Camp power budget calculator

What this calculator does#

It converts your device list into watt-hours per day, multiplies by nights, subtracts a realistic solar harvest, then adds back the losses that make a nominal 37 Wh bank deliver rather less than 37 Wh. The answer is a capacity to buy, in the only unit that compares batteries honestly.

How the maths works#

Daily demand. Phone use is modelled at 4.5 Wh a day in airplane mode, 8 Wh light use and 16 Wh heavy use, per phone. A headlamp at 100 lumens draws about 1.1 W, so hours times 1.1. A GPS watch tracking a full day is about 2.2 Wh, a satellite messenger with tracking about 5 Wh, a camera battery about 7 Wh each. A compressor fridge is modelled at 7.5 Wh per litre of capacity per day in warm conditions, which is why it dwarfs everything else.

Conversion losses. A power bank delivers roughly 85 percent of its rated energy through a cable, so we divide by 0.85.

Cold. Below freezing, lithium cells give up roughly 20 percent of usable capacity, so we divide by a further 0.8 when you tick cold nights. Charging a lithium cell below 0 degrees C damages it, which is a separate and more serious point.

Solar. Rated watts times equivalent full sun hours: 3.5 for an open sky with the panel repositioned through the day, 2.2 strapped to a pack or pitched once, 1.0 in forest, cloud or short winter days. Those figures already include system losses, so do not derate them again.

The two worlds problem#

Look at the result table and you will usually see one of two shapes. Either every line is single digit watt-hours, in which case the honest answer is a small bank and no solar, or there is a fridge, in which case the handhelds are a rounding error and the real question is battery chemistry and charging strategy.

There is very little in between, and buying for the wrong world is the most common mistake in camp power. A 20,000 mAh brick for a two night trip is 500 g of unused capacity. A 20,000 mAh brick for a week with a fridge is a fifth of one day.

Frequently asked questions#

How many watt-hours is my power bank?#

Multiply the printed mAh by 3.7 and divide by 1,000. A 10,000 mAh bank is about 37 Wh, a 20,000 mAh bank about 74 Wh, and a 27,000 mAh bank is right at the 100 Wh airline limit.

Can I take a power bank on a plane?#

In carry-on baggage only, and up to 100 Wh without approval. Between 100 and 160 Wh needs airline permission, and above that they are not accepted. Never in checked baggage.

How much solar do I need?#

Take the daily demand the calculator gives you and divide by the equivalent full sun hours for your conditions. A 10 Wh per day handheld load in good conditions is a 3 W panel, which is smaller than most people buy. Fridges are a different scale, typically 100 W and upwards.

Does cold really halve my battery?#

Not halve, but a fifth of usable capacity is a fair planning figure near freezing, and the capacity comes back when the cell warms. Keeping the bank in an inside pocket or in your quilt overnight recovers most of it.

Do bigger cables or fast charging change the numbers?#

Fast charging moves the same energy in less time and generates a little more heat, so the loss is marginally higher. The much larger effect is cable and connector quality: a poor cable can waste a noticeable share of a small bank over a week.

What about a solar panel that charges the phone directly?#

It works in strong sun and stops the moment a cloud passes, which can leave a phone in a charging loop that wastes energy. Charging a bank from solar and the phone from the bank is more reliable and more efficient in practice.