I've got a pair of doubles and four singles.... For some things it's hadny to have the 40-watt double as it really does get warm, plenty of other stuff is perfectly happy in the pair of 8W singles or the 20W double.....the 11 and 14 watt singels make up the set so they're all different temps and the 40W double is thermostatic and has a thingy.... There's no such thing as one that's too big or too powerful, but any of them are better than nothing.... next year mine are getting joined by a 40-foot thermostatic soil-heating cable in a big bed in the greenhouse..... should keep things nice and toasty.....
chrisc