The operating difference
A fixed-speed compressor commonly cycles between on and off states. A variable-speed or inverter system can reduce compressor speed as the room approaches the target. The resulting energy difference depends on part-load efficiency, sizing, climate, set point, installation, and runtime.
Compare seasonal energy, not only rated watts
Use seasonal efficiency information or measured energy when available. Comparing maximum input ratings can be misleading because the systems may spend different amounts of time and power at part load.
Test the purchase decision
Run low, expected, and high cooling-use cases. Add installation cost and compare the incremental purchase price with annual savings. If the payback depends on an aggressive assumption, measure the present system over representative weather first.
Run your own scenario