The useful answer: Estimate battery runtime from usable watt-hours divided by the station’s average watt draw, then treat the result as a planning value that must be confirmed under load.
What changes in real operation
An HF radio does not draw one fixed current. Receive, audio level, transmit power, mode, tuner activity, accessories, cable loss, and battery-voltage sag all affect the average.
The shortcut that causes trouble
Using transmitter output power instead of DC input power, or assuming 100 percent of rated capacity is usable, produces unrealistically long runtime estimates.
Put it into practice
Measure receive and transmit current, apply the expected duty cycle, include every accessory, derate for battery and weather conditions, and carry a reserve rather than planning to zero.
Practical boundary
Use current equipment instructions, battery and controller limits, measurement references, operating rules, and observed propagation. Planning values and propagation labels are not guarantees.
