Amp-Hours, Watt-Hours and Kilowatt-Hours Explained
Learn the difference between amps, amp-hours, watts, watt-hours and kWh so you can compare 12V, 24V and 48V marine lithium systems.
The five units every boat owner should understand
Volts, V: electrical potential.
Amps, A: current flowing at a particular moment.
Amp-hours, Ah: a measure of charge capacity.
Watts, W: power at a particular moment.
Watt-hours, Wh: energy used or stored over time.
Watts = Volts x Amps
Watt-hours = Watts x hours
For a battery:
Watt-hours = nominal voltage x amp-hours
Battery examples
| Battery | Calculation | Nominal energy |
|---|---|---|
| 12.8V 100Ah | 12.8 x 100 | 1.28 kWh |
| 12.8V 200Ah | 12.8 x 200 | 2.56 kWh |
| 12.8V 400Ah | 12.8 x 400 | 5.12 kWh |
| 25.6V 200Ah | 25.6 x 200 | 5.12 kWh |
| 25.6V 400Ah | 25.6 x 400 | 10.24 kWh |
| 51.2V 200Ah | 51.2 x 200 | 10.24 kWh |
| 51.2V 400Ah | 51.2 x 400 | 20.48 kWh |
Why amp-hours can mislead
A 200Ah 48V bank has about four times the nominal energy of a 200Ah 12V bank.
If a website says only "200Ah battery system" without voltage, you do not know its energy capacity.
Current matters for cabling
At approximately 90% inverter efficiency:
| AC load | Approx. 12.8V current | Approx. 25.6V current | Approx. 51.2V current |
|---|---|---|---|
| 500W | 43A | 22A | 11A |
| 1,000W | 87A | 43A | 22A |
| 2,000W | 174A | 87A | 43A |
| 3,000W | 260A | 130A | 65A |
| 5,000W | 434A | 217A | 109A |
This is one reason high-energy yachts often move away from 12V for large inverter loads.
Apply this guidance to your boat
Use this page as one input to a vessel-specific plan. Record the actual equipment, cable runs, charging sources and operating pattern aboard before choosing hardware or changing the installation.
- Measure the real load and its operating time instead of relying on a label or a generic boat profile.
- Check every proposed current against the battery, BMS, protection, cable and connection limits.
- Confirm the result against current manufacturer instructions and the rules that apply to your vessel.