Incandescent bulbs and new generation halogen bulbs give light almost immediately when switched on. CFLs take time to achieve full brightness (and can take much longer in very cold temperatures).
There are many different light fittings, most of which have been designed to use the traditional incandescent light bulb. But that's not to say you can't use new energy efficient lamps to replace them. In any case, the number of new light fittings designed around energy efficient bulbs and lamps is growing by the day.
Here are the a few things to remember when choosing energy efficient replacements:
1. Choose the right replacement bulb wattage. To maintain existing light levels, use an efficient bulb that provides a similar light output (this should be clearly stated on the packaging). Never exceed the maximum wattage stated on the fitting.
2. Match the light colour to the bulb you're replacing. New generation halogen bulbs and halogen spotlights are a similar colour temperature to standard incandescent bulbs. Look for a CFL with 2700K or 3000K on the packaging (often called 'warm white') for a similar colour temperature.
3. If you can't see the light bulb, it doesn't matter what it looks like. The effect of the light and the light output is what is most important so in a covered fitting a standard CFL is likely to be the most cost effective way to maximise your energy efficiency.
4. Make sure the replacement bulb has the necessary features or functionality. To use a CFL on a dimmer circuit or in a high-switch area, choose one designed for this use. New generation halogen bulbs can be used anywhere a standard incandescent is used.
5. Check the base type. Make sure the base of the new bulb – whether it be bayonet or screw fitting - is the same as the one you are replacing. If you get it home and it doesn’t fit, don’t force it.
6. Just because it fits doesn't mean it is going to give you the light you need. Some fittings such as recessed ceiling lights may only work well with certain bulbs. Choose a replacement bulb that's similar to the original bulb's shape and size - such as the most appropriate new generation halogen bulb or reflector CFL. Alternatively, replace the fitting with one designed to take a the efficient bulb you want to use.