Battery Backup Sizer
Bank capacity, Ah & unit count for outage backup
Free · No sign-up · Computed instantly on your device
Backup Requirement
Size a battery bank for the load you want to keep running during an outage.
Assumes 90% combined inverter + round-trip efficiency and the selected chemistry’s usable depth of discharge. Add headroom for battery ageing (~20% over 5 years) and simultaneous surge loads.
Required battery capacity
4kWh
Delivers 3.2 kWh usable at 90% depth of discharge
Bank capacity
82 Ah
at 48 V system
Battery units
1
× 48 V / 100 Ah (4.8 kWh)
Energy delivered
3.2
kWh to the load
About This Calculator
Battery sizing has one honest formula: the energy your loads need, divided by what the chemistry actually lets you use. This calculator takes your backup load in watts and required backup hours, then returns the gross battery capacity in kWh, the bank size in Ah at your chosen system voltage, and how many standard battery units that means.
The chemistry choice matters more than most buyers realise: lead-acid banks should only be discharged ~50% for acceptable life, while LiFePO₄ safely delivers ~90% — so a lithium bank of half the nameplate capacity can back up the same load. The tool models both, with a 90% combined inverter and round-trip efficiency.
How the Calculation Works
Usable energy needed = load (W) × hours of backup. Gross battery size = usable energy ÷ (depth of discharge × efficiency), because you can only use part of a battery’s rated capacity (about 90% for LiFePO₄, about 50% for lead-acid tubular) and about 10% is lost in conversion. Dividing by system voltage gives the bank in amp-hours.
Sizing the backup is only half the question — whether a battery pays back is the other half. For the economics see is solar battery storage worth it in India, and for a real battery datasheet see our Loom Solar battery review.
Worked example (the calculator’s defaults)
- Load 800 W for 4 hours on a 48 V LiFePO₄ bank (90% depth of discharge, 90% efficiency).
- Usable energy: 0.8 kW × 4 h = 3.2 kWh.
- Gross battery size: 3.2 ÷ (0.9 × 0.9) = 3.95 kWh, or about 82 Ah at 48 V — one 4.8 kWh unit.
- The same backup on lead-acid (50% depth of discharge) needs about 7.1 kWh gross.
Go Deeper
Frequently Asked Questions
- Lithium or Lead-Acid for Home Backup in India?
- LiFePO₄ wins on lifetime cost for daily-cycling use: 3–5× the cycle life, 90% usable capacity, no maintenance. Lead-acid (tubular) still makes sense for rarely-used backup where upfront cost dominates.
- How Much Extra Capacity Should I Add for Battery Ageing?
- Plan for ~20% capacity fade over five years of regular cycling. If your backup requirement is inflexible, oversize the bank by that margin on day one.
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