It's not as simple as that. You can't just tack the cap on and the inverter will suddenly magically be happy. The inverter itself has to from start to finish be designed for that kind of starting load. What you need is an inverter designed to run AC motor loads directly (true sine wave) As far as a modified sine wave inverter goes, a typical AC motor is a dead short to ground on start. A capacitor can buffer that power surge briefly, but a large enough capacitor to do that is going to also cause a significantly higher initial load than even the motor itself.