A solar radio runtime plan separates a confident portable CW activation from watching a low-voltage alarm ruin the best part of the afternoon. Your panel may look capable, and your battery may have an impressive amp-hour label, but neither number alone tells you how long your station will stay on the air. Runtime comes down to real load, usable battery energy, operating style, sunlight, and the losses between every component.
For QRP operators, this is good news. CW is one of the most power-efficient ways to communicate on HF, especially when you pair a compact transceiver with a disciplined operating plan. But efficient does not mean automatic. A 5-watt radio can run all day in one scenario and drain a small battery surprisingly fast in another.
Solar Radio Runtime Starts With Watt-Hours
Amp-hours are familiar, but watt-hours give a more honest picture of available energy. A battery rated at 12.8 volts and 10 amp-hours stores about 128 watt-hours:
`12.8 V × 10 Ah = 128 Wh`
You should not plan to use every watt-hour. Lead-acid batteries last longer when they are not deeply discharged, and voltage sag under load can end an operating session before the label suggests it should. A LiFePO4 battery generally provides more of its rated capacity, maintains voltage better, and is often the better field choice for its weight. Still, leave a reserve.
For practical planning, assume about 50 percent usable capacity for a lead-acid battery if battery longevity matters. For a quality LiFePO4 pack with a proper battery-management system, planning around 80 to 90 percent usable capacity is usually reasonable. Check the battery maker's specifications, especially its low-temperature limits and continuous-current rating.
A 10 Ah LiFePO4 battery might therefore offer roughly 105 usable watt-hours after reserve and normal system losses. That number is the foundation of your runtime estimate.
Measure the Radio, Not Just Its Output Rating
A transmitter's RF output is not its DC power draw. A QRP rig producing 5 watts at the antenna may pull 1.5 to 3 amps or more while transmitting, depending on the radio, supply voltage, final amplifier efficiency, and mode. Receive current also matters. A receiver drawing 150 mA is very different from one drawing 700 mA when you spend long periods listening through a crowded band.
Look in the manual, but verify with a wattmeter or inline current meter when possible. Measure at the voltage your station actually uses. A radio may consume more current as input voltage falls, and digital accessories, displays, audio amplifiers, tuners, and charging devices can add loads that are easy to overlook.
For a compact CW station, start with three numbers: receive current, transmit current, and accessory current. If you run a decoder, tablet, small display, or powered audio device, measure it separately. The radio is not always the biggest draw.
The Duty-Cycle Reality of CW
CW does not transmit continuously. That is a major advantage, but your duty cycle changes with the activity.
Calling CQ, running a pileup, sending long exchanges, and operating contest-style will push transmit time upward. Chasing POTA contacts, listening for weak stations, or making occasional check-ins may put far more time in receive. A practical CW estimate for many portable sessions is 20 percent transmit and 80 percent receive. Busy operating can reach 30 to 40 percent transmit time.
Use this average-current formula:
`Average current = (Transmit current × transmit fraction) + (Receive current × receive fraction) + accessory current`
Suppose your QRP transceiver draws 2.0 amps on transmit and 0.25 amps on receive. At a 20 percent transmit duty cycle, the radio's average current is:
`(2.0 × 0.20) + (0.25 × 0.80) = 0.60 A`
Add 0.10 amps for a small decoder or display, and the station averages 0.70 amps. With a usable 8 Ah battery capacity, the battery-only estimate is about 11.4 hours:
`8 Ah ÷ 0.70 A = 11.4 hours`
That is a planning estimate, not a guarantee. Cold weather, a louder audio setting, a higher transmit duty cycle, or an aging battery can cut it down quickly.
What a Solar Panel Actually Replaces
A 30-watt folding panel does not deliver 30 watts all day. That rating is measured under controlled laboratory conditions with ideal illumination and panel temperature. In the field, panel angle, haze, cloud cover, shade, cable loss, controller efficiency, and the sun's position all reduce output.
