Leistungsschwidler properly debunked: Some Notes

Here can be measured on standard USB-B with the absence PortaPow adapter only standard USB-A, which makes it difficult, for. Instance to measure the output power at the end of a micro USB B. That is why only 4 stars from me.
Matching Adapter as a side dish could have been a very practical to extend the range of applications.

Remember:
Volt (V, voltage) x amperes (A, current) = "Watt" or VA (volt-amps)

1: utilities (. Transformer, eg charger)
As a rule, are a USB charger / utilities from no load between 5,0-5,2V. If a consumer is connected, the voltage (V) is reduced by the stronger, the more the burden of the consumer, the device performs its capacity to the limit. The load limit should not be significantly below 4,85V as consumers react from here increasingly on the excessively low voltage, and either not working properly or reduce charging currents, or other adverse reactions could be provoked.
Under full load, the performance of the provider should correspond approximately to the product of VxA as indicated on the utilities. Especially with solar chargers you will here often find significant shortfalls since their peak performance can only be achieved under laboratory test conditions.

2: Cable
Ultimately it does not matter how much a tested charger makes and how much the consumer can decrease performance if the charging cable is used in between is not suitable for his job.
Relevant for this is the internal resistance of the USB cable used, which increases with the length if the length is not compensated with a larger diameter cable.
Usually most USB cable coming from Chinese Billigstproduktion and consequently with the thinnest possible cable cross-sections. Copper costs now times more than plastic and 10 million cables, saving at the wrong (for the customer) expects spot - for the producer.
The so-called loss of power is lost in the form of radiated heat through the cable. In the overall small USB power transfer but this is usually hardly perceptible.
If you have an adapter, or the consumer can voltage (V) and current (A) specify, so you fair once before and once behind the cable.
If one measures only seeps with PortaPow so lower currents and / or voltage falling at the cable end, as revealed by the PortaPow that the cable is no good, and the power of the charger in the internal resistance of the cable.
Basically, the shorter the cable, the more likely it is appropriate to transfer even larger currents (mA). If you like it so dear "quick 'n' dirty" as long (sam) and comfortable, should have recourse to a 30cm short USB cable. While this is no guarantee of a lossless electricity transmission, but even Chinese Billigsthersteller would have to scramble to build in such thin wires to versaubeuteln on 30cm still.

3: Consumers
The information on the consumer are maximum values. The consumer always decreases only as much power as it needs at the moment. This must be the maximum amount does not necessarily. A consumer with battery for example, invites these potentially with different currents, to find a compromise between charging speed and battery life depending on charge status and used battery management via the charging volume respectively. Time is then also the screen lighter or darker, the processor slaves, or maybe not. This may vary. "Put" an iPad with 1.351mA then need, or maybe wants it just no longer, even if the charger 2.100mA could provide. So be careful before jumping to conclusions.

Conclusion (tl; dr):
Before so perhaps mistakenly suspected a device not to use at all, you should check if the culprit is not actually to look elsewhere.
Some understanding of the physics of electricity to the correct interpretation of the data obtained assuming that PortaPow USB Power Monitor this is a convenient, accurate and reliable help.