They recover some latent heat as they dehumidify. Mine uses about 1kWh per kilogram of water condensed.
Latent heat of evaporation at room temp is about 2.6MJ/kg.
So in recovering 1kg of water it also recovers 2.6MJ of latent heat.
2.6MJ is equivalent to 0.72kWh (1kWh = 3.6MJ)So in using 1kWh it outputs 1kWh of heat and recovers another 0.72kWh of latent heat.
So the effective heat output is 1.72kWh for 1kWh input.
I'm sure someone will correct me if I'm wrong :)
cheers, Pete.