How does APC work?

An APC UPS sits between the wall outlet and your equipment. In normal conditions, it passes clean power through and charges its battery. If the power fails, it switches to battery power in milliseconds so devices keep running.

The main parts

• Battery: stores energy, usually a sealed lead-acid or lithium-ion battery.
• Charger or rectifier: converts AC power to DC to charge the battery.
• Inverter: converts battery DC power back to AC for your devices.
• Transfer switch: moves the load between utility power and the battery.
• Voltage regulator (AVR): raises or lowers voltage in small steps without using the battery on line-interactive models.
• Surge protection: blocks spikes.

Step by step

1. Power from the wall flows through the UPS to your devices.
2. The charger keeps the battery full.
3. The UPS monitors voltage all the time.
4. If voltage sags or rises, the AVR corrects it.
5. If power fails or goes out of range, the transfer switch moves the load to the inverter within a few milliseconds.
6. Your devices run from the battery and the UPS beeps.
7. When power returns, the UPS switches back and recharges the battery.

Different APC designs

• Back-UPS: a standby or line-interactive design for home and office.
• Smart-UPS: line-interactive with pure sine wave output.
• Smart-UPS Online: double conversion, which runs the load from the inverter at all times, so there is no transfer.

What to know

Battery runtime depends on your load. A UPS is designed to give you minutes to save and shut down, not to run a home for hours. Software such as PowerChute can shut your computer down automatically.