Why the Rockpals 300 instead of the more popular (here on Amazon) 250? The 300 really doesn't have that much more capacity, but it has a few features I find worth the extra cost. First, the charger is built in - no power brick to keep up with - just plug in a three-prong AC power cord. Second, it has a built-in 12-volt cigarette lighter jack@ 8 amps. Again, no additional dongle to keep up with. Third, any output can be switched off or on. This means that the inverter is off when not in use along with the power wasting idle current it uses. The USB ports are off when desired, eliminating the small current waste of the 5-volt regulator circuit.
Realistically, the power available from any of these small banks doesn't lend itself to any significant AC usage. Many folks plug in their AC adapters to power low voltage devices like CPAP machines. I understand some offer direct 12 volt DC operation. This would offer the best efficiency since the inverter and its associated power loss would be eliminated. In general, setting yourself practical expectations for such a small bank will avoid a bad experience. Heaters, refrigeration, lighting other than that using LED bulbs - don't count on any real continuous runtime.
I use the bank to power 12-volt lighting. Repurposed lamps and the 12 volt A19-type bulbs available here on Amazon make an efficient combination. Yes, you can charge most of your USB devices, but there are cheaper alternatives. Recently, Rockpals was having a sale and I bought 2 of the 300 models for $270 each.
Runtimes during testing have been good. I've been getting 244 Wh out of both banks. This is close to an 85% discharge efficiency and is a little better than I expected. It seems a discharge efficiency is a moving number depending on the specific chemistry, temperature, discharge rate, design and operation of the BMS, etc. The values I've seen in some white papers range from 82% to 92%, so anything in the middle looks good.
Overall, 300 is excellent for my purposes and expectations. The sale price didn't hurt either. Evaluate your specific needs before investing in one of these power banks and you won't be disappointed.