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    Sizing a UPS

    3 min read

    If a supplier quotes you a UPS after one phone call, they're guessing. Here are the four numbers that need to be on the table first.

    1. The actual load — measured, not estimated

    Nameplate ratings on IT equipment lie by 30-50%. A rack labelled 5 kW may draw 2 kW at typical utilisation. Clamp meters on the existing circuits, or a power quality logger for a week, get you the number that matters. If the equipment isn't installed yet, size on measured similar systems, not on nameplates.

    2. Power factor

    UPS ratings are given in kVA. Your real load is in kW. The ratio between them is power factor. Modern IT loads run at 0.9-0.99 leading — old servers and inductive loads run lower. A 100 kVA UPS at 0.8 PF gives you 80 kW; at 1.0 PF it gives you 100 kW. Assume the wrong PF and you've sized 20% wrong.

    3. Autonomy — how long, at what load

    '15 minutes at full load' is not the same as '15 minutes at half load' — battery discharge curves are non-linear. Decide the autonomy at the load you'll actually run at, at the temperature the battery room will actually reach in August. Then check the runtime table, not the marketing sheet.

    4. Growth — the load in year five

    A UPS is a 10-15 year asset. What does the load look like in five years? Modular UPS lets you grow inside a single frame. Static UPS gets replaced. Decide up front which philosophy fits your growth path — retrofitting the wrong choice is expensive.

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