How do I convert watts to kVA for a UPS?

Divide the real power in watts by the load power factor, then divide by 1000. For example, 1500 W at a 0.9 power factor is 1500 / 0.9 = 1667 VA, or about 1.67 kVA. UPS nameplates are rated in kVA, so this step matters.

Why does depth of discharge change the battery size?

You should not fully drain a battery — doing so shortens its life. If you only use 80% depth of discharge, the nameplate capacity must be larger so that 80% of it still delivers the energy you need. The tool divides the usable energy by the depth-of-discharge fraction.

What inverter efficiency should I use?

Online double-conversion UPS units and inverters typically run 88–95% efficient on battery. A value of 0.90 is a safe default. Lower efficiency means more battery energy is lost as heat, so the battery must be larger.

How is amp-hour capacity calculated?

First find the energy the battery must store in watt-hours, then divide by the battery bus voltage: Ah = Wh / V. A 48 V bank needs far fewer amp-hours than a 12 V bank for the same energy, which is why larger systems use higher DC voltages.

Does this include charging or generator transfer time?

No. This sizes the battery for the discharge runtime only. If your backup must bridge until a generator starts, set the runtime to the generator start-and-transfer window plus a safety margin.

Battery Backup (UPS) Load & Runtime Calculator

When the utility fails, a UPS keeps critical equipment alive on battery until power returns or a generator picks up the load. Getting the battery size right is the difference between 5 minutes and 50 minutes of runtime. This calculator sizes both the UPS (in kVA) and the battery bank (in amp-hours) for your target runtime.

How it works

There are two independent calculations.

UPS sizing (apparent power). A UPS is rated in kVA, but loads are measured in watts. Convert using the load power factor:

S (VA) = P (W) / power factor
kVA    = S / 1000

Designers then add 20–25% headroom for growth and inrush.

Battery sizing (energy). The battery must store enough energy to carry the load for the runtime, accounting for inverter losses and the fact you never fully drain a battery:

usable Wh    = load W × (runtime hours) / inverter efficiency
nameplate Wh = usable Wh / depth of discharge
Ah           = nameplate Wh / battery bus voltage

Worked example

A small server rack draws 1500 W at a 0.9 power factor and needs 30 minutes of runtime on a 48 V battery bus, with a 90% efficient inverter and an 80% depth of discharge.

Apparent power: 1500 / 0.9 = 1667 VA → about 1.67 kVA; with 25% headroom, a 2.1 kVA UPS.
Usable energy: 1500 W × 0.5 h / 0.90 = 833 Wh.
Nameplate energy: 833 / 0.80 = 1042 Wh.
Capacity: 1042 / 48 = 21.7 Ah at 48 V.

Notes and tips

Higher battery bus voltage means fewer amp-hours for the same energy, which cuts cable size and copper cost — one reason large systems use 48 V or higher.
Real battery capacity falls at high discharge rates (Peukert effect) and in cold temperatures. For short, hard discharges, add margin beyond this estimate.
Always confirm the UPS can supply the inrush of any motor or transformer loads, not just steady-state watts.

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