The unit you're probably looking for is
BTU. 1 BTU is the energy necessary to heat 1 pound of water 1 degree F in a constant atmospheric pressure. It's the standard when it comes to describe heat loss/gain.
If you wanted to convert it to the wattage of room/tank heater(s), 1 BTU is 0.293071 watt-hour.
For a particular room, you need a certain amount of BTUs to raise the room temperature over ambient. Then depending on how well insulated the room is and it's difference between room inside and outside temperature, you'll have a certain heat exchange of BTUs (typically for loss because we live in MI).
In a perfect world with no heat loss, yes, the wattage for heaters is the same whether you've got a lot of in tank heaters, or a single room heater.
In the real world, if you heat the tanks, the tanks loose heat to the room('s air), and the room loses heat to outside itself (also fastest through the air). So if you have in tank heaters, the heat in the tanks will be greater than the heat in the rooms air. For example, if the temperature outside the fishroom was 68F and the tanks at 78F, the room temperature might be something like 76F. (Humidity might make it feel hotter FWIW).
Also in the real world, if you heat the room, you're room's air is loosing heat to the tanks (necessary to heat them - we like this) and to the outside. So if you heat the room, the room is typically going to be hotter than the tanks. For example, if the temperature outside the fishroom is 68F, to keep the tanks at 78F you might need to heat the room to 80-81F.
If heating the room, the more you insulate the room, the less heat loss and the less gap between the room temperature and the tank temperature.
FWIW, I started with heaters in all my tanks and slowly one by one began unplugging them over a period of days after adding a dehumidifier (since it adds heat as well when running). I started removing the heaters from upper tanks since heat rises. I also started with the smaller tanks since they have less volume, so less heat retention IMO. All my tanks below waist high right now are running ~80F. The room air is 78F. The tanks nearer the ceiling are 77F and those in the middle run about 76F.
With my room, working backwards was easiest. At this point I can add in additional tanks and anything at waist level or above simply doesn't need a heater (saves me from the expense of buying one, nor does it cost anything more to heat the room at this point with more tanks. If anything it actually save money per greater heat retention!).
If I move to a room heater, I'd add the heater, the in-tank heaters won't run if it's how enough, and then I'll start working backwards unplugging more tank heaters.
