I have a question about a project I am working on. I am making a spotlight and the battery I have is an 18650 battery rated for 3.7 volts and 2000 mah. The light bulb is 1000 watts. My question is: how long can I run the light bulb for on just this one battery at a full charge. Also, how many of this specific battery will I need to purchase in order to reach a maximum use time of 1 hour? Thank you in advance for helpful answers.
I'm sorry, but you've made a big mistake.
First, a basic fact. Power (electrical power, that is) is equal to voltage times current. Now let's look at what you are trying to do.
A 1 kW bulb operated at 3.7 volts will require (1000/3.7) or 270 amps. I expect you have no idea what this implies, so I suggest you Google "ampacity", find a table, then look up how big a wire you'll need to carry 270 amps. The wire will need an area of 300 to 400 kcmil. You can do the math (and the research to find out the numbers), but let me put it this way: you'll need wires about 1/2 inch in diameter (for copper - aluminum will be bigger).
Of course, finding a battery which will supply 270 amps is just not going to happen, so you don't really need to worry about wire size. And you're not going to find a 3.7 volt, 1 kW bulb, either.
So, let's say you made a battery string which produces 120 volts, which is a nice standard voltage. How many will you need? That's simple, 120/3.7 equals about 32. Current is now 1000/120, or just about 8 amps, and you should have no trouble finding a battery which will do that.
Now, how long will such a battery last? With a capacity of 2 A-hr, at 8 amps the battery will discharge in 15 minutes (2 A-hr / 8 A equals .25 hours).
And how many batteries will you need to last one hour? 4 strings of 32 each, or 128.