How Big a Power Bank Do You Need for Backpacking?

The right backpacking power bank is not really a phone-charging question. It is an energy-budget question.

A five-day trip with a phone mostly in airplane mode can need less stored energy than a two-day trip with constant GPS recording, satellite tracking, video, navigation and cold-weather battery losses.

That is why a useful estimate starts with the devices you actually carry.

Use the free Backcountry Power & Battery Planner to build the calculation in your browser. It uses US weight units by default and can switch to metric.

The short answer

For a rough starting point:

• a light weekend setup may fit inside a 5,000–10,000 mAh-class bank;
• several days with a phone plus satellite messenger often pushes the plan toward 10,000–20,000 mAh;
• long stretches without reliable charging, heavy camera use or multiple devices can exceed 20,000 mAh.

Those are only classes, not guarantees. Two products with the same advertised mAh can deliver different usable energy after voltage conversion, cable losses and device charging behavior.

Why mAh alone is misleading

Power banks are normally marketed in milliamp-hours. The problem is that mAh does not describe energy unless voltage is also known.

Watt-hours do:

Wh = volts × amp-hours

That makes Wh much better for mixing devices with different batteries.

For example, a phone, satellite messenger and headlamp can all be converted into one daily Wh budget even though their battery capacities are advertised differently.

The GentleTools planner keeps the calculation in Wh internally, then shows common power-bank classes as an easy comparison.

Step 1: estimate daily use for each device

List every rechargeable item you expect to depend on:

• phone;
• Garmin inReach or other satellite messenger;
• GPS watch;
• headlamp;
• camera;
• earbuds;
• rechargeable water-treatment device;
• other electronics.

For each one, estimate how much of its battery you normally use in a day.

If your phone battery is approximately 17 Wh and you typically use 40% per trail day, that is roughly:

17 × 0.40 = 6.8 Wh/day

Do the same for the rest of the device list.

If you have measured real charging energy with a USB meter, use that instead. Real measurements are more useful than generic internet averages.

Step 2: multiply by days between reliable charging

The important number is not total trip length when you can recharge halfway through.

It is the longest stretch between charging opportunities you are willing to rely on.

If your daily electronics budget is 12 Wh and the longest stretch is five days:

12 × 5 = 60 Wh

That is still not the final answer.

Step 3: account for charging losses

Energy is lost between the cells inside the bank and the battery inside your device.

Losses come from:

• voltage conversion;
• cable resistance;
• charging electronics;
• heat;
• device behavior near full charge;
• battery condition.

Instead of pretending a 72 Wh bank gives you exactly 72 Wh at the device, use a conservative efficiency assumption.

The Backcountry Power & Battery Planner lets you set this yourself rather than hiding a fixed assumption.

Step 4: add your own reserve

A reserve is not the same thing as claiming a battery will perform a certain way in bad conditions.

It is simply an extra planning margin for uncertainty.

Examples of things that can increase use:

• more navigation than expected;
• weak cell service before you switch fully offline;
• taking more photos or video;
• a longer-than-planned day;
• extra satellite messaging;
• charging a companion’s essential device;
• battery behavior changing with temperature.

The tool separates your normal reserve from an optional cold/uncertainty reserve so you can see exactly what assumption is increasing the result.

5,000 vs 10,000 vs 20,000 mAh for backpacking

The real tradeoff is usually energy versus carried weight.

A smaller bank may be enough if:

• the trip is short;
• your phone stays mostly offline;
• navigation is on a separate low-power device;
• you do not record much video;
• you can recharge reliably during the route.

A larger bank becomes more attractive when:

• the no-charge stretch is long;
• the phone is your main navigation device;
• you run satellite tracking frequently;
• several devices share the bank;
• you need larger planning margins.

Do not choose the biggest bank automatically. Extra capacity that you never use is still weight carried every mile.

A better comparison: Wh per ounce

For US users, one of the most useful shopping comparisons is:

nominal Wh ÷ ounces carried

Metric users can do the same as Wh per gram.

That helps separate genuinely energy-dense banks from products that are simply larger.

The planner shows common capacity classes and estimated carried weight together so you can see the tradeoff rather than comparing mAh in isolation.

What about a Garmin inReach?

Treat the inReach as its own line item rather than assuming it is “basically free” because its battery lasts a long time.

Daily use can change substantially with:

• tracking interval;
• message volume;
• screen use;
• Bluetooth connection;
• weather requests;
• temperature;
• device model and battery age.

For a more specific walkthrough, see Phone + Garmin inReach Battery Planning for Multi-Day Hikes.

Build the plan before buying another battery

The most useful result is not “everyone needs a 20,000 mAh bank.”

It is:

My devices use approximately X Wh per day, my longest no-charge stretch is Y days, and with my chosen losses and reserve I want approximately Z Wh of nominal bank capacity.

That gives you a number you can explain and adjust.

Open the Backcountry Power & Battery Planner to calculate it, save several trip versions, compare power-bank classes and print or export the finished plan.