How to Size a UPS Correctly
Q: How do I size a UPS correctly?
A: Sizing depends on four inputs: (1) the accumulated inverter load to be connected; (2) the type of loads (electronics, motors, lighting, etc.); (3) the required backup duration — minutes for graceful shutdown vs. hours to ride through load shedding; (4) load criticality, which determines whether parallel-redundant configurations are needed.
Choose topology to match: Offline Topology for least protection and short 5–30 minute backup; Line-Interactive for moderate protection (typically with AVR); Online Double-Conversion for full protection, zero transfer time, and either short or extended battery backup. Pricing rises with topology.
Headroom rule: allow 30% on top of calculated power draw to absorb inrush during load start-up. To convert from Watts to VA divide by 0.7. Targeting 60–75% load is most efficient — running a UPS at 25–50% load is inefficient and increases running cost.
Form-factor break: UPS units up to 3 kVA are plug-and-play; 4 kVA and above require an electrician to hard-wire the unit into the building supply.
Connections
- Offline Topology — referenced, source: https://standbysystems.co.za/ups-batteries-questions-and-answers/
- Line-Interactive — referenced, source: https://standbysystems.co.za/ups-batteries-questions-and-answers/
- Online Double-Conversion — referenced, source: https://standbysystems.co.za/ups-batteries-questions-and-answers/
- Parallel Redundancy — referenced, source: https://standbysystems.co.za/ups-batteries-questions-and-answers/