UPS & Battery Sizing

UPS power in W and VA, and preliminary battery energy.

EstimatePreliminary result, not a design verification.

Before you calculate

UPS power and preliminary battery energy

Two inputs: the critical load and the backup time. Power (W and VA) and energy are sized separately; the manufacturer runtime chart decides the real autonomy.

Learn more — complete guide →

Step 1

Load and backup time

The expected running load of the equipment that must stay on, not the breaker rating.

Critical load (W)

Backup time (minutes)

Advanced: assumptions and a candidate UPS (optional)

Load power factor (reference assumption 0.9)

UPS sizing headroom (%)

Battery-to-AC efficiency (%)

Usable battery fraction (%)

End-of-life capacity (%)

Candidate UPS rating (W)

Candidate UPS rating (VA)

Candidate battery nominal energy (Wh)

Step 2

Results

Check before relying on this result

Preliminary energy estimate. The real autonomy must be checked on the manufacturer's runtime chart at the actual load (short, high-rate discharges in particular do not scale with kWh).

UPS rating needed1.50 kW · 1.667 kVABoth ratings must be met; the load alone is 1333 VA.

Energy delivered to the load600 WhDrawn from the battery ≈ 667 Wh

Preliminary battery nominal energy1.042 kWhReference estimate; runtime verification required.

Rating class to investigate2.0 kVANot a recommended product: check its W rating and runtime chart.

Not covered: inrush and motor loads, crest factor, transfer behaviour, redundancy, battery voltage and BMS limits, temperature. Life-safety systems need specialist design.

1. UPS active and apparent power

The load with the sizing headroom; the apparent power uses the LOAD power factor.

P_{UPS}=P(1+h),\quad S_{UPS}=\frac{P}{PF}(1+h)

P_{UPS}=1200\times1.25=1500\ \text{W},\quad S_{UPS}=1666.67\ \text{VA}

2. Energy delivered to the load

The current load for the backup time. The headroom is NOT included: it sizes the UPS, not the energy.

E_{AC}=P\,\frac{t}{60}

E_{AC}=1200\times\frac{30}{60}=600\ \text{Wh}

3. Preliminary battery nominal energy

Battery-to-AC efficiency η, usable fraction u and end-of-life capacity a are design assumptions, not fixed values for a chemistry.

E_{nom}\approx\frac{E_{AC}}{\eta\,u\,a}

E_{nom}\approx\frac{600}{0.9\times0.8\times0.8}=1041.67\ \text{Wh}

Calculations use full precision internally; the displayed numbers are rounded for readability.