Its been my experience using AGM batteries only last about 3-5 years installed in the typical UPS battery backup. I've been burned several times when there is a power failure the UPS fails to supply the necessary load or only has enough reserve for a few seconds. The UPS I have is supposed to check the condition of the batteries every so often, and even that is not a guarantee the batteries are good. Measuring voltage when the UPS is in standby show 13+ volts but disconnecting them and then measuring each battery I usually find a bad one somewhere in the bank.
I stopped using those store bought UPS's in favor of a stand alone power inverter along with a smart charge control and two independant battery banks using deep cycle marine batteries. Just before switching in a battery bank I do load test, similar to those used for testing car batteries, before switching the bank into service. Each month I repeat the process. That way I find the defective battery before putting it into service.
Of coarse this is overkill for just a sump pump but if you have things you cannot afford to loose power its a good setup to have, plus using a regular power inverter you can get much more runtime than any consumer grade UPS.
