Step 1: A 12 V, 100 Ah lead-acid battery powers a 120 W load. How long does it last (usable 50% depth of discharge)? Step 2: Total stored energy = V × Ah = 12 × 100 = 1200 Wh. Step 3: Usable (lead-acid shouldn't go below ~50%) = 600 Wh. Step 4: Time = 600 Wh/120 W = 5 hours. Step 5: Compare a lithium battery of same size: usable ~80–90% (960–1080 Wh) → 8–9 hours, at ~1/3 the weight. Result: 5 hours usable. Depth-of-discharge limits are why lithium displaced lead-acid in phones, EVs, and solar homes despite higher upfront cost.