Transients, by definition, are very short period. And (in the context of the vague scenario you describe) they're the effect of either infrequent load switching, or highly periodic 'switch-mode' noise that may result from use of a DC-to-AC inverter, for example, or an actual load drawing non-DC or non-sinusoidal current. The former can probably be ignored without a second thought, due to the comparative rarity of them.
As for highly periodic 'switching noise', well, "it depends" on the specifics of the equipment (inverter?) & load in use. If it were me, I'd want to see at least a voltage waveform, & ideally a current waveform, with the system under various loads, to "see if you have a problem" that needs to be accounted for in your measurements, or not. But my gut feeling is that the energy in any such 'noise' is probably going to at least 2, probably more like 3- orders of magnitude less (one hundredth or one thousandth) energy than the actual steady-state (DC or sinusoidal) load - if that guess is actually the case, then you can make a call as to whether that matters to your desired measurement accuracy. In a "regular household", I'd expect not :)
Having designed a number of energy meters over the years, I can say with authority that reactive energy (Q) is not the same as exported energy (PE), in fact it has basically nothing to do with it. What you need is an energy meter that has separate counters for imported real energy (P) and exported real energy (PE).
Reactive power is basically only relevant for industrial metering, but in case you're interested, an energy meter can additionally have up to four counters for reactive energy (Q), these being, in order of real world relevance, inductive (QIND) and capacitive (QCAP), exported inductive (QEIND) and exported capacitive (QECAP). When there are six registers in total, the sign of real power is used to select which registers are accumulated.
Most important, however, is that when money is involved (the meter is used for billing), the exact meter brand and model must be accepted by your power grid company. If money is involved, they may also come and seal the meter themselves with their own seal, although the practice varies from company to company in different countries.
When you're reading the registers, be sure to note the difference between instantaneous power (P) and accumulated energy (W). The register names for instantaneour powers might be something like P, PE, Q, QIND, QCAP and the energies might be something like W, WE, WQ, WQIND, WQCAP etc.
Best Answer
Full scale is the maximum value that the register or channel can holdor display.
section 5.1 on page 16 says
Current may be sensed eg with an inline shunt rsistor.
By choosing a shunt resistance or other means the current which causes 250 mV peak to occur at the current input is the full scale current.
Voltage may be sensed with a resistive divider.
The input voltage which when applied to the installed resistive divider causes 250 mV peak to occur at the voltage input for is the full scale voltage.