What this tool is, and who it is for
Off-grid power planning is dominated by two errors: people forget devices until they are on the trail, and they size their power bank against the optimistic battery specs the manufacturer publishes. This planner fixes both. It starts from a realistic device list with measured daily consumption values, applies cold-weather derating for any Li-ion cell operating below about 5 degrees Celsius, and adds a 25 percent reserve so that the bank does not run flat on the last night. The output is a single capacity figure plus the per-device breakdown that produced it.
Used to derate the Li-ion bank for cold-weather capacity loss.
Adjust the daily Wh for each device, or untick what you won't carry.
Across 4 active devices.
5 days × 10.8 Wh/day.
Includes 25% reserve, 5% cold derate at 2 °C, 85% usable fraction.
At this size, the bank sustains 6.3 day(s) on a single charge.
Step-by-step user guide
How to operate the calculator in your browser, what to enter, and what to do with the result.
- 01
Add each device you plan to carry. The defaults cover phone, headlamp, GPS watch, sat communicator, camera, GPS unit, inReach mini and a small action camera. Edit the values to match your own gear.
- 02
Set the trip duration in days. The planner multiplies daily consumption by trip length and adds reserve.
- 03
Set the expected overnight temperature. Below 5 degrees Celsius, the planner applies a cold derating factor to the power bank capacity because Li-ion cells lose usable energy in the cold.
- 04
Read the total daily energy requirement and the minimum recommended bank capacity. Add 25 percent if your trip includes any night with no recharging opportunity.
- 05
Round up to the next standard bank size — 5,000, 10,000, 20,000 or 40,000 mAh. Carrying more is harmless; running out is not.
What the calculator actually does with the numbers you enter
- 01
Device consumption values come from manufacturer datasheets cross-checked against independent teardown reviews, in Wh rather than mAh to handle mixed voltages cleanly.
- 02
Phone and tablet consumption is modelled against mixed use — navigation, camera, occasional transmission — not continuous screen-on or continuous streaming, which would be substantially higher.
- 03
Cold derating follows the Li-ion capacity-versus-temperature curve from the battery literature: roughly 80 percent at 0 degrees, 60 percent at -10, and 40 percent at -20.
- 04
A 25 percent reserve is added to cover unplanned charging cycles, device firmware updates that increase drain and the dead-band at the bottom of the discharge curve.
Published sources behind the formulas
- [1]ANSI/NEMA FL 1 — flashlight basic performance standardNational Electrical Manufacturers Association · 2024
- [2]Lithium-ion battery performance at low temperatureJournal of Power Sources · 2024
- [3]Field power management for backcountry electronicsBackpacking Light · 2025
Frequently asked questions
Short, sourced answers to the questions readers send us most often about this calculator.
Why do you measure in watt-hours rather than milliamp-hours?
Because mAh is only meaningful at a specific voltage. A 10,000 mAh power bank at 3.7 V delivers 37 Wh; at 5 V output it delivers about 25 Wh after conversion losses. Watt-hours lets the calculator compare a phone battery and a sat communicator on the same scale without lying about either.
How do solar panels change the math?
Solar panels add energy under direct sun but are unreliable: panels typically deliver 60 to 80 percent of their rated output in field conditions, less in cloud or at low sun angles. If you are carrying a panel, enter it as a partial recharge rather than a guaranteed one — say 20 to 30 percent of daily requirement in fair weather, zero in poor weather.
Is airplane mode really necessary?
It is the single biggest power saver on the trail. A phone in airplane mode with GPS only uses roughly a quarter of the energy of the same phone in normal mode. The planner assumes airplane mode by default; if you need connectivity, add 30 to 50 percent to the phone figure.
What about lithium primary batteries?
Lithium primary (non-rechargeable) AA and AAA cells deliver roughly three to four times the energy of alkaline at low temperatures and weigh less per unit energy. For headlamps, GPS units and emergency devices on a winter trip, primaries are often a better trade than a second power bank. Enter the device with its lithium primary consumption profile if your device supports it.