The price is primarily driven by the battery chemistry, with Lithium Iron Phosphate (LiFePO4) being the premium choice due to its 6,000+ cycle life and safety, compared to cheaper Lead-Acid alternatives. For a 100W system (often referring to 100Ah capacity), ensure the Depth of Discharge (DoD) is at least 80-90% for lithium. Additionally, verify the Built-in Battery Management System (BMS) features, which protect against overcharge, short circuits, and thermal runaway, as these directly impact the unit's longevity and cost.