ATM90E32 total energy reading
The datasheet says that energy registers are cleared after reading, so that means each read returns the accumulated energy since the last read. If you want to continuously monitor the amount of energy used over a longer period (eg. to show how much power has been used so far during the day) then you will have to accumulate the individual reads. 

The value in the register is a 16 bit integer representing 100ths of a CF, and there are 3200 CFs in a kWh so at most it can store ~0.2kWh without overflowing. Therefore you will need to read the register often if the power consumption is high.

Alternatively you could just count CF pulses via eg. an external interrupt, and divide by 3200 to get kWh.