Board Game Arena

If I choose a 3 power action (or any odd number action space), can I use the 1 left over power in the conversion screen before ending the turn? I feel like there should be a 'confirm you want to waste this power' box when finishing the turn.

Nope. Rules say excess payment is lost, same thing with the power stone. There's no warning because it's a standard rule and, as you've pointed out, pretty common when paying.