the power bank's stored energy that can actually be delivered to the device after accounting for conversion losses.

  • Effective Energy: Total Energy (Wh) × Efficiency
    • 7.4 Wh * 0.85 ≈ 6.29 Wh

5. Calculate Runtime

  • Runtime (hours): Effective Energy Available (Wh) / Device Power Consumption (W)
    • 6.29 Wh / 5 W ≈ 1.26 hours

Conclusion

A 2000mAh, 3.7V power bank can theoretically supply a device drawing 1A at 5V for approximately 1.26 hours, or about 1 hour and 15 minutes.

Disclaimer: This is an estimate. Actual runtime depends on factors such as:

  • The precise efficiency of the specific power bank.
  • The age and health of the battery cells.
  • The quality of the charging cable (resistance losses).
  • Ambient temperature.
  • Whether the device's power draw is constant or fluctuates.

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