With a 4.5kw system, to get a reasonable ROI it will need some leverage. A good portion of that leverage could be invested in sweat equity up front, but to make a lasting difference, something must be done to offset your existing electricity consumption.
On cost savings alone, in the present economy, natural gas is highly competitive with electricity. Compared to straight resistance heating, it is more economical. With phase change heating and cooling, overall system efficiency comes into play. With dated systems, both gas and phase change, there is a balance point in the operating envelope where one method is more effective than the other. With modern systems, there exists a huge gray area in the operating envelope where it is very difficult to tell if one is better than the other just by looking at spec sheets. Features such as condensing furnaces and variable speed heat pumps are higher in efficiency than old technology, but they aren't so easy to compare against each other. In many installations, the balance point must be found through trial and error.
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