About the Autonomy / Backup Time
Autonomy (or backup time) is how long a battery energy storage system can supply a given load before the battery is exhausted. For off-grid and backup applications it is the primary sizing metric — IS 16270 sets minimum backup requirements for critical loads in India.
Formula
Backup time (h) = (Usable capacity kWh) ÷ Load (kW)
- Usable capacity
- Nameplate capacity × DoD × efficiency factor (kWh)
- DoD
- Depth of Discharge — fraction of capacity actually used (e.g. 0.80 for 80% DoD)
- Load
- Average power draw during backup (kW) — may vary hour by hour
How to calculate it
- 1Profile the critical load: list all appliances that must run during backup, their power (W), and operating hours.
- 2Sum to get average load in kW and total backup energy needed (kWh).
- 3Choose the battery technology and DoD: LFP at 80–90% DoD; lead-acid at 50% DoD.
- 4Calculate required nameplate capacity: backup energy needed ÷ DoD ÷ round-trip efficiency.
- 5Verify against IS 16270 minimum backup hours for the load category.
Worked example
A critical telecom tower load: 2 kW continuous, 8-hour backup required. Energy needed = 2 × 8 = 16 kWh. LFP at 90% DoD, 95% efficiency: nameplate = 16 ÷ 0.90 ÷ 0.95 = 18.7 kWh → round up to a 20 kWh LFP battery bank.
What you can calculate
- Backup hours
- DoD impact
- Load profile
- IS 16270
Standards & references
Frequently asked questions
How long can a 10 kWh battery power a typical Indian home?
A typical Indian urban home uses 3–5 kW during peak hours and 0.5–1 kW at night (refrigerator, fans, lighting). A 10 kWh LFP battery at 90% DoD provides ~9 kWh of usable energy — about 8–18 hours of night loads or 2–3 hours of peak usage. Pairing with solar ensures the battery recharges each day.
Why is LFP preferred over lead-acid for backup in solar systems?
LFP (lithium iron phosphate) offers 80–90% DoD (vs 50% for lead-acid), 4,000+ cycle life (vs 500–800 for VRLA), no maintenance, faster charging, and better thermal stability. The higher upfront cost is recovered through longer life and lower replacement frequency. For solar residential systems, LFP's 10-year effective life typically beats two or three replacement cycles of lead-acid.
What does IS 16270 specify for BESS backup?
IS 16270 covers battery energy storage systems for solar PV applications in India, specifying safety, performance, and labelling requirements. For critical loads, it aligns with IEC 62619 (safety for secondary lithium cells) and sets minimum requirements for state-of-charge accuracy, thermal protection, and overcharge/over-discharge protection.
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