Nothing ends a POTA activation or a field deployment faster than running out of power halfway through. Guessing at battery capacity is how that happens, actually running the numbers is how you avoid it.
Start with your radio’s actual current draw
Every radio’s manual lists current draw figures for receive, standby, and transmit at various power levels, and these numbers are your starting point. Transmit draw at full power is dramatically higher than receive draw, often 5 to 10 times higher, which matters because your actual operating pattern is mostly receive with intermittent transmit, not full-power transmit the entire session.
Estimating your duty cycle
A realistic SSB ragchew duty cycle runs maybe 40 to 50 percent transmit during active conversation, factoring in listening while the other station talks. FT8 has a fixed, predictable duty cycle since you know exactly how many of every 15-second cycle you’re transmitting versus receiving. Once you know your radio’s receive and transmit current draw and a realistic duty cycle for your operating style, multiply out the average current draw over your planned session length to get a real watt-hour or amp-hour requirement.
Building in margin
Whatever number your calculation produces, add real margin, I’d target at least 30 to 50 percent extra capacity beyond your calculated need. Cold weather reduces usable battery capacity, sessions run longer than planned more often than they run shorter, and having genuine reserve capacity is the difference between a minor inconvenience and cutting an activation short. A battery that’s rated for exactly your calculated need is a battery that will disappoint you eventually.
Related reading: Solid-State vs Tube Amplifiers · Buying a Used HF Radio · Understanding ITU Regions