About the Off-Grid Autonomy
Autonomy (days of backup) is how long an off-grid battery bank can power the load through cloudy days with no solar input. It is the usable battery energy divided by the daily load, and it is the key sizing number for any off-grid or backup solar system.
Formula
Autonomy (days) = Usable battery energy (kWh) ÷ Daily load (kWh)
- Usable battery energy
- Bank capacity × depth of discharge × system efficiency (kWh)
- Daily load
- Energy the loads consume per day (kWh)
How to calculate it
- 1Tally the daily load in kWh (each appliance's watts × hours of use).
- 2Compute the bank's usable energy: nameplate capacity × allowable depth of discharge (≈80–90% for lithium, ≈50% for lead-acid) × round-trip/inverter efficiency.
- 3Divide usable energy by daily load for days of autonomy.
- 4For true off-grid, size for 2–3 days of autonomy to ride out cloudy spells.
Worked example
A 10 kWh lithium bank at 90% DoD and 90% efficiency gives ~8.1 kWh usable. For a 3 kWh/day critical load that is about 2.7 days of autonomy with no sun.
What you can calculate
- Autonomy days
- Critical loads
- Monsoon buffer
- Genset backup
Standards & references
Frequently asked questions
How many days of battery backup do I need?
Grid-tied backup systems usually target a few hours to one day of critical-load autonomy. True off-grid systems are sized for 2–3 days so they survive consecutive cloudy days; more than that gets expensive, so a small generator is often paired for rare long spells.
Why can't I use the full battery capacity for backup?
Fully discharging a battery shortens its life, so a usable depth of discharge is applied — around 80–90% for lithium (LFP) and only ~50% for lead-acid. Inverter and round-trip losses further reduce the delivered energy, which is why usable energy is less than the nameplate rating.
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