UPS Runtime Calculator: Battery Backup Time and VA Sizing

Estimate blackout backup duration in minutes for connected hardware (PC, NAS, router) and compute recommended VA capacity.

UPS battery runtime: formula, VA sizing and example

UPS runtime is battery capacity (Wh) × efficiency ÷ connected load (W) × 60, in minutes. With 216 Wh and 223 W of IT equipment, the tool returns about 49 minutes. The usable output of the UPS is its VA rating × 0.65.

Formulas used

  • Maximum UPS output: max W = VA × 0.65 (fixed power factor, typical of consumer units).
  • Load = connected watts ÷ max W.
  • Runtime (min) = Wh × efficiency ÷ W × 60, with 85% efficiency, reduced to 76.5% (× 0.90) above 50% load and to 69.7% (× 0.82) above 75% load to reflect losses at high discharge rates.
  • Recommended UPS: VA ≥ W × 1.25 ÷ 0.65 (25% margin).

Example with the default values

An office PC (90 W), two 27-inch monitors (2 × 35 W), a fibre gateway (18 W) and a NAS (45 W) draw 223 W. On a 1,500 VA UPS (975 usable watts), the load is 22.9%, so efficiency stays at 85%. With a 216 Wh battery, runtime is 216 × 0.85 ÷ 223 × 60 ≈ 49.4 minutes. The smallest recommended unit is 429 VA. The same equipment on a 650 VA model (84 Wh) reaches 52.8% load and drops to 17.3 minutes.

Limits

  • The power factor (0.65) and efficiency (85%) cannot be changed: if your UPS datasheet gives a watt rating, compare your load with that figure directly.
  • Device wattages are averages; a PC under full load or a brighter monitor draws more.
  • Battery ageing, temperature and the real cut-off voltage are not modelled; a 3- or 4-year-old battery may have lost a significant share of its capacity.

For a home battery system with solar panels, use the solar, inverter and battery sizing tool instead, which works in kWh and days of autonomy.

Frequently Asked Questions

How long will a 1500 VA UPS last?

It depends on the battery and the load, not on the VA rating alone. With 216 Wh and 223 W connected, the tool estimates about 49 minutes; at 450 W, runtime falls below 25 minutes.

What is the difference between VA and watts on a UPS?

VA is apparent power, watts are the power actually available. The tool uses a 0.65 power factor, so a 1,500 VA UPS delivers at most 975 W.

How do I choose the right UPS size?

Add up the watts of the devices to protect, add a 25% margin and divide by 0.65. For 223 W, that means at least 429 VA.

Why does runtime drop faster when the UPS is heavily loaded?

At high discharge rates the battery and inverter lose more energy as heat. The tool cuts efficiency by 10% above 50% load and by 18% above 75% load.

Is this tool really free?

Yes, all tools on Kristools are 100% free and accessible without registration.

Is my data protected?

Yes. All processing is performed directly and securely inside your browser. No personal data is sent to our servers.