Depends on where you live (lowest temp and amount you are raising lowest temp), amount of insulation you have and square feet, along with ceiling height.
Also, what fuel is available and how much is electricity in your area? Do also need a/c?
All factor into required btus and type of unit.
You're getting all sorts of answers because everyone has differing answers to the questions above.
Do a btu calc first.
^^^ This is the only real advice in this thread.
Insulation is measured by its R-value. The R-value lets you calculate how much heat will move through a certain wall area depending on the temperature difference between the indoor and the outdoor air.
So let's say you have a wall that contains R-10 insulation in the United States. Let's say the wall measures 8 feet high by 10 feet long, or 80 square feet. Let's say it is 70 degrees F indoors and 30 degrees F outdoors, or a 40 degree F temperature difference. The calculation looks like this:
80 square feet * 40 degree F difference / 10 = 320 BTUs
In other words, you would need a heater producing 320 BTUs to compensate for the heat loss through that wall, and that heater would be running continuously.
If you are not in the United States, you do the same kind of calculation but you do it in SI units, using degrees C, meters, square meters and SI R-values. What you get out is watts. So if you have a wall that is 2.4 meters high and 3 meters wide (7.2 square meters), and the outside temperature is -1 degrees C while the inside temperature is 21 degrees C (22 degrees C temperature difference), and the insulation is SI R-1.8, you get:
7.2 square meters * 22 degrees C difference / 1.8 = 88 watts
If you multiply watts by 3.4 you get approximate BTUs and vice-versa.
So now let's look at a house. Let's imagine an idealized 2,500 square foot single-story house. It is a square that has 4 walls that are each 50 feet long and 8 feet high. The ceiling and floor are 50 x 50 foot squares. The ceiling has R-30 insulation. The floor has R-15. The walls have R-10. It is 70 degrees F inside and 30 degrees outside. How much heat is escaping through the insulation?
Each wall is 50 x 8 = 400 square feet for a total of 1,600 square feet. With a 40 degree F temperature difference and R-10 insulation you get:
1,600 * 40 / 10 = 6,400 BTUs
The ceiling has 2,500 square feet at R-30, so:
2,500 * 40 / 30 = 3,330 BTUs
The floor has 2,500 square feet at R-15, so:
2,500 * 40 / 15 = 6,660 BTUs
So the whole house needs about 16,400 BTUs (or 4,820 watts) to maintain its temperature.
No real house is ideal like this, however. A real house has windows that might be at R-2. And doors that might be R-4. And there are little leaks that allow cold air infiltration, plus less insulation around outlets, etc. All in all, a real 2,500 square foot house might need 20,000 to 25,000 BTUs (depending on how many windows/doors and how "tight" it is) to maintain its temperature.
Chris