What Is a UPS?
A UPS (Uninterruptible Power Supply) provides battery backup power when the main electrical supply fails. It protects servers, network equipment, medical devices, and other critical systems from data loss or hardware damage caused by sudden power outages. Most home and office UPS units provide enough runtime to safely shut down equipment or ride out brief outages.
Battery Runtime Formula
Runtime (min) = (Battery Wh × Efficiency%) ÷ Load W × 60
Example: 100 Wh battery, 50 W load, 90% efficiency → (100 × 0.9) ÷ 50 × 60 = 108 minutes. Inverter efficiency is typically 85–95% for consumer UPS units.
Converting V × Ah to Wh
If your battery is labeled in volts (V) and amp-hours (Ah) rather than watt-hours: Wh = V × Ah. A 12V 9Ah battery has 12 × 9 = 108 Wh of capacity.
UPS Sizing Guide
- Total connected load should be 80% or less of the UPS's rated watt output.
- Example: PC (200W) + monitor (30W) = 230W total → choose at least a 300VA/180W UPS.
- UPS batteries typically last 3–5 years and need periodic replacement testing.
- Online (double-conversion) UPS provides the cleanest power but costs more than standby UPS.
Frequently Asked Questions
VA is apparent power; W is real power. Actual watts = VA × power factor (0.6–0.9 for most UPS). Make sure connected device wattage does not exceed the UPS's rated watt output.
Runtime (minutes) = (Battery Wh × efficiency) ÷ Load W × 60. Example: 100 Wh, 50 W, 90% efficiency → 108 minutes.
Add up the wattage of all devices to protect, then choose a UPS where that total is 80% or less of the rated watt capacity. This leaves headroom and extends battery life.