Using a copy of [SPICE] a free circuit simulation application, I have created several sine wave sources and summed them together. Seen here, the waves combine into a square wave with it looking ...

so it may be wise to invest in one of Ryobi's pure sine wave inverters instead. Just because the Ryobi 40V 300-Watt Power Source inverter is small, doesn't mean it can't get the job done.