Um, you can’t ^^’. The key is to use the rewind power very sparingly, only as long as it’s required to destroy the enemy. This way you’ll guarantee you will be in the net positive energy generation for the first 2-3 waves. You should be able to afford the 2nd tower by the end of Wave 2.
I was barely using the backspace, I even tried letting most of the enemies through, but I never built up enough energy for a second tower.