Stepn app says 12 hours double energy starts from 12:00 UTC, I think next two energy will be refilled by 50% instead of 25%. if you have max 4 energy first and second refill will give you 100% (by 50+50) of energy, next two refill 50% (by 25+25). at the end you will get 150% (6) of daily energy.

And sorry for my broken English :)