How do we calculate a imput wattage on a fixture?

Wattage is a result, not an input value.

The simple formula is P= I*E. The translation is Power or Watts is equal to I (Amperage) * E (Voltage)

For example, a 15 Amp circuit using 120 V will yield 1800 Watts, 15*120 = 1800.

I say “simple” because there is another way of calculating the wattage by including the "pf" (Power Factor) in the formula.

What you need to be asking is what the listed amperage of the light is or the "VA" (Volt Amps).

The listed wattage of the lamp will tell you what size circuit needs to supply the lamp. For example, by code the smallest sized power branch circuit in a residence is 15 Amp and the standard voltage here in the U.S.A. is 120 V. As before, that equates to an 1800 Watt circuit. If the lamp uses 700 watts then all is fine. However, if the lamp uses 2200 Watts then you would need to put the lamp on a circuit no smaller than 20 Amp / 120 V.

For example: 20*120= 2400 VA or Watts.

There is no such thing as input wattage for a fixture. Wattage is a measurement of draw, or load.

What is it you actually want to know?

Ohm's Law (http://www.google.com/search?q=ohms+law&ie=utf-8&oe=utf-8&aq=t&rls=org.mozilla:en-US:official&client=firefox-a#hl=en&client=firefox-a&hs=bbw&rls=org.mozilla:en-US:official&&sa=X&ei=ArfqTPq2D8XflgfczqCRDA&ved=0CBkQBSgA&q=ohm%27s+law&spell=1&fp=269988b335e72e7d) might help you.

Are you looking for how much wattage an item uses when on or how much wattage an item uses over a period?