I tend to drop small hints before leaving them without power. Something like having the ring announce that it's down to 30% power.
In one instance the heroes had fight Grundy and Black Adam, which was mostly diversion for the real attack. Near the end I announced the ring was at 30% capacity right before having batman call in help dealing with Sinestro.
Had the player flown off to combat Sinestro without recharging, choosing instead to recharge at a more dramatic moment I might have let him do more damage.
In another instance the heroes were trapped in an alternate dimension that blocked outside energy from getting in, so the GL was running off one charge and had to do things like walk, and punch. Also as a side note they were assisted by a young boy named Billy who had been trapped there for a bit and was really eager for them to bring down the energy barrier.
Someone told me to write a blog about movies and RPGs...So here it is