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.

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.