Answer

 The 'Standard' energy meter is fine to use with a 50W immersion heater being run at 12 Volts. The energy meter has a specification of 25 Volts d.c. and 7 Amps, so it will cope fine running an immersion heater as above, which will draw around 4.2 A.
 Please note that the immersion heaters are normally designed to run at around 12V.