I recently bought a few powerbanks to replace by self-made battery-packs and to have an “emergency” power supply for my iPad. (I always forget to charge my iPad during the day and get angry about myself in the evening.)
To test the power consumption on my devices (without always hanging a volt meter in series to my devices) I’ve bought a USB voltage and current meter. The device is simple to use – just stick it in between the device you want to test and your computer/ charger/ powerbank. Its OLED display shows the voltage, current, power, and capacity (nice, but why?) of the attached device. 
I’m pretty sure the meter is not 100% accurate and so far have not tested it by hanging a volt meter in series. But for me it’s enough to have a rough estimate if a device is limited to 0.5 W or actually pulls 1.5 W from a powerbank or charger.
Just in case you’re wondering what happens if your connection is rotated 180° (compared to my pictures): there is a tiny button, really difficult press, on the back side that rotates the display.
The USB meter is available from multiple vendors on eBay and AliExpress. I’ve got mine from the (sold out) eBay auction: 0-3A OLED USB Detector Tester.
| Weight | 9 g |
| Display | 0.91" OLED screen |
| Size | 43 x 20 x 10 mm (LxWxH) |
| Voltage measurement range | DC 3.50 – 9.99 V |
| Voltage accuracy | ± (0.3% + 1 word) |
| Current measurement range | DC 0.00 – 3.00 A |
| Current accuracy | ± (0.5% + 2 words) |
| Power measurement range | 0 – 30.00 W |
| Capacity measurement range | 0 – 99999 mAh |