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*'''Electrical Power Consumption''': The commonly used unit for small electrical devices is Watt (W) or Milliwatt (mW). 1000 mW equals 1 W. For DC consumers the power consumption can be calculated by multiplying the supply voltage with the required electrical current. In example a GPS device that is powered with two AA NiMH batteries (2 x 1.25 V) and draws a current of 50 mA has a power consumption of 125 mW (or 0.125 W). Manufacturers sometimes provide the power consumption in Ampere (A) or Milliampere (mA) what is scientifically incorrect but not an issue if the voltage is known. The conversion between current (A) and power (W) is made by multiplying the current with the voltage.
*'''Electrical Energy Consumption''': The technical correct unit is Watthour (Wh). Note the difference between power and energy: power (W) is the rate of consumption while energy (Wh) is the accumulated consumption. So the energy consumption is the summed-up power consumption over a given time span. If you want to compare power and energy with hiking than power is like the speed and energy is like the distance covered.
*'''Stored Electrical Energy''': The technical correct unit is Watthour (Wh) but battery manufacturers often use Milliamperehour (mAh) to indicate the capacity of a battery. This is scientifically incorrect but when comparing batteries of the same type this works. But when comparing different battery types the unit mAh is misguiding. A Lithium-Ion battery with a capacity of 1000 mAh stores more energy than a 2000 mAh NiMH battery because the Voltage of a Lithium-Ion battery is higher. A Lithium-Ion battery with a 1000 mAh capacity contains roughly 3.7 Wh while a 2000 mAh NiMH battery contains only 2.4 Wh. Note that USB power banks are not rated based on the output voltage but based on the capacity of the internal Lithium-Ion battery.
*'''Specific Stored Energy:''' The technical correct unit is Watthour per Kilogramm (Wh/kg). This number tells how much electrical energy is stored in relation to the weight of the battery or power pack. You can calculate the specific stored energy of a battery by dividing the stored electrical energy (Wh) by the weight of the battery (kg). As a hiker you obviously want this number to be high, to have as much as possible energy packed into as little as possible weight. But the battery chemistry puts physical limits to this. Understanding this helps to identify obviously false advertisements. I.e. if someone offers an AA NiMH battery with a capacity of 3500 mAh he scams. See the table about battery and power bank specifications for more detailed information.
*'''Energy Transfer Efficiency''': When charging an electrical device i.e. with a power bank some losses occur. A part of the energy will no end up in the charged device i.e. the smartphone battery but get lost in form of heat while charging. The common unit for efficiency is Percent. An energy transfer efficiency of 75% means that only ¾ of the stored electrical energy ends up in the charged battery. When charging a device from the grit in a town you do not need to worry about this, but when charging a device on the trail with a power pack this becomes relevant.
*'''Effective Stored Electrical Energy''': Unit: Watthour (Wh). This is the amount of the electrical energy of a power bank that ends up the in the charged device when considering the losses of the energy transfer process.
*'''Generated Electrical Power''': Unit: Watt (W). Today a wide range of outdoor gear promises a free recharge of your electrical consumers on the trail. Most common are solar panels but also water and wind turbines pushed on the market. The most weird gear I have seen is a stoves that promise to produce electrical energy using the thermoelectric effect. The power might be free but always comes with a weight penalty. Therefore the following is of importance:
*''Specific Power Generation: Unit''': Watt per Kilogram (W/kg). This is the actual archived power output divided by the weight of the device. This number shows the difference between a gadget that just appears cool and an actual useful piece of gear.
===Electrical Energy Consumption===
The actual electrical energy consumption during a long-distance hike depends on the electronic consumers and their use.