Electrical Calculators is the best app for an electrician,it has many calculators that can help you in your work.

Amps to KW Calculator
Amps to KVA Calculator
Amps to Volts Calculator
KW to Amps Calculator
KVA to KW Calculator
KW to KWH Calculator
KW to HP Calculator
HP to KW Calculator
HP to Amps Calculator
Power Factor Calculator
Power Factor from KW & KVAR