It just depends on the rate of heat loss from your aquarium (a function of surface area divided by litres), and the temperature difference between the desired temperature of the tank and its surroundings. The first is constant for your tank, so the only variable is the room temperature.
It will take more energy to maintain the tank at 24 degrees if the room is 16 degrees than it would if the room is 20 degrees.
The size of the heater doesn't make any difference (as long as it's strong enough to raise the water to the desired temperature). If you had a 400W heater it would just be on for half the time your 200W one is.
So the electricity consumption is going to go up and down as the room's temperature changes - unless, of course, the room is held at exactly the same temperature all year round. Unless your room is air conditioned, your heater will probably hardly ever be on at the moment, so not a lot of point looking at it.
Good luck with any measurements or calculations. If you can't find the answer, don't worry about it too much - a 96l aquarium won't be taking much power to maintain it at 25 degrees in a normal heated room.
Let us know if you find anybody who's done the calculations for us!!
Best regards,
Mark