You’re liable for paying your energy bills as soon as your contract starts.
However, it is possible that your landlord has different rules for paying energy over the summer, so it’s worth double checking with them exactly when you’re liable from.
It is your right as a tenant to switch energy suppliers. That's right! People assume they can’t switch providers because they’re renting but that’s not the case. However, if your bills are included in your rent, then the landlord gets to choose who you’re with.
When the old tenant moved out, they should have taken a final meter reading. But it is still worth you taking a reading, sending it to your current provider and letting them know you’ve moved in as soon as possible, to make sure that you’re not paying for the last tenants’ energy.