Actually, using a washer dryer uses LESS energy than separate washing and drying:
according to the Euro energy label
-a A class washer uses 0,19 kW/kg of laundry
-a B class condenser dryer uses 0,64 kW/kg of laundry (witha rough subdivision of 60% B class, 30% C class, 10% A class heat pump driven, all vented dryers are C class or less)
- Total 0.83 kW/kg of dry clean laundry
- a A class washer dryer (most of the current European range is A class) uses 0,68 kW/kg for washing and drying, if the W/D were B class that would be 0,81 kW/kg, less than the 0,83 needed for separate machines.
The results are another thing even if I must say I never had problems, just be careful to put the right quantity of clothes in the drum while drying.