Calculating the runtime for a Ham radio is a bit more nuanced than a lightbulb because radios don’t pull a constant amount of power. You have to account for the Duty Cycle—the ratio of time you spend transmitting (high power) versus receiving or idling (low power).
1. Establish Your Capacity
First, convert your battery’s Amp-hours into Watt-hours (Wh) to make the math easier. Since LiFePO4 batteries have a very flat discharge curve, we usually calculate using a nominal voltage of 12.8V.
12.8 x 20Ah = 256Wh
The “Real World” Adjustment:
Even though LiFePO4 can handle deep discharges, it’s best practice to assume 80% usable capacity to prolong battery life and account for wiring losses.
256Wh x 0.80 = 204.8Wh usable
2. Determine Current Draw (Amps)
A “100W” radio refers to the RF output power, not the power consumed from the battery. Radios are not 100% efficient.
- Receive (RX) / Idle: Most HF rigs pull about 1A to 2A just sitting there.
- Transmit (TX): To push 100W of RF out the antenna, a typical radio pulls about 18A to 22A from the battery.
3. Apply the Duty Cycle
Unless you are keyed down constantly (like in FM or some digital modes), you aren’t pulling 20A the whole time. A common “casual” operating ratio is 20/80 (20% Transmitting, 80% Receiving).
The Calculation Formula:
- Average Hourly Draw: (TX Amps x TX %) + (RX Amps x RX %)
- Runtime: Capacity ÷ Average Draw
Example (Standard SSB Voice):
- TX: 20A (at 20% of the time) > 4 A
- RX: 1.5A (at 80% of the time) > 1.2 A
- Total Average Draw: 5.2 Amps per hour
4. Final Estimated Runtime
Using your 20Ah LiFePO4 battery (and that 80% safety margin of 16Ah):
16 Ah / 5.2 A = 3.07 hours