It will cost exactly the same, either way. Assume it would normally take 1 hour to heat the water with the 4500 Watt (4.5 kW) element. That would be 4.5 kWh of billed electrical usage (4.5kW x 1 hour).
A 3500 Watt element is going to take 29% longer to heat the water to the same temperature, or 1 hour, 17 minutes (1.2857 hours). 3.5kW x 1.2857 hours = 4.5 kWh.
The short answer is NO!!! Save your money and puy back what the designed called for. It's all about recovery rate (Time) and the temperature set point. The smart money saving thing to do is to simply lower your water temperature a little or simply use less hot water.. Ed
Well there's an up and a down side. The 4500 w are going to use a little more power but heat the water quicker. The 3500's are not going to use as much power but have to stay on longer. So they end up using a little more power. I would say its going to depend on how much hot water you use. If you don't use a lot, they will not have to stay on very long and you might save some power. If you use a lot, then they are going to stay on a lot to reheat the water. I changed 2 side by side 30 gal tanks 4500w elements to 6000 w elements so they would heat water quicker due to a whirlpool tub that kids were using all the time. And it still couldn't keep up with the demand of the kids. But there kids, and don't know about that yet.