Power Bank Calculator

How to use the power bank calculator

1. Enter the trip length in days (1 to 14).
2. Enter your phone battery in Wh. Typical phones have 3,000–5,000 mAh at 3.85 V, which is about 12–19 Wh; the exact figure is in the manual or the technical data.
3. Choose your phone use per day: careful (about 40 % of the battery – flight mode, offline map, screen dimmed), normal (about 70 %) or heavy (about 100 % – lots of photos, navigation with the screen on, messages).
4. Choose your headlamp: USB rechargeable, AA/AAA cells or none.
5. Add other USB devices in Wh per day – a sports watch is about 1–2 Wh.
6. Pick the weather: mild adds 25 %, frost 50 %.
7. Leave devices fully charged at the start ticked if you set off with full batteries.

The result shows the recommended power bank with its Wh and weight, the capacity needed in mAh and printed Wh, the energy per day, a bar chart by device and a table of how far each bank size goes on this trip.

Worked examples

The weekend: three days, careful use, mild weather

This is the worked example from our power guide. A phone with a 15 Wh battery, used carefully, a USB headlamp, no other devices, a 3-day trip in mild weather, everything charged at the start. The calculator shows:

• Per day: 8.0 Wh – phone 6.0 Wh plus headlamp 2.0 Wh, with 2 days from the bank
• Needed: 8,300 mAh – 31 Wh printed capacity
• Power bank: 10,000 mAh – 37 Wh, typically 180–250 g

Day one runs on the full devices, so the bank covers two days: 16 Wh, plus 25 % = 20 Wh that must arrive in your devices, and 20 Wh ÷ 0.65 ≈ 31 Wh printed. The table shows the reserve in days: a 5,000 mAh bank lasts 1.2 days at your use, a 10,000 mAh bank 2.4 days and a 20,000 mAh bank 4.8 days. The 10,000 mAh bank fits with a little room to spare.

The same trip in frost

Switch the weather to frost and everything else stays the same. The daily need is still 8.0 Wh, but the margin rises to 50 %: 16 Wh × 1.5 = 24 Wh, ÷ 0.65 ≈ 37 Wh. The calculator now shows needed: 10,000 mAh (37 Wh printed) and still recommends a 10,000 mAh bank – in the table it lasts exactly 2.0 days. That is the full bank with nothing spare. In winter, take the next size up or a second small bank. A frost tip appears as well: keep phone and bank warm, and charge only above 0 °C.

Setting off with half-empty devices

Untick devices fully charged at the start for the weekend trip in mild weather. Now the bank has to cover all 3 days: the calculator needs 12,500 mAh (46 Wh printed) and recommends a 20,000 mAh bank (74 Wh, 300–450 g). Charging everything the night before saves you a whole bank size and its weight.

Five days with normal use and a watch

A 5-day trip, the same 15 Wh phone used normally, a USB headlamp and a sports watch at 1.5 Wh per day, mild weather. The result:

• Per day: 14.0 Wh – phone 10.5 Wh, headlamp 2.0 Wh, other devices 1.5 Wh, with 4 days from the bank
• Needed: 29,100 mAh – 108 Wh printed
• Power bank: 2 × 20,000 mAh, with the note “or AA devices / solar on long trips”

A single 20,000 mAh bank lasts only 2.7 days at this use. Because the total is above 100 Wh, the calculator adds the flying note: airlines often limit banks to around 100 Wh each without approval, and all banks go in hand luggage. Each of the two 20,000 mAh banks has 74 Wh. Before you carry 600–900 g of power banks, look at the daily need: switch the phone use to careful and the calculator shows 9.5 Wh per day, 19,800 mAh needed (73 Wh printed) and a single 20,000 mAh bank – just enough, so the 20–30 % reserve on the phone matters even more.

AA headlamp and a one-day hike

With an AA headlamp on a 4-day trip (careful use, mild), only the phone draws on the bank: 6.0 Wh per day, 3 days, 9,400 mAh (35 Wh printed) – a 10,000 mAh bank, plus the tip to carry spare cells. On a 1-day trip with full devices the calculator shows no power bank needed: a one-day trip runs on full devices.

How it is calculated

The calculator follows the method in four lines from our power guide:

• Daily need = phone battery (Wh) × share used per day + 2 Wh for a USB headlamp + other devices. The shares are typical values: careful 40 %, normal 70 %, heavy 100 %. The 2 Wh for the headlamp is half of a 4 Wh battery per evening.
• Days from the bank = trip days − 1 if the devices start full, because day one runs on the devices themselves.
• Energy that must arrive = daily need × days × margin. The margin is 1.25 in mild weather and 1.5 in frost; it covers detours, photos and cold.
• Printed capacity = that ÷ 0.65. About a third of the printed capacity is lost to voltage conversion and heat, so the tool plans with 65 % (the real range is about 60–70 %).
• mAh = Wh × 1,000 ÷ 3.7 V, the voltage of the cells inside the bank.

The recommendation is the smallest of 5,000, 10,000 and 20,000 mAh that covers the need; above that the calculator combines 20,000 mAh banks. The “days at your use” column in the table is the bank’s usable energy (65 %) divided by the margin and by your daily need – for the 10,000 mAh bank in the weekend example: 24 Wh ÷ 1.25 ÷ 8.0 Wh ≈ 2.4 days.

What the calculator cannot tell you

• Your phone’s real consumption. The shares of 40, 70 and 100 % are typical values. Your phone may use much more or less – check its battery statistics on a normal day or on a test walk and adjust the use setting.
• How cold your bank really gets. The 50 % frost margin assumes you keep the batteries warm. A phone strapped to the outside of the pack can switch off long before the battery is empty. More in phone battery in cold weather and in our guide to winter camping.
• The emergency reserve. The calculator sizes the bank for your planned use. The 20–30 % reserve for an emergency call still has to stay on the phone – when it gets there, the phone is no longer your map.
• Unplanned nights. A delay or bad weather adds a day the plan does not contain.
• Solar. Read the honest assessment of solar chargers before you rely on one.
• Airline rules. The flying note is a reminder; rules differ between airlines and countries.

Save power before you buy capacity

Flight mode, offline maps and a dim screen save more than any bigger bank. Download maps before you leave and carry a paper map and compass as a backup – our guide to navigation gear and offline maps explains how. For the headlamp, a red-light mode and a low setting in camp stretch a charge a long way; see choosing a headlamp. In autumn and winter the evenings are long, so the daylight calculator helps you judge how many hours of headlamp you will need. If you travel alone, power is part of your safety plan: read safety on solo trips and how making an emergency call works when the battery is low. Packing for a bushcraft trip? The sharpening angle calculator covers the other piece of kit that needs maintenance on the way.