How to determine electrical energy and the convert it to heat?
You don't start by calculating the power so much as you start by calculating the current (Either nodal analyis with KCL or mesh analysis with KVL will work). You are going to need to find the current in both loops and calculate the power for each resistor individually using the calculated currents.

\$P = I^2R\$ which is Watts. Watts is Joules per second. You just want Joules and you know you have 1 hour (3600s) Can you take it from there?

Hint: You have two current sources. They determine the current flowing in their branch, and you just so happen to need the current in each loop. So you could do it by inspection without KVL or KCL if you know what you are looking for. The problem is actually much simpler than if it were two voltage sources.  That said, you won't learn too much by doing it by inspection since it won't help you solve these circuits in general.