Energy lost in in Peltier device?
Lets suppose water (in a perfectly insulated bath) is heated with a resistor (where there will be 100% electric energy to heat) and you put in 1 Joule of energy (1 Watt/second). With a peltier that is connected to something colder than the water, one could get 5 to 8% of the energy back, or around 0.05J. 

But this is assuming all of the heat in the water will go back through the peltier, which is not possible in the real world. A good insulated water bath will be needed if you want to store energy in this way. It would be better to use a resistor to heat the water, if you use a peltier to do the heating, there could also be an efficiency loss through the heatsink or whatever other thing its connected to. 

It may also be advantageous to heat 1 water bath and cool another, and insulate both. One problem with thermal storage is heat leakage as any heat stored will want to equilibrate with any temperature outside of the heat storage container.  So it makes up for a poor long term energy storage solution.