FWIW, "upgrading" to save money isn't really possible. A watt is a watt.
The tipping point in heating the room IMO is when you've reached a point where the tanks you currently heat, when all placed in a room, hold the room temperature well enough that you can add another tank without a heater, yet have it remain warm enough. Once you reach that point, you can pack the room full of tanks with no extra heating costs!