How can you determine the length of time that equipment can be powered from a battery?
Why Battery capacity is rated in ampere-hours (Ah), which is literally current multiplied by time. So if you know how many amps your radio draws on average, dividing capacity by that draw gives hours of operation: a 20 Ah battery running a radio that averages 2 A lasts roughly 10 hours. Use the average draw, not the transmit peak, since a typical station spends most of its time receiving.
Watch out Using watt-hours divided by peak power is the wrong pairing twice over: it assumes worst-case draw the whole time, and battery capacity is normally published in ampere-hours anyway.
Amp-hours divided by amps equals hours. Units cancel, and that is the only choice where they do.