Two alternatives to clamp on ammeters jaws, One you can buy current transformers that can be installed without disturbing the conductors or two, hall efect sensors will detect the magnetic flux around the conductor.

After you have conditioned your signals you need to compute the phase angle and include it in your calculation, as a simple v*a*1.73... returns kilowatts plus kilo volt amp reactive The decision to correct will depend on your needs, but if you have enough memory left it would make a much more versatile instrument.

BTW.Your utility company probably already has these available as calibrated meters- They are installed in large industrial sites and can be read remotely. the basic instrument will be off the shelf from the instrument supplier that they use, and you could just ask your utility where to get one. You just supply the inputs. Terry