Pretty sure you are doing it wrong. I'd be more willing to bet you are currently seeing .82A for startup, .33 running. However... I don't trust your meter. Since you've already killed and skinned a power cord, separate out enough insulated wire in it to get several turns around the clamp, and try again. Each conductor that passes through the meter loop multiplies your value, so if you manage to get 5 turns you would divide the face value of the meter by 5. That should get the meter into the meat of its measuring range, where it is more accurate. 10 turns would be nice, but may not be doable. You need to also measure the voltage accurately, not just guess. Volts * Amps is the number we want, not watts.
Now, the second part of your question. UPS's aren't commonly quoted in watts, as the VA number is bigger, and it's actually more accurate for what's being measured. The stated rating of the UPS is the maximum peak power it can supply, but has little if any correlation to runtime. Runtime is a measure of usable battery capacity and efficiency, and is rarely stated clearly because the variables are too hairy.
Battery chemistry in most UPS's is pretty basic (actually very acidic) and hasn't changed in over a hundred years. Because lead is the heaviest ingredient in the battery and is responsible for overall capacity, you can bet that if you have 2 UPS's rated for the same rough peak capacity and one is twice as heavy, it will run your system for significantly longer. Bigger, heavier batteries = longer runtime. Conversely, if 2 UPS's are the same rough size and weight, the higher peak capacity will probably have shorter runtime under equivalent load because of inefficiencies in power conversion.