I've bought 5 of these $40 Port-a-Pow units:
http://www.amazon.com/PortaPow-Premium-Monitor-Multimeter-Ammeter/dp/B00LZ07BG0/ref=pd_sim_sbs_468_1?ie=UTF8&dpID=41JZ7oyd4TL&dpSrc=sims&preST=_AC_UL160_SR160%2C160_&refRID=02E0YS8KK5QS1SG3H315Here's my review on Amazon:
KEY ADVANTAGES
The key advantages of this unit over the other, cheaper USB power meters are that:
(1) it has it's own internal, rechargeable battery that lasts for many hours of use.
(2) It measures HOW LONG (in hours, minutes and seconds) that current has been flowing through it. And it stops counting time if current flow is interrupted, then restarts the count if current flow resumes. This means that it reports the true amount of time that current was actually flowing through it.
(3) It measures the total amount of POWER that has flowed through it, both in Watt hours and Amp hours. And it remembers this reading until you turn it off or the battery runs down; so you can let an experiment run and come back hours later to view the readings. And, like the time count, you can start and stop the current flow and it will start and stop measuring the power consumption, giving you a true measure of the total power that has flowed through the meter.
(4) It is quite accurate.
FURTHER COMMENTS
I did a series of experiments with four of these meters to determine how well they met their specifications. I found that voltage and current measurements are typically within 0.1% of the true value (as measured by a high precision digital multimeter), which is an order of magnitude better than the 1% accuracy claimed by the manufacturer. I also found that the voltage burden with half an Amp flowing through them was only about 55mV.
I did not measure battery life, but I have run overnight experiments with these units with no problems; and I think that in one of my experiments a unit ran for two days before the battery ran out. And remember that these can be recharged from any USB port that supplies power.
The units have the look and feel of a home-brew device. Of the five I've bought, one came with a banana jack that internally had a loose locknut because the threads were poorly formed on the jack. I had to find a matching locknut, open the unit, unsolder the back of the jack, screw down the first locknut, then add a second one behind it, then re-solder the banana jack. Other than that they have all been fine.
Although marketed for USB measurements, the specs say these meters will measure up to 50 Volts, up to 5 Amps, and up to 250 Watts; so they should be useful for lots of other projects. With this in mind the units come with digital multimeter style test leads and some general-purpose connectors so that you can use them for lots of general electronics projects.
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