Why do both watts and VA matter?
Every UPS has a real-power limit in watts and an apparent-power limit in VA. The connected equipment must stay within both limits. Power factor and load behaviour determine which limit is reached first.
A rating such as 1,500 VA therefore means neither 1,500 watts nor the amount of energy stored in the battery. Power capacity and runtime are separate parts of the sizing process.
Which load should be used for sizing?
The actual power drawn at the mains input is what matters. A server with a 1,000-watt power supply does not necessarily consume 1,000 watts continuously. For an existing installation, a measurement is usually more reliable than adding all nameplate ratings.
New projects use realistic load profiles. Typical load, possible peaks, startup behaviour and planned expansion should all be included.
How much reserve is sensible?
Reserve should cover specific uncertainties such as load changes, measurement tolerances or expected growth. A fixed percentage is not suitable for every application.
An unnecessarily large UPS may need more space and a larger electrical connection, but it does not automatically provide more runtime. Battery configuration is assessed separately.
How does runtime affect the choice?
A UPS can have enough power capacity and still provide too little backup time. The required response must be clear: bridge a brief interruption, shut systems down safely or keep them running until a standby supply starts.
This target determines the battery size, possible external battery modules, space, weight and recharge time.
What information does ERREPI need?
A list of the connected equipment, measurements or realistic load assumptions, the intended response to a power failure and the required backup time provide a useful starting point. Connection, form factor, installation site, monitoring, bypass and maintenance requirements complete the picture.
Use the runtime simulator for an initial estimate, or start with UPS consulting for a specific selection.