UPS and Battery Sizing Guide for Pakistan
Size inverter VA, battery Ah and expected backup time without confusing watts, volt-amperes and watt-hours.
Add simultaneous running watts, allow surge and safety headroom to select inverter VA, then calculate battery energy from load watts and desired hours. Apply depth-of-discharge and conversion losses before converting watt-hours into amp-hours.
What to remember
- ✓ Running load determines energy use; startup surge can determine inverter size.
- ✓ Battery Ah has meaning only with voltage and usable depth of discharge.
- ✓ Lead-acid and lithium batteries should not be compared by Ah alone.
Build a realistic load list
List only appliances that must run during an outage and record quantity, running watts and likely simultaneous use. Nameplate values are a starting point; measured input is better where available.
Refrigerators, pumps and some air conditioners have a startup surge. Lights and electronics may have smaller peaks, but poor power factor can still make VA demand higher than watts.
- Essential running watts
- Largest startup surge
- Hours required for each load
- Loads that can be switched off
Choose inverter capacity
A simplified estimate divides watts with headroom by power factor to obtain VA. For example, 800 W with 25% headroom and a 0.8 power factor suggests at least 1,250 VA before surge verification.
Check continuous watts, surge rating, DC battery voltage, charging current, waveform and manufacturer limits. A large VA label does not guarantee that every motor will start reliably.
Convert backup hours into battery capacity
Required usable energy is load watts multiplied by hours. Battery nominal energy is voltage multiplied by amp-hours, but only part may be usable after depth-of-discharge limits, inverter loss, battery age and high discharge current.
A planning formula is Ah = required Wh ÷ (battery voltage × usable fraction × inverter efficiency). Use conservative values and avoid routinely discharging lead-acid batteries too deeply.
Compare battery technologies
Lead-acid batteries are commonly rated at a specified discharge rate and may deliver less capacity at high current. Lithium products can offer more usable depth, cycle life and stable voltage, but require compatible battery management and charging settings.
Confirm cable size, fuses, ventilation, earthing, polarity and safe installation with a qualified technician. Battery banks can deliver dangerous fault current even at low voltage.
Limitations and responsible use
This guide cannot verify battery health, motor surge, wiring, protection or product compatibility. Electrical installation should be checked by a qualified professional.
Sources and further verification
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