How many hours will a 100Ah battery run an uninterruptible power supply (UPS)?

A 12V 100Ah LiFePO4 battery providing 918 usable Watt-hours will run a 100W load for approximately 9.1 hours. A standard lead-acid 100Ah battery with a 50% depth of discharge will run that same 100W load for approximately 5.1 hours.

What size inverter do I need for my UPS backup?

Under National Electrical Code (NEC) continuous duty guidelines, size your inverter at least 25% larger than your total continuous wattage load (Continuous Load × 1.25). For example, a 400W load requires at least a 500W rated continuous inverter.

Why does my lead-acid UPS battery die faster than the calculator says?

Lead-acid batteries suffer from Peukert's effect: the faster you discharge them (high wattage loads), the lower their effective capacity becomes. If you pull a heavy load in under 2 hours, effective capacity can drop by up to 30%.

Can I replace standard UPS lead-acid batteries with LiFePO4 batteries?

In many consumer UPS units, drop-in replacement 12V LiFePO4 batteries work well if the built-in charging voltage profile is compatible (typically 13.8V float). Verify that the battery's Battery Management System (BMS) can support the maximum discharge current.

UPS & Battery•Uninterruptible Power Supply Hours•Updated September 2026

UPS & Battery Backup Run-Time Calculator

Calculate exact backup run-time hours for any uninterruptible power supply (UPS), inverter battery bank, or portable power station based on appliance wattage, battery voltage, and Amp-hour capacity.

Input Parameters

Quick Appliance Presets

Total Appliance Power / Load *Watts (W)

Enter the total running wattage of all connected devices.

Battery Bank Voltage

Most standalone UPS units use 12V or 24V banks.

Battery Capacity *Amp-Hours (Ah)

Amp-hour (Ah) rating printed on your battery label.

Battery Chemistry & Type

Modern deep-cycle lithium standard; safe down to 80–90% discharge.

Estimated Backup Run-Time

6 hr 7 min

Under continuous 150W load at 90% Depth of Discharge

Usable AC Energy

918Wh

0.92 kWh delivered

Total Stored Capacity

1200Wh

At 12V nominal

DC Current Draw

14.7Amps (A)

From battery terminals

Min. Recommended Inverter

200Watts

250 VA continuous

Calculated using 85% inverter efficiency.

Compatible Hardware Reference

Amazon Affiliate

Hardware examples matching your 12V setup and 150W draw:

12V 100Ah Deep Cycle LiFePO4 Batteries

High cycle life (4000+ cycles) with built-in BMS

Pure Sine Wave Inverters (200W+)

Safe clean power for electronics & CPAP machines

Calculation Methodology & Formula

The run-time of an uninterruptible power supply or battery backup depends directly on the total chemical energy stored in the battery bank, the safe discharge limit of the battery chemistry, the DC-to-AC electrical conversion efficiency of the inverter, and the total continuous power draw of the connected devices.

Formula

T (Hours) = [ V (Volts) × Ah (Amp-Hours) × DoD (%) × η (%) ] / P (Watts)

Variables & Constants

Run-Time DurationT (Hours)

The estimated period the battery system can sustain continuous AC output before reaching low-voltage cutoff.

Nominal Battery VoltageV (Volts (V))

Operating DC voltage of the battery pack (typically 12V, 24V, or 48V).

Battery CapacityAh (Amp-Hours (Ah))

The volume of electric charge the battery can deliver over its nominal discharge rating.

Depth of DischargeDoD (Decimal (0.50–0.90))

Safe discharge limit. 50% for standard Lead-Acid/AGM, up to 90% for Lithium Iron Phosphate (LiFePO4).

Inverter Efficiencyη (Decimal (0.80–0.95))

Conversion losses when transforming DC battery current into 120V/240V AC power. 85% is typical.

Connected LoadP (Watts (W))

Sum of continuous real electrical power drawn by all connected equipment.

Peukert's Law: For lead-acid batteries subjected to heavy loads (>0.2C), effective capacity decreases non-linearly.
Quiescent Inverter Draw: Even with no load connected, inverters typically consume 5W to 25W just remaining active.

Step-by-Step Worked Example

Example Scenario: A home office user needs to keep a laptop (65W), dual monitors (50W), and a fiber Wi-Fi router (15W) running during a blackout using a 12V 100Ah LiFePO4 battery bank and an 85% efficient pure sine wave inverter.

Calculate Total Load in Watts

P = 65W + 50W + 15W = 130 Watts

Add up the measured or labeled continuous wattage of every device connected to the backup system.

Calculate Total Stored Energy

E_total = 12V × 100Ah = 1,200 Watt-hours (Wh)

Multiply the nominal DC voltage by the battery capacity to find total raw energy.

Calculate Usable AC Energy Delivered

E_usable = 1,200 Wh × 0.90 (DoD) × 0.85 (Efficiency) = 918 Wh

Apply the 90% safe discharge limit for LiFePO4 chemistry and the 85% inverter conversion factor.

Determine Run-Time Hours

Runtime = 918 Wh / 130 Watts = 7.06 Hours

Divide usable Watt-hours by continuous Watts. 7.06 hours equates to approximately 7 hours and 4 minutes.

Result: The 12V 100Ah LiFePO4 system will power the 130W home office workstation continuously for approximately 7 hours and 4 minutes.

Calculation Assumptions & Real-World Variables

Every battery backup system operates in dynamic physical environments. This calculator uses standard industry engineering baselines as detailed below.

Parameter	Model Default	Typical Field Range	Impact on Runtime
Lead-Acid Safe DoD	50%	40% – 50%	Discharging lead-acid past 50% causes plate sulfation and permanently shortens lifespan.
LiFePO4 Safe DoD	90%	80% – 95%	Modern lithium chemistry sustains deep discharges without rapid cycle degradation.
Inverter Conversion Efficiency	85%	80% – 92%	Lower efficiency inverters lose energy as heat, reducing available operational minutes.
Ambient Temperature	77°F (25°C)	32°F – 104°F	Freezing temperatures reduce effective chemical capacity of lead-acid by 20–40%.

Electrical Safety & Engineering Disclaimer

This calculator provides theoretical run-time estimates based on constant nominal power consumption and manufacturer standard capacity ratings.
Actual runtime will vary based on battery state-of-health (SoH), ambient operating temperature, surge/startup inductive loads (compressors, motors), and inverter quiescent idle current.
High DC current draws create substantial fire risks if undersized wire gauge or incorrect fuse ratings are installed. Always consult the National Electrical Code (NEC Article 480 / 706) and local regulations.
For critical life-support, medical equipment, or high-availability data infrastructure, consult a licensed electrical engineer or certified installer before relying on backup sizing.

Frequently Asked Questions

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