Battery Size for Common Backup Scenarios
| Backup Scenario | Load | 4-hr Lithium | 4-hr Lead-Acid |
|---|
| Essentials (fans, lights, Wi-Fi) | ~400 W | 2.2 kWh | 3.8 kWh |
| Essentials + fridge + TV | ~800 W | 4.4 kWh | 7.5 kWh |
| Above + 1 ton AC | ~1,700 W | 9.4 kWh | 16 kWh |
| Whole home with 1.5 ton AC | ~2,500 W | 13.8 kWh | 23.5 kWh |
Running ACs on battery is expensive: every hour of a 1.5 ton AC consumes as much stored energy as an entire
evening of essentials. Most homes back up essentials only and leave heavy loads on grid or daytime solar.
Lithium vs Lead-Acid for Solar Storage
- Usable capacity: lithium (LiFePO4) delivers about 85% of its rating; lead-acid only about
50% before lifespan suffers. That is why the lead-acid bank above is nearly double the size.
- Cycle life: lithium lasts 3,000 to 6,000 cycles (10+ years of daily use); lead-acid
typically 800 to 1,500 cycles (3 to 5 years).
- Cost: lithium costs about ₹18,000/kWh installed versus ₹9,000/kWh for lead-acid, but per
usable kWh over its life, lithium is now cheaper for daily-cycling solar use.
- Charging: lithium accepts fast charging from solar; lead-acid needs a slower, staged
charge. Check how long your panels take to refill the bank with the
battery charging time calculator.
Decide What Actually Needs Backup Before You Size
Battery budgets are made or destroyed by one decision: the backup list. Walk the house during an imagined
evening power cut and sort loads into three buckets:
- Must run: fans, lights, Wi-Fi, phone charging, the fridge. Typically 600 to 900W. This
list alone keeps battery costs sane.
- Nice to run: TV, a computer, the water pump for one fill. Add 300 to 900W and size the
hours honestly; a pump runs minutes, not hours.
- Luxuries that break budgets: ACs and geysers. Each AC hour drains 1.2 to 1.5 kWh; backing
one up for 4 hours nearly triples the bank. Most homes leave heat and cooling off the backup bus.
Then pick hours from your outage reality: urban feeders with 1 to 2 hour cuts need 3 to 4 hours of autonomy;
rural evening-outage areas need 6+. Build the wattage list precisely in the
solar load calculator first if you have not.
Worked Example: 6-Hour Backup in Gurugram
A Gurugram home wants fans, lights, fridge, TV, Wi-Fi and one computer (about 1,100W) protected for 6-hour
evening cuts. Energy needed: 6.6 kWh. After inverter losses and safe depth of discharge, that is
9.1 kWh of lithium (about 190 Ah at 48V) or a hefty 15.5 kWh of lead-acid. Indicative cost:
roughly ₹1.65 lakh lithium versus ₹1.4 lakh lead-acid, and the lithium bank will outlive two lead-acid
replacements, which is the real comparison. Halving the backup window to 3 hours halves every number above:
hours are the cheapest thing to negotiate with yourself.
After Sizing: Buying the Battery Right
- Read warranties in cycles, not years. "5 years or 3,000 cycles at 80% DoD" tells you the
real service life; a year figure alone hides the cycling assumption.
- Match bank voltage to the inverter. 24V suits small backup; 48V is the modern default
above 2 kWh because it halves currents and cable sizes (see the
wire size calculator).
- LiFePO4 is the lithium to buy. For homes, its thermal stability and 3,000 to 6,000 cycle
life make it the standard; other lithium chemistries belong in vehicles.
- Plan the recharge. A bank your panels cannot refill by evening protects you exactly once;
check the loop with the charging time calculator.
Battery Sizing Mistakes
- Backing up the whole house "to be safe". Whole-home backup triples cost for loads nobody
misses during a two-hour cut; the must-run list is the honest baseline.
- Sizing lead-acid like lithium. Lead-acid delivers only half its label safely; a 150Ah
tubular is not 150Ah of usable anything.
- Ignoring inverter losses. The 15% conversion loss compounds daily; this calculator
includes it, but hand-sized banks usually do not.
- Oversizing hours for a rare storm. Designing for the once-a-year 12-hour outage doubles
the bank you cycle daily; a small generator or graceful load-shedding covers the black swan cheaper.
How This Battery Calculator Works
Energy needed = Backup load (W) × Hours ÷ 1000
Battery kWh = Energy ÷ 0.85 inverter efficiency ÷ DoD (0.85 lithium / 0.50 lead-acid)
Capacity (Ah) = Battery kWh × 1000 ÷ Bank voltage
Assumptions
- Inverter efficiency: 85% from battery terminals to appliances
- Depth of discharge: 85% lithium (LiFePO4), 50% lead-acid (tubular)
- Costs: ~₹18,000/kWh lithium, ~₹9,000/kWh lead-acid, installed, 2026 indicative
Pair the battery with the right electronics using the
solar inverter size calculator.
