A radio is only as useful as the power behind it. In a long outage, the operators who stay on the air are usually the ones who did the arithmetic beforehand. This guide shows a simple way to estimate the amp-hours your station needs for three days, then size a battery and charging setup to match.
Decide What Your Station Must Do
Start with the job. Will you mostly listen to a net and transmit occasionally, or run a busy station? Will you power a handheld, or a 100 W HF radio with a computer and antenna tuner? Write down every device and how many hours a day you expect to use it on receive and on transmit.
Listening needs no licence, but transmitting requires the right licence for your country, and Canada and the USA have different rules. Emergency circumstances don't remove that requirement, so operate within the privileges you hold and follow local rules.
The Amp-Hour Formula
Amp-hours (Ah) equal current in amps multiplied by hours of use. A radio drawing 2 A for 5 hours uses 10 Ah. Watt-hours (Wh) equal Ah multiplied by volts, which is handy for comparing batteries and solar panels. Work out receive, transmit and accessories separately, add them for one day, then multiply by three.
Measure, Don't Guess
Your manual lists receive and transmit current, but real figures depend on the model, output power, display brightness and any noise-reduction features. Many 100 W HF transceivers draw roughly 1 to 3 A on receive and around 20 A or more at full power. Treat those as approximate. A handheld draws far less.
Verify your own numbers with an inline power meter or a battery monitor with a shunt. Mode matters too: SSB speech is peaky, so average current is lower than the keyed-down figure. FM and most digital modes draw close to full transmit current whenever you key up. Reducing output power lowers current noticeably.
A Worked Example
These numbers are illustrative only. Suppose a 100 W radio draws 1.5 A on receive and 20 A while transmitting. You listen 6 hours a day and transmit 30 minutes in total. A laptop, tuner and lighting add roughly 1 A for 8 hours.
- Receive: 1.5 A x 6 h = 9 Ah
- Transmit: 20 A x 0.5 h = 10 Ah
- Accessories: 1 A x 8 h = 8 Ah
- One day: 27 Ah. Three days: 81 Ah
- With a 25 percent margin: about 101 Ah
Notice how receive adds up. Leaving that same radio on receive for 24 hours would use 36 Ah a day, more than this entire example day.
Choose a Battery Type
Sealed lead-acid (AGM) batteries are common and affordable, but for a long life you generally shouldn't discharge them below about half their rated capacity. A 100 Ah AGM battery therefore gives roughly 50 Ah of usable energy. Voltage also sags under heavy transmit current.
Lithium iron phosphate (LiFePO4) 12 V batteries are lighter, hold their voltage steadily and can usually deliver most of their rated capacity. Their built-in battery management system may shut the battery off abruptly when it is depleted or overloaded. Most lithium batteries should not be charged below freezing unless they are specifically designed for it.
Size the Battery With Margin
Take your three-day total plus margin, about 101 Ah in the example. A LiFePO4 battery rated at 100 Ah is borderline, so step up to the next size. With AGM, plan on roughly double, around 200 Ah. Size for the battery alone, since a cloudy stretch can leave solar with little to give. Charging then extends your runtime instead of being a requirement.
Add Charging: Solar, Vehicle and Generator
The example day uses about 27 Ah, or roughly 350 Wh. A solar panel rarely produces its rated output, and a typical day might equal only a few hours at full rating, less in winter or under cloud. As a rough guide, that suggests something like 150 to 200 W of panel. Use a charge controller set for your battery chemistry.
A vehicle can charge through properly fused wiring or a DC-DC charger, but run the engine outdoors only. Run any generator outdoors, well away from doors and windows, because carbon monoxide is deadly. Chargers and generators can add noise to receive, so try them in advance.
Wire It Safely
- Fuse the positive lead close to the battery, rated for the wire and the radio.
- Use wire thick enough for your transmit current, and keep runs short to limit voltage drop.
- Use polarised connectors and double-check polarity. Reversed power can destroy a radio.
- Secure the battery upright and, for vented lead-acid types, ventilate it.
- Keep antennas well clear of power lines and people when you raise them.
Stretch Your Energy
Use only as much power as a contact needs, since many contacts don't need 100 W. Dim the display, and switch the radio off between scheduled check-ins. Use a handheld for local nets. Look at the live HF propagation page before calling so you don't burn capacity on a band that is closed. Keep a paper log during the outage, then enter it into the free online logbook afterward.
Common Mistakes
- Ignoring receive current, which is small per hour but runs all day.
- Forgetting accessories such as computers, tuners, lights and phone charging.
- Treating a battery's Ah label as fully usable, especially with lead-acid.
- Skipping the fuse, or using undersized wire.
- Never testing the setup, or leaving the battery half-charged.
- Assuming panels will deliver their full rating.
Your 72-Hour Checklist
- Measured receive and transmit current for your radio
- Daily Ah worked out, multiplied by three, with margin
- Battery sized for real usable capacity
- Charger, controller or DC-DC unit matched to the battery type
- Fused, polarised, correctly sized wiring
- Paper log, pencil, and a printed ham radio quick reference
- Battery fully charged and checked every month or two
Wrapping Up
Do a dry run: spend a day running your station only from the battery, with a monitor watching the numbers, then compare it to your estimate and adjust. A local club is a great place to practise, so find a club near you for field days and drills. For the rest of your kit, see our guide to a ham radio go-bag. Three days of power is just a plan you have written down and tested.
How HamRadioList can help
Once your power plan is on paper, keep your practice sessions organised in the free HamRadioList logbook so you can see how your station behaves off the grid. Check the live HF propagation page before you use precious battery capacity, and browse the ham radio quick reference to build a one-page printout for your emergency kit.
- Free online ham radio logbook - log every contact, import and export ADIF, sync with QRZ.com and keep your awards organised.
- Live HF propagation - see current band conditions before you call CQ.
- Ham radio quick reference - bands, modes, phonetics and Q-codes on one page.
- Find a club - locate a local amateur radio club.
- Ham radio FAQ - plain answers to the questions new operators ask most.
New here, or getting back on the air? Create your free HamRadioList account to log contacts, check band conditions and practise for your exam, or browse more guides from the blog. HamRadioList is built by amateur radio operators in Nova Scotia for operators everywhere.