I'm not sure that termotanques are that inefficient. It's probably just a resistance coil with all energy going to heat. So it's likely very efficient.
If your tank is located outside than you might be spending energy to warm it up that the wind and environment then takes out of it. I would see about an enclosure or moving it inside if you have space. Otherwise, I would still recommend doing an audit of electrical consumption. In other countries I've heard of similar issues and it ended up getting resolved by the electric company changing the meter.
