The two most important formula that you need are:
V = IR
1 Watt = 1 Joules/Second
Joules is Energy, so Power is the rate of Energy consumption (or production)
How much current does a 100W household light globe use?
Power = 100 Watts
Voltage = 230 Volts
Current (I) = P/V = 100/230 = 0.43 Amps
An industrial plant uses a 1000 kWh in an 8 hour day, on average what is the required power?
1000 kWh = 1000000 Wh
P = Energy (kwh) / time (h) = 1000000/8 = 125000 Watts = 125 kW avg.