UPS & Battery Backup Run-Time Calculator
Estimate backup run-time hours for an uninterruptible power supply (UPS), inverter battery bank, or portable power station from appliance wattage, battery voltage, and Amp-hour capacity.
Quick Appliance Presets
Enter the total running wattage of all connected devices.
Most standalone UPS units use 12V or 24V banks.
Amp-hour (Ah) rating printed on your battery label.
Modern deep-cycle lithium standard; safe down to 80-90% discharge.
Under continuous 150W load at 90% Depth of Discharge
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.
Variables & Constants
T (Hours)V (Volts (V))Ah (Amp-Hours (Ah))DoD (Decimal (0.50-0.90))η (Decimal (0.80-0.95))P (Watts (W))- 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
Add up the measured or labeled continuous wattage of every device connected to the backup system.
Multiply the nominal DC voltage by the battery capacity to find total raw energy.
Apply the 90% safe discharge limit for LiFePO4 chemistry and the 85% inverter conversion factor.
Divide usable Watt-hours by continuous Watts. 7.06 hours equates to approximately 7 hours and 4 minutes.
Calculation Assumptions & Real-World Variables
Battery runtime depends on discharge rate, chemistry limits, conversion losses, and ambient temperature:
| 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
How many hours will a 100Ah battery run an uninterruptible power supply (UPS)?
What size inverter do I need for my UPS backup?
Why does my lead-acid UPS battery die faster than the calculator says?
Can I replace standard UPS lead-acid batteries with LiFePO4 batteries?
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UPS Battery Runtime Calculation: Discharge Curves & Inverter Efficiency
Uninterruptible power supply runtime depends on connected equipment wattage, internal battery chemistry, DC bus voltage, and inverter conversion efficiency during grid outages.
The Universal UPS Battery Runtime Formula
Battery backup runtime is calculated by dividing total usable energy in watt-hours by connected electrical power draw, adjusted for internal inverter losses:
UPS Runtime Table by Unit Size & Connected Appliance Load
Estimated backup duration across standard line-interactive consumer and professional UPS units under real-world household and office loads:
| UPS Capacity | Max Watts | Internal Battery | WiFi Router (20W) | CPAP Machine (45W) | Office PC (100W) | Gaming PC (350W) |
|---|---|---|---|---|---|---|
| 450 VA | 255 W | 1x 12V 5Ah (60 Wh) | 1 hr 45 min | 40 min | 14 min | Overload |
| 600 VA | 360 W | 1x 12V 7Ah (84 Wh) | 2 hrs 20 min | 1 hr 0 min | 20 min | 4 min |
| 850 VA | 510 W | 1x 12V 9Ah (108 Wh) | 3 hrs 15 min | 1 hr 20 min | 32 min | 7 min |
| 1000 VA | 600 W | 2x 12V 7Ah (168 Wh) | 5 hrs 0 min | 2 hrs 10 min | 50 min | 13 min |
| 1350 VA | 810 W | 2x 12V 7.2Ah (172 Wh) | 5 hrs 30 min | 2 hrs 25 min | 58 min | 16 min |
| 1500 VA | 900 W | 2x 12V 9Ah (216 Wh) | 7 hrs 0 min | 3 hrs 0 min | 1 hr 18 min | 20 min |
| 2000 VA | 1,400 W | 4x 12V 7Ah (336 Wh) | 11 hrs 0 min | 5 hrs 0 min | 2 hrs 15 min | 36 min |
Essential Electrical Factors Influencing UPS Battery Life
Volt-Amps (VA) vs. Real Power (Watts) in UPS Sizing
UPS units display capacity in both volt-amps (apparent power) and watts (real power). Computer equipment with active power factor correction operates near unity power factor (0.95 to 0.99).
Budget UPS units often carry a lower power factor rating between 0.60 and 0.70. A 1,000 VA budget UPS with a 0.60 power factor safely delivers only 600 watts of real power.
Always verify that total connected load wattage never exceeds either the watt rating or the volt-amp rating of the unit.
Peukert Effect on Sealed Lead-Acid Batteries
Standard sealed lead-acid (SLA) batteries are rated at a gentle 20-hour discharge rate. Discharging that same battery rapidly in 15 to 30 minutes reduces effective chemical capacity by 20% to 35%.
Modern LiFePO4 lithium iron phosphate replacement batteries maintain virtually flat discharge curves with zero Peukert losses. They deliver full rated capacity even under rapid heavy loads.
Outage Strategy: Extending Internet & Communications Runtime
During prolonged severe storm outages, desktop computers and monitors draw unnecessary power that drains small batteries within minutes.
Disconnect large monitors and desktop towers to run solely your fiber ONT terminal and WiFi router (15 to 25 watts total).
A modest 850 VA consumer battery backup can sustain critical home internet and emergency smartphone charging for over 3 to 4 hours.