Off-Grid Solar Battery Sizing & Autonomy Days (Holdover)
Core Question: How do you size a solar battery bank to maintain continuous power through 3 to 5 consecutive sunless or stormy days?
Executive Technical Summary
Autonomy days (holdover) define how long a standalone solar system can run exclusively on stored electrochemical energy without any solar input. Sizing requires combining daily load watt-hours with Peukert effect and temperature derating buffers.
Detailed First-Principles Explanation
For a remote dwelling using 2,000 Wh daily on a 24V bus with 4 days of required winter holdover, total stored energy needed is 8,000 Wh. Divided by 24V equals 333 Ah. If utilizing lead-carbon batteries limited to 50% max depth-of-discharge, the bank must be sized to at least 666 Ah (333 / 0.50). For LiFePO4 operating at 80% DOD, 416 Ah is sufficient.
Assuming a sunny 100 Ah battery delivers 100 Ah at high current; discharging a lead-acid battery in 2 hours instead of 20 hours reduces usable capacity by 25% due to the Peukert effect.