Capacity vs usable capacity is the first trick. A 10 kWh ESS might be advertised as 10 kWh nominal and 8 kWh usable at 80 percent depth of discharge. That is fine — but some sellers advertise the nominal number and quietly set the BMS to discharge to 90 or 100 percent to claim "more capacity." That works for the first year, then the cells degrade fast.
Cycle life conditions are the second trick. A spec sheet that says "6,000 cycles" without telling you the depth of discharge, the C-rate, and the temperature is hiding something. The honest number is "6,000 cycles at 80 percent DoD, 0.5C, 25 degrees Celsius." Anything less specific is a marketing claim.
BMS features that matter: cell balancing, low-temperature charge protection, and CAN or RS485 comms. If the BMS does not have active balancing, the cells drift apart over time and the bank loses capacity. If it does not block charging below 0 degrees Celsius, the cells will plate lithium and lose capacity in a cold morning.
What does not matter as much: the brand of cells inside. Most Tier 1 Chinese cells (CATL, EVE, CALB) are good. The brand matters less than how the pack is assembled, how the BMS is configured, and how the factory tests the finished unit.
Our rule: ask for a one-page test report from the factory showing the actual measured capacity of the specific batch you are buying, not the catalog number. A good factory will have this ready.