Is the 12HX300 a 70 Ah or 71 Ah battery?

EnerSys's current range summary publishes 71 Ah at the 8-hour rate to 1.75 volts per cell; size from that figure. Older literature carried a 70 Ah rounding, and it survives in some listings, this page's address among them. One amp hour changes no sizing decision in this class, and UPS work sizes from the 284 watts per cell at the 15-minute rate in any case.

Can I step up to the 12HX330 later for more runtime?

Only as a full string swap, and only if the tray allows it: the 330 runs 11.8 inches long against this model's 10.2, an inch and a half a packed shelf may not have. It buys 83 Ah and 336 watts per cell against 71 and 284. If growth is on the roadmap, measure for the larger case now and quote both models; carrying the extra length from day one costs less than rebuilding shelves later.

What float voltage does an HX string want?

EnerSys publishes 2.25 to 2.28 volts per cell at 77°F, which is 13.5 to 13.68 per 12V bloc, and the HX design also accepts recharge at high current rates after a discharge. Confirm the UPS's float setpoint lands in that window at commissioning and temperature-compensate where the equipment supports it. A string floated outside the published band ages faster than any spec sheet predicts.