Off-grid systems live or die on stored energy between sun or generator windows. Estimate battery capacity from nightly load, desired autonomy days, and bus voltage—then stress-test with winter and cloudy-week margins.
Benefits
- Translates daily kWh or Wh load into bank Ah at your inverter bus voltage.
- Supports multi-day autonomy targets (1–3+ days) common in cabin and homestead planning.
- Highlights DoD and chemistry choices that change nameplate vs. usable capacity.
How it works
- Inventory 24 h loads—or worst-case overnight + cloudy-day bundle—in watts.
- Multiply by autonomy days to get total Wh before charging opportunity.
- Convert to Ah at system voltage; apply DoD and efficiency factors before ordering modules.
FAQ
How do I estimate off-grid battery capacity?
Sum energy use for the autonomy period you must survive without input (solar or generator). Example: 2 kWh/day × 2 days = 4 kWh (4,000 Wh). At 48 V that is about 83 Ah delivered; size nameplate higher for DoD and losses.
Should off-grid banks be larger than backup-only sizing?
Usually yes. Off-grid designs target multiple days of autonomy and deeper cycling. Backup calculators may assume hours; off-grid estimators should include cloudy-week buffers and seasonal load spikes (heating, pumps).
How does solar change the estimate?
Solar reduces how often you hit full depth on the bank, but capacity must still cover nights and low-production stretches. Use daily yield tools first, then size Ah for the gap between consumption and average harvest.
Technical specifications
- Energy: Wh_autonomy = daily_load_Wh × autonomy_days.
- Capacity: Ah = Wh_autonomy ÷ voltage_V ÷ allowable_DoD.
- Margins: +10–25% for inverter loss; +seasonal factor for heating loads.
- Companion: solar-battery-bank and battery-bank-size calculators for cross-checks.
Autonomy days vs. overnight hours
Grid-tied backup often sizes for hours until utility returns. Off-grid sites size for days without meaningful solar. State your autonomy target explicitly—one cloudy weekend can double the Wh requirement versus a single-night model.
Lithium vs. lead-acid nameplate
A 400 Ah lithium bank at 80% DoD delivers more usable Wh than 400 Ah flooded lead at 50% DoD. Estimators should output usable Wh first, then map to chemistry-specific nameplate ordering.