The simple answer is this, you cannot dim a compact fluorescent lamp with any type of dimmer cheap or expensive, you either have them on or off. The amount of electricity that a low energy lamp uses is most of the time 75% less than a conventional 60 watt GLS lamp.
Now when it comes to outside lights it's better to leave the light on all night to illuminate the area you want to so you get the effect of the light output of a low energy compact flourescent as in the winter the lamp needs about 4 minutes to warm up, also if you want to fit a "Photo electric cell" to the fitting so the light stays on all night and off in the day. They are inexpensive and all electrical wholesalers have them. The ones I buy are about �9 + vat and are very small and can be fitted in most outside light fittings ask them for the Eterna EMPCR photo cell.
Can you tell i'm a sparky!!!