Fundamentally, you will not be making use of much of the energy coming from electricity firms so there is a chance that your monthly expenses will be lowered by more than half. The preliminary expenditure for renewable energy sources can be high, but it's well worth the money. 

You are undoubtedly trying to preserve money, but you can save more over time if you replace them. You may expect that your bills will go down as the new models do not consume a lot of energy.
You will have to spend money for this, but you can preserve more if you will replace them straight away. The new models do not consume plenty of energy so you can expect that your monthly bills will go down.
Lighting is usually ignored because a number of people believe that you can turn on the lights for 24 hours without worrying about your electricity. Your bulbs might be small, but they consume a large amount of energy.