My first thought was 1000 watts was high, like Harriet. But if you concider the "General rule is to have three to five watts per gallon for every 10 deg F above room temperature that you wish to have the tank water temperature."
So if you want your tank 86 and your room 68 and you use an average 4 watts and 150 gal tank, you could calculate,
86 - 68 = temp / 10 x 150 x 4 = 1080 watts.
You will have to substitute your tank temp and room temp and aquarium size for different situations. You could be pretty close Eileen, you may be a genius.
http://forum.simplydiscus.com/images/smilies/smile.gif Bill