Battery Backup Hours Calculator 2026

Backup capacity

Backup at Your Load

15.6 hrs

@ 500W continuous

Usable Energy

7,776 Wh

7.78 kWh

Bank Nominal

9.6 kWh

200 Ah @ 48V

If running only these appliances…

4 LED bulbs + 2 ceiling fans

186 W

41.8 hrs

1 refrigerator + lights

250 W

31.1 hrs

1 LED TV + WiFi

80 W

97.2 hrs

1.5 Ton 5-star AC (inverter)

1250 W

6.2 hrs

Home office (laptop + monitor + router)

150 W

51.8 hrs

1 HP water pump

750 W

10.4 hrs

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How to calculate battery backup hours

Step-by-step guide to computing battery backup from Ah, voltage, DoD, and load.

Enter your battery bank specs

Ah rating and voltage from the battery datasheet or inverter nameplate.

Pick chemistry

Lithium for 90% DoD, tubular for 60%, lead-acid for 50%.

Enter running load

Sum of wattage of appliances you plan to run during outage — not all connected loads.

Read backup hours

Calculator outputs hours for your exact load plus 6 reference appliance scenarios.

Adjust load if needed

If backup is too short, consider running fewer appliances (e.g. turn off AC during outage).

Battery backup math is deceptively simple

Every battery spec sheet quotes an Ah rating at nominal voltage — but the usable energy is much less. After factoring in depth of discharge, inverter efficiency, and cable losses, a 150 Ah lithium battery at 48V delivers about 5.8 kWh (not 7.2 kWh). Our calculator runs this math correctly for you, including reference scenarios like “fridge + fan + lights” and “1.5 Ton AC alone.”

If the backup hours look short, size up the battery with our Battery Size Calculator, or measure and trim your real load using the Appliance Load Calculator.

Frequently Asked Questions

How do I calculate battery backup hours?

Backup hours = (Battery Ah × Voltage × DoD × inverter efficiency) ÷ Load Watts. For a 200 Ah × 48V LiFePO₄ bank running a 500W load: (200 × 48 × 0.9 × 0.9) ÷ 500 = 15.5 hours.

What is depth of discharge (DoD)?

The usable percentage of a battery's nominal capacity. Lithium = 90%, lead-acid = 50%, tubular = 60%. A 200 Ah lead-acid actually delivers only 100 Ah of usable energy; 200 Ah lithium delivers 180 Ah.

How much backup do I actually need?

Urban India with 1-2 hr outages: 4-6 hrs covers 95% of cases. Semi-urban with 4-6 hr outages: 8-12 hrs. Rural with 8+ hr outages: 16-24 hrs. Going beyond 24 hrs means multi-day autonomy — usually wasteful for grid-connected homes.

Can I run AC on battery backup?

Yes, but AC eats a lot of battery. A 1.5 Ton inverter AC (1250W) runs for 7-8 hours on a 200 Ah × 48V lithium bank (assuming light + fan + fridge are off). Running AC + other appliances simultaneously cuts backup to 4-5 hours.

Do backup hours decrease as battery ages?

Yes. Lithium: 80% capacity after 6-8 years. Lead-acid: 80% after 2-3 years. Always size the bank with 20-30% extra capacity to maintain useful backup in later years of life.

Why do my actual backup hours seem less than calculated?

Common causes: (1) inverter efficiency drops at low load (use 80% not 90% for very light loads), (2) cold weather (lithium loses 15% capacity below 5°C), (3) old/mismatched batteries, (4) continuous ghost loads (router, TV standby) that you didn't count.

What's the difference between running load and peak load?

Running load is the steady-state wattage (AC compressor running). Peak load is the instantaneous surge (AC compressor starting = 3× running wattage). Inverters must handle peak; battery backup is calculated on running load.

Should I add more battery or more solar panels?

For grid-tied homes: more panels (net metering pays back excess). For off-grid: more battery (to bridge cloudy days). For hybrid: typically the bank is sized for 4-8 hours of critical load, not daily autonomy.

Battery Backup Hours Calculator

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