I have 4 of the the 24v Techdirect/BigBattery pre-assembled wheeled units - I am rating them at 3kwh per unit. They are wired in parallel to an Schneider Conext SW4024 system and three solar arrays totaling 4.5kw. The units have been in operation continuously for about 4 months - and are operating flawlessly. The units were charged/discharged separately - and capacity tested - before installation. I am totally off-grid so dependability is key for me. As to the expected lifespan - difficult to say - but so far I am impressed. Caveat: I am using conservative charging specs - 27.2 bulk, 26.7 float, 26.4 bulk recharge reset, and I have plenty of charging capacity. I routinely run 1kw AC, washer/dryer, refrigerator, microwave, tv and associated household loads. I also have a Chevy Volt which is currently fully charged. Your mileage may vary, of course. Best.