How is battery backup runtime calculated accurately?

Basic runtime is calculated as: Runtime (Hours) = (Battery Ah × Voltage × DoD × Inverter Efficiency) / Load in Watts. When Peukert's Law is enabled, the calculator further accounts for the internal resistance and chemical diffusion limits of lead-acid batteries, which reduce available capacity under high discharge rates.

What is Depth of Discharge (DoD) and why does it differ across chemistries?

Depth of Discharge (DoD) represents the fraction of total stored energy that can be safely withdrawn without accelerating cell degradation. Flooded Tubular Lead-Acid batteries should not be discharged beyond 50% DoD to preserve cycle life, whereas Lithium Iron Phosphate (LiFePO4) can easily tolerate 90% to 95% DoD daily.

What is Peukert's Law and how does it affect battery backup time?

Peukert’s Law (t = H × (C / (I × H))^k) models how the effective capacity of a battery decreases when discharged at higher currents. Lead-acid batteries have a high Peukert exponent (k ≈ 1.25), meaning a heavy 1,000W load will deplete the battery much faster than a 100W load. Lithium batteries have k ≈ 1.02, making their capacity virtually independent of discharge current.

How do I choose between a 12V, 24V, and 48V battery bank?

For loads under 800W, a 12V system (1 battery) is cost-effective. For loads between 800W and 2,000W, a 24V system (2 batteries in series) halves the DC discharge current, reducing wire resistive heat losses. For heavy loads (>2,000W or solar installations), a 48V DC bus (4 batteries in series) is industry standard.

Why does ambient temperature impact battery capacity?

Electrochemical reactions slow down at lower temperatures. At 0°C (32°F), traditional lead-acid batteries deliver only ~70% of their rated 25°C capacity. LiFePO4 and Sodium-Ion batteries maintain significantly higher cold-weather efficiency, though lithium batteries must not be charged below freezing without low-temp BMS heating.

Is LiFePO4 cheaper than Tubular Lead-Acid over a 10-year lifespan?

Yes. While LiFePO4 has a higher initial upfront price, its 5,000+ cycle life lasts over 10 to 15 years without replacement. A lead-acid bank must be replaced 2 to 3 times over that same period and requires periodic distilled water maintenance, making LiFePO4 approximately 30% to 40% cheaper on a 10-year Levelized Cost of Storage (LCOS) basis.

Battery Backup Calculator

Free online Battery Backup & Inverter Sizing Calculator. Calculate backup runtime hours, required Ah capacity with Peukert's law, inverter efficiency, solar recharging, and 10-year Lead-Acid vs LiFePO4 TCO.

⚡ Interactive Calculation Engine Active

Please enable JavaScript in your browser to run live computations, 2D envelope diagrams, and export PDF reports.

How to Use Battery Backup Calculator

Choose your preferred mode: Runtime Estimator (given battery & load) or Target Ah Sizer (given required backup hours).
Input your continuous running electrical load in Watts or aggregate multiple appliances in the Load Builder tab.
Select your battery chemistry (Tubular Lead-Acid, AGM, GEL, Lithium-Ion NMC, LiFePO4, or Sodium-Ion) to auto-apply realistic Depth of Discharge (DoD) and Peukert constants.
Configure the DC Bus Voltage (12V, 24V, 48V) and inverter/wiring loss efficiency percentages.
Inspect the Charging & Solar Sizing tab for mains charger amps and rooftop solar PV panel recommendations, or compare 10-Year Total Cost of Ownership.

Battery Chemistry Specifications & Cycle Life Profiles

Selecting the correct battery chemistry depends on backup frequency and budget. Tall Tubular lead-acid batteries offer reliable, low-cost storage for regions with occasional grid cuts. In contrast, Lithium Iron Phosphate (LiFePO4) provides maintenance-free operation, fast 2-hour charging, zero acid fumes, high thermal stability, and 5,000+ deep discharge cycles.

Inverter VA Sizing vs True Wattage & Power Factor

Apparent Power (VA) equals Real Power (Watts) divided by the Power Factor (PF ≈ 0.8 to 0.85 for inductive loads like ceiling fans and refrigerator motors). When sizing an inverter, ensure the continuous VA rating accommodates inductive reactive power and transient starting surges.

Frequently Asked Questions

How is battery backup runtime calculated accurately?: Basic runtime is calculated as: Runtime (Hours) = (Battery Ah × Voltage × DoD × Inverter Efficiency) / Load in Watts. When Peukert's Law is enabled, the calculator further accounts for the internal resistance and chemical diffusion limits of lead-acid batteries, which reduce available capacity under high discharge rates.
What is Depth of Discharge (DoD) and why does it differ across chemistries?: Depth of Discharge (DoD) represents the fraction of total stored energy that can be safely withdrawn without accelerating cell degradation. Flooded Tubular Lead-Acid batteries should not be discharged beyond 50% DoD to preserve cycle life, whereas Lithium Iron Phosphate (LiFePO4) can easily tolerate 90% to 95% DoD daily.
What is Peukert's Law and how does it affect battery backup time?: Peukert’s Law (t = H × (C / (I × H))^k) models how the effective capacity of a battery decreases when discharged at higher currents. Lead-acid batteries have a high Peukert exponent (k ≈ 1.25), meaning a heavy 1,000W load will deplete the battery much faster than a 100W load. Lithium batteries have k ≈ 1.02, making their capacity virtually independent of discharge current.
How do I choose between a 12V, 24V, and 48V battery bank?: For loads under 800W, a 12V system (1 battery) is cost-effective. For loads between 800W and 2,000W, a 24V system (2 batteries in series) halves the DC discharge current, reducing wire resistive heat losses. For heavy loads (>2,000W or solar installations), a 48V DC bus (4 batteries in series) is industry standard.
Why does ambient temperature impact battery capacity?: Electrochemical reactions slow down at lower temperatures. At 0°C (32°F), traditional lead-acid batteries deliver only ~70% of their rated 25°C capacity. LiFePO4 and Sodium-Ion batteries maintain significantly higher cold-weather efficiency, though lithium batteries must not be charged below freezing without low-temp BMS heating.
Is LiFePO4 cheaper than Tubular Lead-Acid over a 10-year lifespan?: Yes. While LiFePO4 has a higher initial upfront price, its 5,000+ cycle life lasts over 10 to 15 years without replacement. A lead-acid bank must be replaced 2 to 3 times over that same period and requires periodic distilled water maintenance, making LiFePO4 approximately 30% to 40% cheaper on a 10-year Levelized Cost of Storage (LCOS) basis.