Battery Backup for AC Calculator

Battery Backup Planning

Plan your battery backup system for uninterrupted AC operation during power outages. Proper battery sizing ensures you get the backup duration you need without damaging the batteries.

Battery Sizing Formula

Calculate the battery capacity needed:

• Basic formula: Ah = (Watts × Hours) ÷ (Voltage × DoD × Efficiency)
• DoD: Depth of Discharge (50% for lead-acid, 80% for lithium)
• Efficiency: 80-85% for inverter losses

Example Calculation

For 1.5 ton AC (1,500W) running 3 hours:

• Energy needed: 1,500W × 3h = 4,500 Wh
• With efficiency (80%): 4,500 ÷ 0.8 = 5,625 Wh
• At 24V, 50% DoD: 5,625 ÷ 24 ÷ 0.5 = 469 Ah
• Practical: 4 × 200Ah batteries in 24V configuration

Battery Types Compared

• Lead-acid (flooded): Cheapest, needs maintenance, 50% DoD, 3-5 year life
• Lead-acid (sealed/AGM): Maintenance-free, 50% DoD, 4-6 year life
• Lithium-ion (LiFePO4): 80% DoD, 10+ year life, 3x cost, best for AC
• Tubular batteries: Better for deep discharge, 5-7 year life

Battery Requirements by AC Size

Approximate battery bank for 2-hour backup (24V system):

• 0.75 ton: 200-250 Ah (2 × 150Ah)
• 1.0 ton: 300-350 Ah (2 × 200Ah)
• 1.5 ton: 400-500 Ah (4 × 150Ah)
• 2.0 ton: 550-650 Ah (4 × 200Ah)

Important Considerations

• Inverter AC uses less: Calculate 20-30% lower for inverter models
• Temperature matters: Cold reduces battery capacity
• Recharge time: Need adequate charger capacity
• Cycle life: Frequent deep discharge shortens battery life
• Ventilation: Lead-acid batteries emit hydrogen gas

Cost Analysis

Typical costs for 2-hour backup of 1.5 ton AC:

• Lead-acid: $400-600 for battery bank
• Lithium-ion: $1,500-2,500 for battery bank
• Cost per outage hour: Lead-acid cheaper initially, lithium cheaper long-term