Home energy · backup power
Emergency radio and flashlight battery calculator.
Enter the device draw, the hours you want it to run and the battery capacity to get the number of sets and cells to store.
Inputs
Batteries to store
Enter the numbers above.
How many batteries for an emergency radio or flashlight?
Public Safety Canada and Ready.gov both put a battery-powered or crank radio, flashlights and extra batteries in the basic emergency kit, and Public Safety Canada says to be ready for at least 72 hours. Hours per set = battery mAh x usable share / device mA. Sets needed = hours wanted / hours per set, rounded up. Cells = sets x cells per device x devices.
Worked example. A radio draws 100 mA on 3 AA cells of 2,000 mAh with 80 percent usable: 2,000 x 0.80 / 100 = 16 hours per set. To run 72 hours: 72 / 16 = 4.5, rounded up to 5 sets, which is 15 AA cells.
Find the draw in the device manual or on its label; if it only lists runtime, divide the battery capacity by the hours. Capacity depends on the battery type and brand and falls at high drain and in the cold, which is why the usable share is an editable setting rather than a fixed figure. A manually powered (wind-up or crank) device avoids the problem; Public Safety Canada suggests extra batteries or the manually powered kind. Use a flashlight rather than candles to lower the risk of fire.
Frequently asked questions
How many AA batteries do I need for a 72-hour emergency kit?
It depends on the devices. With the example of one 100 mA radio on 3 cells of 2,000 mAh, you need 15 cells for 72 hours. Enter your own draw and capacity.
Where do I find the current draw?
On the device label or manual. If you only know the runtime, current draw is roughly battery mAh divided by runtime in hours.
Why is there a usable share?
Batteries rarely give their full rated capacity, especially at high drain or in the cold. 80 percent is an editable planning value, not a published standard.
Should I use candles instead?
Public Safety Canada says to use flashlights instead of candles to lower the risk of fire.
What about a wind-up radio?
Public Safety Canada suggests including a crank-operated or battery-powered radio in case the internet is down, and extra batteries or the kind that can be manually powered.
Sources
Checked
- Public Safety Canada, Get Prepared, "How to prepare before a power outage" (page dated 12 March 2026), opened 8 October 2026. crank or battery radio, flashlights with batteries, use flashlights instead of candles
- Public Safety Canada, Get Prepared, "Emergency kits", opened 8 October 2026. at least 72 hours, extra batteries or manually powered devices
- Ready.gov, "Build A Kit", opened 8 October 2026. battery-powered or hand crank radio, flashlight, extra batteries
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Scope
Capacity, usable share and device draw are inputs you control; real battery life varies with brand, temperature and load. Keep spare batteries stored as the manufacturer directs.