I've been running at home for years a slightly smaller unit than your requirement, a BEST FERRUPS 1.4kVA/1.0kW unit, factory configured for external batteries, 15amp internal charger option and cord and plug connection. This unit is spec'd to handle a 150% surge (no duration given) and 125% for 10 minutes if online and 110% if running on inverter. Needless to say on a hard surge (for example the inrush from starting a motor too large for the unit) shutdown is extremely fast, I believe faster than typical magnetic breakers which open very quickly on overload.. Another major reason for installing this unit is surge protection, spec'd at 2000:1 with the ferro magnetic transformer based design.
As noted the big costs are the batteries, and I'm pretty sure I have a noticable hit on the power bill as this older technology unit is only in the mid 80% efficiency range, with constant load on it.
What helped with my costs for many years was working in the commercial UPS/DC power plant business. The UPS unit came from a customer site that needed to upgrade and good used batteries (for many years) came from a few customer sites that due to extreme availability requirements meant switching out 10 year design life batteries every 3 years. The rather old pic below shows 6 6v batteries strapped for the 12v battery bank operation. We've had week long power outages and by dropping load back, it had sufficient batteries at the time to carry the load for 3 days; This is a nice system to have even if you have a generator.. With the flexibility it added, we would only run the generator around 2 hours in the morning, and then in the evening for (all electric) well water, showers, hot water, cooking and battery charging.. except for the trusty wood stove for heat.
Not pictured (and getting off topic) is another battery bank and outboard charging system to add more capacity (have had up to fourteen 12V 70AH batteries in the system). In addition to the long run time these were used to do some long term battery charging methodology studies over a 15 year period using charging methodology similar to as described in this Eaton video
and most critical, a relatively constant temperature environment no higher than 68 deg. It yielded a 10-14 year battery life out of most of the 10 year design life batteries in the study. Have dealt with a number of larger UPS systems with integrated battery banks get barely 3 -5 years life out of 10 year design life batteries due to the UPS heat cooking the batteries.
Most of the in house electronics (large screen TV, network equipment, computers, DECT phones, etc), a subset of cord and plug connected lighting (sufficient for power outages) and a rather sizeable security system are powered from this unit. It powers an Article 702 (US NEC) - Optional Standby System branch ciruit with marked UPS outlets at multiple locations, installed next to selected original house outlets as itsmoked did. Nice to be able to use the computer to log on to the power company website and report a power outage!
One challenge I ran into when I originally installed this system was trying to find reasonably priced, appropriately rated (both ampacity and UL listed) hardware for the battery banks. The commencial market stuff is just too big and incredibly expensive so I went with good marine application low voltage, suitable ampacity UL listed stuff. Note the marine battery disconnect switch. I also wanted more interbank battery fusing than we even had on commercial sites. It makes nice disconnects for testing and swapping out batteries.
