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We have tested Xantrex power hub 1800 in combination with APC UPSes to insure continuous power source to servers during extended power outages.
We bought ours at 899$ ( here ). The link above is only for documentation purposes. 
Tested APC UPSes ( Back-UPS XS 1300, Back-UPS ES 550, Smart UPS 750 ) (http://www.apc.com/) 
 
IMPORTANT NOTE ON UPS AND SINE WAVE: Since the Xantrex Power Hub 1800 constitute a relatively affordable solution ( ~ 1000 $ without the deep cycles batteries), it doesn't produce a true sine wave for AC output when running on batteries, the output is a modified sine wave. This caused the APC UPSes to keep switching on and off batteries when the UPSes were getting their AC power from the Xantrex Power Hub 1800 running on batteries. Setting the UPSes sensitivity to LOW (or medium on some models) either with apctest on Linux or using APC Power Chute on Windows solved this problem. On Linux, you must shutdown apcupsd before running apctest. On the sine wave matter, it is also interesting to note that only the APC Smart-UPSes line of products provide a true sine wave on AC output. Cheaper models output AC current with a modified sine wave when running on batteries.
Why use both the Xantrex Power Hub 1800 and APC UPSes in combination ? 1) APC UPSes are not designed to run on batteries for extended period of time. Neither are they designed to run with bigger batteries than the recommended ones. 2) Xantrex Power Hub 1800 doesn't have any way to provide battery or power status to the computers. It would therefore be impossible to do a clean shutdown of your computers when battery power is exhausted if your computers were connected directly to the Xantrex Power Hub 1800. 3) On power failure, the Xantrex Power Hub 1800 switches to batteries in 40 ms (milli second) while the APC UPSes do that much faster, in 4 ms. Having your computer plugged to the APC UPS insure that your data is safe when a power failure occurs. 4) The life of your UPSes batteries will be extended because they will only get drained after the Xantrex deep cycle batteries have been drained. 5) APC UPSes consider a battery with a 11.8-12 V DC voltage to be exhausted and turn themselves off while the Xantrex will drain the deep cycle batteries until they reach a voltage of 10 V DC, giving you more run time especially with small loads. Xantrex Power Hub 1800 highlights : 1) With 4 deep cycle batteries (4X100 amp-hour), the Xantrex Power Hub can provide power to a single server for more than 2 days if the display monitor (screen) in turned off. You provide your own batteries so the Xantrex Power Hub is not limited to a specific type of proprietary batteries. 2) Xantrex Power Hub 1800 is designed to run with deep cycle batteries. It features a 3 stage battery charger that will keep your deep cycle batteries healthy. It can run with 1 to 4 deep cycle batteries with a maximum of 400 amp-hour storage capacity. 3) The Xantrex Power Hub 1800 has 2 DC inputs so you can plug-in a wind turbine or solar panels into it to recharge the batteries even while running on batteries. These additional inputs may keep you running even longer in the event of a power failure. 4) You may also switch to an AC power generator ( example: gas or diesel ) if you think that the power failure will last longer than your deep cycle battery capacity. While running on the AC generator, the Xantrex will recharge your deep cycle batteries while providing power to the UPSes. Power failure scenario: 1) Power goes off. 2) UPSes switch to their battery power. 3) 40 ms later, Xantrex switch to battery power. 4) UPSes switch back to AC power source ( AC is provided by the Xantrex running on batteries ) Power comes back scenario: 1) Xantrex waits for 20 seconds to insure power is now stable, then it switches to line power and starts recharging the deep cycle batteries. 2) APC UPSes quickly switch on and off batteries when the Xantrex switches back to line power. Deep cycle batteries exhausted scenario: 1) When deep cycle battery voltage drops to 10 V DC, the Xantrex Power Hub shuts down. 2) UPSes switch to battery power. 3) When UPSes batteries are exhausted, apcupsd ( or Power Chute on Windows ) cleanly shuts down the computers. 4) When power comes back, the Xantrex will automatically send the line power to the UPSes again. 5) Although Xantrex always forward line power to the UPSes (even if the Xantrex is turned off), you may have to manually turn on the Xantrex when it shuts down due to low deep cycle battery voltage in order to have it recharge the batteries and enable the capacity to run on batteries again. Conclusion: If you are looking for an affordable, flexible, yet robust solution to insure continuous power to your computers when an extended power failure occurs, we recommend this setup because we are satisfied with the results we obtained during our tests.
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