VA and W rate capacity; Wh measures energy
UPS VA is apparent-power capacity; W is the real-power output limit. Neither rating may be below the connected load. Battery V×Ah provides a starting energy estimate, but a larger VA rating does not automatically mean longer runtime.
Pre-purchase sequence
- Connect only essential devices; measure actual W or add maximum draws.
- Divide load W by power factor to estimate required VA.
- Add rating headroom for growth and peaks. Eaton recommends at least 15% above total requirement.
- Set a runtime target and check the model’s official load/runtime curve.
- Check whether the battery is replaceable and what replacement costs.
Why a simple estimate may be too long
Lead-acid Ah ratings are measured under specific discharge conditions. High load, cold, battery age, UPS cutoff voltage and inverter losses reduce usable energy. The estimator exposes efficiency and usable battery factors but does not reproduce a product discharge curve.
Decision rule: If runtime matters operationally, choose from the model’s official runtime curve at your load—not from this simple estimate alone.
Sources
- Eaton, UPS sizing guide — capacity versus runtime and at least 15% growth allowance.
- Schneider Electric/APC, UPS buying guide — VA, W, power factor and the load/runtime relationship.