Question

How to calculate input power of an electric heater?

Answer 1

There's little need to calculate the power of your heater, since you have already said it's a 1000W heater.

However, to address your confusion, your choice of which to use will depend on what values you know. They will all give the same result, and they are related by Ohm's law. Here's how:

You know Ohm's law, voltage equals the product of current and resistance:

E=IR

And you know that power is the product of voltage and current:

P=EI

Let's say you have a resistor, and you know the voltage across it, and you want to know the power dissipated in that resistor. P=EI, but you don't directly know I, but you can calculate it with Ohm's law:

I=ER

substituting:

P=E(ER)

which simplifies to:

P=E2R

If you have a 1000W heater which runs at 220V, I can calculate the resistance your heater must have:

1000W=(220V)2R

R=(220V)21000W

R=48.4Ω