Competency 8's method: follow the energy. No power at night but fine by day = battery problem (sulfated, undersized, or loads too big). Slow charging = dirty panels, shade creep (that mango tree grew), loose MC4, or a PWM controller pulling the string below its power point. Controller error codes are your friend — photograph them before resetting. And measure before replacing: half of 'dead inverters' are a blown battery fuse.