You can consider the Silverstone Strider Essential 500W for RM199. An alternative would be the Corsair Gaming Series 600W power supply for RM299. Your system's power consumption is estimated to be around 250W or so, so with a 500W power supply you should have it running at close to its best efficiency since it's being half loaded. Most power supplies have the best efficiency at half load. You can go with the Corsair Gaming Series 600W power supply if you intend to use the system for a long time and plan to upgrade in the future.

It's just slightly better, but if you want it to be the best then that's one of the options you can turn to.

And electrical bill wise, it should lower your electrical bill since a power supply with a higher efficiency wastes less power compared to one with a lower efficiency. Bear in mind that although you have a 1000W power supply, if 500W is all your system requires that that is what it will use. The power supply does not supply 1000W all of the time unless you are using that amount.