How the calculation works
Nominal energy (Wh) = voltage × amp-hours. Delivered usable energy = nominal energy × starting charge fraction × usable share × efficiency fraction. Load watts = entered watts, or amps × voltage. Runtime hours = delivered usable energy ÷ load watts.
Worked example
At 12 V and 30 Ah, nominal energy is 360 Wh. With 100% starting charge, a 20% usable-share assumption, and 90% delivery efficiency, delivered usable energy is 64.8 Wh. A steady 100 W load gives an estimated 0.648 hours, about 38.9 minutes.
What the battery-type choices mean
The percentages beside each type are illustrative planning defaults, not manufacturer recommendations. Changing a type fills that starting assumption; you can override it. A lithium label alone does not establish its chemistry, permitted discharge, BMS current limit, or compatibility with your charging system.
Why amp-hours can mislead
Ah capacity is specified at a particular discharge rate and endpoint. Heavy loads can produce less usable capacity, especially with lead-acid batteries. The calculation uses a simple energy budget and does not implement a battery-specific discharge curve or Peukert model. Use published battery performance tables when runtime is critical.
Further reading: Trojan’s explanation of rated amp-hour capacity. Follow the documentation for your actual equipment or loan.
Frequently asked questions
Does efficiency include inverter losses?
It can represent your combined delivery-loss assumption. Use an appropriate measured or documented value, and avoid counting the same loss twice.
Can I run a winch from this result?
A continuous-load energy budget does not establish cranking or winch-current capability. Check surge current, wiring, battery limits, and winch duty cycle separately.
Does the result preserve enough power to start?
No. Remaining energy is not equivalent to available cranking current. The tool cannot guarantee a restart.