OT: Continuous mode UPSes

[Bruce Dawson]

I'm having a hard time making sense of all the sales speak on the
various web sites.

I need a continuous mode UPS that will be used 24x365 and it needs to
supply at least 500 watts for 15 minutes. Note: This needs to be
*continuous mode*; line-interactive and standby will *not* work (we have
too many phase changes here). The usual contenders such as APC,
TrippLite, ... don't seem to have continuous mode UPSes.

Line and battery monitoring would be nice but is not mandatory. If it
can use my batteries (deep-discharge marine type), that would be preferable.

I've tried building my own with inverters and chargers, but have had
problems with the batteries dying prematurely (min: 3 months, max: 9
months).

Does anyone have any recommendations?

--Bruce