How do the type of drying technology and the dryer size impact on energy use?
What type tumble dryer?
There are two main types of technology used in tumble dryers – vented tumble dryers and condenser tumble dryers.
Vented (or ventilated) dryers collect the steam generated by the drying process and vent it out through a hose. The vast majority of vented tumble dryers are electric, but there are some gas-operated models available. Vented tumble dryers need to be positioned near a window or door to release the damp air away from indoors.
Condenser dryers remove the moist warm air generated in the drying process from the drum and cool it inside the machine. The resulting water is collected in a removable reservoir. Condenser tumble dryers can be installed anywhere because water from the drum is collected, but the reservoir needs to be emptied regularly. Poorly designed condenser dryers may release steam into the surroundings and cause damp.
Condenser tumble dryers with heat pumps make up the largest number of A-class models currently on the UK market - 32 of 38 tumble dryer models currently on sale. 
What size tumble dryer?
If you are planning to buy a new tumble dryer, think about what best suits your needs. The drum size of the model will have an effect on both the upfront price and the annual running costs. Available drum sizes range from a compact 3kg up to a family-friendly 10kg, which can dry nearly two washing machine loads in one go. The most common size on the market is 7kg.
The average household does over five washes a week and a larger tumble dryer would be appropriate to cut down on the number of times needed to dry all of the clothes washed. A single-person household will have smaller individual loads than a family, so a smaller drum is likely to suffice.
In any case, look out for the Energy Label – the best performing models currently on sale in the UK will be rated A and you can see the annual average energy consumption on the label.
 GfK market data, December 2012