Screw volts. If you are talking about house wiring, then use 120 volts.
Watts roughly equals volts times amps. I say roughly because it gets more complicated for things like motors. But for lamps it is OK. If you have a 1 amp lamp, then it is using 120 watts. Roughly.
Since lamps usually aren't rated in amps, you have to go the other way. Amps equals watts divided by volts. Roughly. So, if you have a 100 watt light bulb on a 120 volt circuit, you are pulling a little over .8 amps. Roughly.
Two 100 watt bulbs? Then it would be roughly twice that amount. And so on.
I keep saying roughly because of other factors. Lamps draw a lot more current (amps) at turn-on than they do while running.