First, what you do mean by a 1000 watt computer? Do you mean a desktop with a 1000 watt power supply, but you have not measured actually power draw (in watts) of the computer? Or do you mean a desktop computer where you have measured the actual power draw in watts and it is 1000 watts?
I ask because desktop computers frequently have power supplies that have watt ratings that are significantly higher than what the hardware will actually draw.
If you KNOW that the computer is drawing 1000 watts, then you can use any number of UPS selectors to enter in that wattage and get a recommendation. For example, here is APC's selector (this is the brand I tend to use for my UPS devices):http://www.apc.com/us/en/tools/ups_selector/US/en/server/load
That selector will recommend this if you input 1000 watts:http://www.apc.com/us/en/tools/ups_selector/US/en/server/load
It should run about 7 minutes under that load according to that selector, but costs $560.
If you are talking about a desktop with a 1000 watt power supply, then you need to get some sort of measurement of the actual power draw in watts. The best way to do this would be to get a device that will measure the power draw such as a Kill A Watt (https://www.amazon.com/P3-International-P4460-Electricity-Monitor/dp/B000RGF29Q
) and then run the computer doing the most power intensive thing (for example, if you game, then run a game) and see how many watts it draws. If you don't want to go that route, then you can estimate you watt draw by using a power supply calculator. You can find one here at NewEgg, but there are many others out there:http://images10.newegg.com/BizIntell/tool/psucalc/index.html?name=Power-Supply-Wattage-Calculator
Enter in the specs of your computer (CPU, graphics card, etc) and see what power supply it recommends. And even then, keep in mind that most selectors will "pad" their recommendation to leave you some headroom for other stuff you might have forgotten to include and safety factor (I believe it is generally recommended to not get a power supply that exactly meets your power requirements, but leave some room for a number of reasons).
You will find that unless you are running a desktop with two powerful GPU cards, you will likely be no where near a recommendation for a 1000 watt power supply.