How to size an inverter and battery
Choose a home inverter VA rating and battery Ah capacity from your appliance load, starting surge, backup time and visible loss settings.
List only the loads you need
Add the running watts of the lights, fans, router, television and other items you want during a power cut. Motors and compressors may need extra power when they start.
Size the inverter for load and surge
Watts describe real power. Inverter capacity is often shown in VA. A planning calculation divides the peak watt load by power factor and then adds a safety margin. Confirm both continuous and peak ratings on the product sheet.
Size the battery for time
Battery energy starts with volts × amp-hours. You cannot normally use all of that energy. Depth of discharge, inverter efficiency, battery age, temperature and load all affect delivered backup time.
Check compatibility before buying
Confirm battery chemistry, bank voltage, charging current, wiring, protection and manufacturer compatibility with a qualified installer. A calculator provides a planning size, not an installation design.