There's a lot of compromise in UPS design. For example, a common topology in offline UPS is that the inverter and the charger are the same circuit driven diffe

There's a lot of compromise in UPS design. For example, a common topology in offline UPS is that the inverter and the charger are the same circuit driven differently[0] - so you can't charge the battery while the inverter is carrying load. This is popular at the low-end because you have literally half as much UPS, but makes what you're describing impossible. Another common issue at the low end is that the inverter isn't thermally sized to run non-stop, they know they can cut a corner because your battery presents a finite and known duty cycle. There are ways around this, but at some point you end up fixing the wrong problem - eg, it's cheaper, safer, and more resilient to buy a transfer switch instead of uprating two UPS to be capable of daisy-chaining. [0] https://patents.google.com/patent/US5302858A/en https://patents.google.com/patent/US5302858A/en