What's the rule of thumb re: kilograms per kWh

[voyager]

It used to be around 6 kg per kWh.

 

[CowFreedom]

It depends on what kind of energy storage you consider (Nickel Iron, lead acid, lithium ion etc.). Here is a handy table comparing values.
Depending on the application, instead of specific energy (energy per unit of mass), the energy density (energy per unit of volume) might also be of more interest.