This may be a noob question, but I have been searching off and on for two days now.
I picked up a used RB2011UAS-2HND-IN unit, but needing to power it.
I understand that ideally the 18POW power supply model would be the best choice.
But what is the difference between RBGPOE and the RBPOE injectors? I can find very little difference about/between them.
What is the difference? When is each used? When is one prefered over another?
Thank you for your time,
Kevin
RBGPOE is for powering Gigabit ethernet ports. RBPOE is for powering 10/100 ports.
RB2011 has Gigabit ports, so you need RBGPOE
Well, that is simple then. 
Thank you very much.
Hopefully this can help somebody else searching for it
how much does the GB POE really differ from a simple POE ??
RBGPOE is completely different from RBPOE. It has two complete L/C filters in order to allow power and data into 4,5 and 7,8 couples of cat. 5e cable.
RBPOE instead uses only 1,2 and 3,6 couples for data, and 4,5 - 7,8 couples for power, and so it only allows 100 base TX connection.
Other differences are:
RBPOE has ground connection only from F and M RJ45 ports, instead RBGPOE has also the ground connection with power plug, and that is a major issue, because often we connect ground with the power supplies and we need to solder a ground cable to use RBPOE in this way.
Those poe adapters are useful in centralized power supply in base stations, but they are very uncomfortable in single subscriber installations, so we buy ubiquiti active poe injector for subscribers and cannot use the standard power supply provided by Mikrotik.
I hope that in the future Mikrotik would provide a compact poe unit with ground connection, similar to ubiquiti ones.
Can i use RBGPOE on hap AC lite? It has 10/100 and original line from switch is gigabit.
Yes, you can.
Thanks!
You can combine gigabit ports with fast Ethernet ports (10/100) but on gigabit side you can’t use the port with full features, but yes, it works anyway.
Regards.