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.
A small worked load list
List each required appliance once: for example, lights, fans, router and television. Add running watts for the normal load. Keep motor or compressor surge separate, because it affects inverter selection differently from battery energy.
Avoid a false exact answer
The calculator can show a planning range, but equipment ratings, battery chemistry, cable protection, ventilation and local installation practice must be confirmed before buying or installing.
Worked inverter VA path
Suppose the required running load is 600 W and the largest starting combination is represented by a 1.4 surge factor. The peak planning load is 600 × 1.4 = 840 W. At a 0.80 power factor, that is 840 ÷ 0.80 = 1,050 VA. Adding a visible 20% planning margin gives 1,260 VA. Each step has a different purpose, so surge, power factor and margin should never be collapsed into one unexplained multiplier.
Worked battery Ah path
For a 600 W load needed for four hours, delivered energy is 600 × 4 = 2,400 Wh. If the planning assumptions are 85% inverter efficiency and 50% usable depth of discharge, nominal stored energy is 2,400 ÷ 0.85 ÷ 0.50 = 5,647 Wh. At 24 V, the mathematical capacity is about 235 Ah. This is a planning result; the supported battery configuration, chemistry, discharge rate and charging arrangement still need equipment-specific confirmation.
Keep the three checks separate
Running watts answer whether the inverter can carry the steady load. Starting surge answers whether it can start the most demanding combination. Watt-hours and usable battery energy answer how long the load may run. Passing one check does not imply the others pass. For example, a battery bank can contain enough energy while the inverter is still unable to start a motor, or a large inverter can run a load but exhaust a small battery quickly.
Information to collect before selecting equipment
Record each appliance’s running input, known start or compressor information, required simultaneous use, desired backup time and any priority loads that can be switched off. Then collect the inverter’s continuous and surge ratings, supported battery voltage, charger limits, battery chemistry requirements and protection guidance. Cable size, isolation, earthing, ventilation and protective devices are installation decisions for a qualified person, not values a browser calculator can approve.