Engineering guide

    UPS & Battery Sizing

    Two different capacities: how much power the UPS can deliver, and how long its battery can deliver it.

    KiloVolt Engineering Toolkit

    A 2 kVA UPS does not mean thirty minutes

    A UPS supplies critical equipment when normal input power is lost. Two capacities must be checked: the power the UPS can deliver at any moment, and how long its battery can deliver it. KiloVolt keeps two main inputs, the critical load in watts and the backup time in minutes; everything else is an editable assumption in Advanced.

    1. Start with the real critical load

    Add the wattages of the equipment that really needs uninterrupted supply, as expected simultaneous demand, not the sum of breaker ratings. Consider startup and inrush separately where relevant.

    2. Watts and volt-amperes

    Sload=PloadPFS_{load}=\frac{P_{load}}{PF}

    A 1,200 W load with PF 0.90 is about 1,333 VA. A UPS has a separate maximum W rating and maximum VA rating, and both must be met; the UPS's own output power factor is not the load power factor. With 25 % headroom:

    PUPS=1200×1.25=1500 W,SUPS=1500/0.9≈1667 VAP_{UPS}=1200\times 1.25=1500\ \mathrm{W},\qquad S_{UPS}=1500/0.9\approx 1667\ \mathrm{VA}

    A 2 kVA class is a useful shopping filter, but a specific product still needs enough W output and compatible characteristics.

    3. Energy and autonomy

    EAC=P tE_{AC}=P\,t

    1,200 W for 30 minutes means 600 Wh delivered to the load. The battery must provide more, because of conversion losses and usable-capacity limits:

    Ebattery,nominal≈EACη u a=6000.9×0.8×0.8≈1042 WhE_{battery,nominal}\approx\frac{E_{AC}}{\eta\,u\,a}=\frac{600}{0.9\times 0.8\times 0.8}\approx 1042\ \mathrm{Wh}

    η is the battery-to-AC efficiency, u the usable fraction at commissioning and a the remaining end-of-life capacity. None is fixed across chemistries and models: the result is a preliminary nominal energy, not a guarantee of 30 minutes.

    4. Why runtime charts are necessary

    The energy a battery delivers at a given discharge rate is not proportional to its nominal label. Lead-acid capacity falls substantially in short, high-current discharges; cold, ageing and the UPS's DC cut-off reduce it further. Lithium packs depend on their battery-management limits, temperature and UPS compatibility. Confirm the product on its runtime chart at about the actual load.

    5. Headroom is not extra energy

    The 25 % headroom sizes the UPS power rating for growth. It does not increase the energy used by the 1,200 W load entered today. To model a future higher load, enter it as a second scenario; never multiply the battery energy by the margin silently.

    6. Choosing a UPS

    Check the load W and VA, the backup time, output voltage and waveform, transfer behaviour, bypass, redundancy and environment. Motors, compressors, high-inrush and non-linear loads need attention to crest factor and overload performance. Without manufacturer runtime data the KiloVolt result stays a preliminary energy estimate.

    7. Questions

    • Is a 2 kVA UPS always a 2 kW UPS? No: read both ratings.
    • Does a 1.0 kWh battery last one hour at 1 kW? Only as an ideal starting point; conversion and discharge limits usually make it shorter.
    • Why is the battery energy larger than the load energy? It includes conversion and usability allowances.
    • Can this size an emergency or life-safety system? Not by itself: those need specialist design and approvals.

    Related: the PV & Hybrid guide (batteries in a different context) · Eaton UPS sizing guide

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