F2P energy refills 1 per 10 mins. Buying membership will increase the rate to 1 every 7.5 mins.
You can get a 'free' refill by levelling up.
Other than that the only other way to get more energy is to use energy elixirs. These can be bought from the crown shop (clicking 'refill' if you are engaged in an activity that requires energy will auto-direct you to it). You can also get energy elixirs from playing Grub Guardian on the FreeKIGames.com website. If you do an online search you will find guides to drops and strategies to achieve this. I'm not sure which free maps drop them, the 'best' map* (Tanglewood Way, Wysteria) will cost a few crowns but will pay off in 1-3 days of use depending on how lucky you get.
I hope this helps!
*many people consider Tanglewood Way, Wysteria Map to be the 'best' for energy elixir drops (silver medal) but this is opinion rather than solid fact and drops are subject to change so this may be out-of-date info. check online to see what is currently considered 'best'
