Battery Backup Calculator

Why Battery Backup for AC Takes More Capacity Than You Expect

A simple calculation might suggest that running a 1,000 W AC for 4 hours needs 4 kWh of battery. In practice you always need more, for three reasons: the depth of discharge limit means you cannot use all the energy stored; the inverter wastes some energy in conversion; and battery internal resistance under load also takes a small share. Together these factors mean the bank must hold 20 to 80% more than the bare load calculation, depending on battery type and inverter quality.

How to Use This Calculator

1. Enter the AC running wattage from the energy label and any other simultaneous loads.
2. Set the target backup duration in hours.
3. Choose the battery type. LiFePO4 lithium allows up to 80% discharge; lead-acid only 50%, meaning you need nearly twice the capacity for the same runtime.
4. Select the system voltage. 48 V is strongly recommended for AC loads; 12 V at high currents wastes energy and requires very thick cables.
5. Enter the individual battery or module size in Ah. The tool calculates how many units are needed.

Battery Types Compared

LiFePO4 is the preferred chemistry for residential AC backup due to safety, cycle life, and high discharge current tolerance.

Type	Usable DOD	Cycle life	Best for AC backup?
Lead-acid (FLA / AGM)	50%	300 to 500 cycles	Low cost but heavy, needs twice the capacity
LiFePO4 (lithium iron)	80%	2,000 to 6,000 cycles	Best choice for AC backup
NMC lithium (phone / laptop type)	80 to 90%	500 to 1,000 cycles	Not recommended for high-current AC loads

System Voltage and Cable Size

Higher voltage halves the current for the same power, halving cable losses and allowing thinner wiring.

Voltage	Current for 1,000 W load	Cable size needed	Verdict
12 V	~83 A	Very large (35 to 50 mm²)	Impractical for AC loads
24 V	~42 A	Large (16 to 25 mm²)	Workable for small systems
48 V	~21 A	Manageable (6 to 10 mm²)	Recommended for AC backup

Worked Example: 4 Hours of AC Backup

A 1,000 W inverter AC plus 200 W of fans and lights for 4 hours. LiFePO4 batteries at 48 V, 100 Ah each, 90% efficient inverter.

• Total load: 1,200 W.
• Energy needed: 1.2 kW times 4 hours equals 4.8 kWh usable.
• Bank capacity: 4.8 divided by (0.80 DOD times 0.90 eff) equals about 6.7 kWh total.
• Ah at 48 V: 6,700 divided by 48 equals about 140 Ah.
• Number of 100 Ah batteries: ceiling of 140 divided by 100 equals 2 batteries.
• Actual runtime with 2 batteries (200 Ah, 9.6 kWh gross): 9.6 times 0.72 divided by 1.2 equals about 5.8 hours, exceeding the 4 hour target.

Common Battery Backup Mistakes

• Using 100% of battery capacity. Discharging lead-acid beyond 50% or lithium beyond 80 to 90% sharply reduces cycle life.
• Using a 12 V system for AC loads. The resulting currents are very large and require extremely thick, expensive cable with significant losses.
• Forgetting inverter losses. An 85% efficient inverter wastes 15% as heat; the battery must supply the extra.
• Confusing Ah and kWh. Ah depends on voltage; kWh does not. A 100 Ah battery at 12 V holds 1.2 kWh; at 48 V it holds 4.8 kWh.

Frequently Asked Questions

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