UPS Runtime Calculator

Estimate runtime from load and UPS/battery specs. Includes efficiency, reserve, and derating factors plus Show Work.

How to Use

  1. Enter your load as Watts or VA. If using VA, set Power Factor.
  2. Enter UPS/battery capacity using Battery Voltage × Ah or Total Wh.
  3. Set Efficiency and optional Reserve + Derating factors.
  4. Read the runtime estimate and open Show Work for the math in base units.
Runtime View
Live estimate using usable battery energy and load power.
Load
—
Usable Wh
—
Runtime
—
Est. @ 50%
—
Margin: —
— Higher load = shorter runtime.
Tip: Use Reserve to keep battery above a cutoff (e.g., 10–20%) and Derating for age/temperature.
Inputs & Settings
Enter load + capacity. Results update instantly. Use “Share Link” to generate a URL.
If using VA, set Power Factor below to convert to watts.
Typical: 0.6–0.9. If Load is watts, PF is informational.
Most small UPS units: sealed lead-acid (SLA) packs like 12V 7Ah/9Ah, often 24V (2×12V in series).
Examples: 12V, 24V, 36V, 48V.
Common SLA: 7Ah, 9Ah. External packs may be larger.
If you know battery energy in Wh, enter it directly. (Tool will ignore V/Ah.)
Typical: 80–92% depending on load and UPS design.
Keeps some energy unused (battery cutoff / safety margin).
Accounts for battery age, temperature, high discharge rate, etc.
Used only for margin guidance (not required).
Show Work (step-by-step)
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Work is shown using base units (W, Wh). Battery math uses Wh = V × Ah unless Total Wh is provided.

Reference

  • Load conversion: if you enter VA, watts ≈ W = VA × PF.
  • Battery energy: Wh = V × Ah (nominal; real usable depends on discharge rate and cutoff).
  • Usable energy: usableWh = Wh × (eff%) × (1 − reserve%) × (1 − derate%).
  • Runtime: hours = usableWh / W.

Real runtimes vary by UPS model, battery chemistry, discharge rate, and low-voltage cutoff. This tool is for solid estimates, not exact manufacturer curves.

FAQ

Why does my UPS runtime drop fast at higher loads?

Battery capacity effectively shrinks at high discharge rates and the UPS runs less efficiently. Add derating to match reality.

What efficiency should I use?

A practical starting point is 85%. If you have measured data for your UPS at your typical load, use that.

My UPS is rated in VA, but my load is watts. Which matters?

For runtime, watts matter (real power). VA matters for capacity limits. This tool converts VA using PF if you provide it.

Can I use this for lithium battery packs?

Yes—enter total Wh if known. Lithium often has better usable capacity than SLA at high loads, so derating may be lower.

Tool Info

Last updated:

Updates may include UI improvements, unit support, and edge-case handling.