The one you built is designed just like an audio amplifier with heat generating resistors to ground.
.... It consumes way too much energy from the battery.
With a 500W load, each 0.1 ohm emitter resistor dissipates 10W half the time. Their total power dissipation is only 40W (8% of the load's power). Then the transistors don't need to be matched to have the same current gain. Remove the resistors if you can buy matched transistors.
With less load then there is less power in the equalizing emitter resistors.
Mosfets are much better than old transistors in a simple square-wave inverter. This circuit is old and is used where Mosfets are not available.
Others have emailed me to let me know ...
They didn't e-mail me and I developed the modification. But they e-mail you?