Why Simple Battery Backup Math Fails and How to Model It Accurately

Standard battery runtime calculations rely on linear division, but real-world electrochemical and power electronics behavior causes those estimates to overstate actual backup time by 30% to 50%. Three key factors drive this gap: Peukert's Law, which describes how higher discharge currents reduce usable battery capacity non-linearly; inverter tare losses, where idle power draw of 15W to 55W persists regardless of connected load; and depth-of-discharge limits, beyond which lead-acid and lithium batteries suffer irreversible degradation. A DEV Community article translates these physical principles into a deterministic TypeScript calculation engine using strictly typed, immutable inputs. The engine models effective capacity using Peukert exponents specific to battery chemistry, ranging from 1.03 for LiFePO4 to 1.30 for flooded lead-acid cells. The approach is particularly relevant for low-load scenarios, such as overnight CPAP or router use, where inverter tare losses can account for more than half of total battery drain.