The relevant number is daily energy harvest, not the panel's headline wattage. If a 30-watt panel receives four effective peak-sun hours, its theoretical harvest is 120 watt-hours. After realistic losses through the controller and wiring, you might plan on 75 to 95 watt-hours on a favorable day. Under trees, during winter, or with a panel laid flat, output can be far lower.
A simple field estimate is:
`Panel watts × peak-sun hours × 0.70 = usable solar watt-hours`
That 70 percent factor is intentionally conservative. It accounts for the conditions that make field power unpredictable. If your station needs 70 watt-hours for the day and your panel realistically contributes 80 watt-hours, you have a workable plan with modest margin. If your calculation says the panel will barely meet your expected consumption, bring more battery or reduce your operating demand.
Controller Choice and Voltage Compatibility
A solar charge controller is not an optional convenience. It protects the battery and manages charging correctly. Match it to the battery chemistry. A controller profile intended for lead-acid charging can be wrong for LiFePO4, while a lithium profile with inappropriate voltage settings can shorten battery life or trigger battery-management-system shutdowns.
PWM controllers are simple and economical when panel and battery voltages are closely matched. MPPT controllers cost more, but can recover more energy from higher-voltage panels and are often worthwhile when panel area is limited or conditions vary. The gain depends on the panel, battery, weather, and wiring. Do not assume MPPT creates power from nowhere, but it can make better use of what the panel is producing.
Keep cable runs short and use adequately sized wire. A few tenths of a volt lost in undersized cable matters when a 12-volt radio is already operating near its preferred input range.
Build a Runtime Budget Before You Leave
The most dependable portable operators plan energy like they plan frequencies and antennas. Write down the usable battery watt-hours, expected station average draw, expected operating hours, and conservative solar harvest. Then include reserve.
A useful target is to size the battery so it can cover the planned session without any solar input. Treat the panel as a runtime extender and a way to recover energy over a multiday operation. This approach prevents a passing cloud bank, a shaded campsite, or a badly oriented panel from ending your communications window.
For example, a 5-hour CW activation at 0.70 amps from a nominal 12.8-volt battery uses approximately 45 watt-hours. A 10 Ah LiFePO4 battery has enough reserve for that job, even before solar contribution. A 20-watt or 30-watt panel may replenish much of the used energy during the day if it has clear sun, but it should not be the only reason the station remains viable.
For an overnight preparedness station, change the assumptions. There is no solar harvest after sunset, receive time may dominate, and you may need to monitor frequencies for hours before transmitting. In that case, low receive current can matter more than peak transmit current. Use headphones or an efficient speaker when practical, dim displays, and turn off nonessential accessories.
Field Habits That Protect Your Power Margin
Solar performance is won before you call CQ. Set the panel in direct sunlight early, then re-aim it as conditions change. Avoid even partial shade from branches, guy lines, vehicles, or a folding chair. A small shadow across one section of a panel can make output collapse far more than expected.
Run the panel through the charge controller to the battery, then power the radio from the battery. Do not depend on a panel to directly feed a transceiver through improvised wiring. Clouds can cause voltage swings, and a battery is the buffer that keeps your radio stable during transmit peaks.
Watch voltage, but do not treat voltage as a perfect fuel gauge. LiFePO4 batteries hold a fairly flat voltage curve until they approach discharge, while lead-acid voltage varies with load and state of charge. An amp-hour counter, watt-hour meter, or battery monitor gives more useful information during extended operation.
Finally, test your exact station at home. Key the transmitter into a proper dummy load, run the same accessories, and log current on receive and transmit. MorseNexus operators who build their station around efficient QRP CW gear can gain meaningful field time, but measurement turns a hopeful estimate into a mission-ready power plan.
The sun is a powerful partner for portable radio, but the battery is your operating reserve. Size that reserve honestly, protect it with a proper controller, and let solar energy extend your time on the key instead of deciding when the session ends.