someone in another thread said that certain power supplies react differently to True Sine Wave and Simulated Sine Wave
That sort of thing isn't an issue these days
modern PSU's handle it better, and modern UPS output better
When they werent compatible, it basically lost some power to efficiency and you needed a higher output unit to compensate.
I went for the highest capacity cheap unit i could afford (price difference was small to go up in capacity) and i get anywhere from 3 minutes to 55 minutes, according to my UPS
(That surge to 684W is unexpected, as my CPU and GPU used <400W combined - I may have done something dumb and plugged something like my speakers into the UPS by accident)
(4K 140FPS gaming, vs web browser on desktop)
You can also set it so that when on battery windows power plan changes, for example locking your CPU to 50% on the power saver plan with short timers for screen power off and HDD power down - that'll instantly extend the runtime.