Hi, that is a tough question to answer without knowing more details about your pump. Water pumps come in a range of sizes and some smaller pumps can run on a small 2000 watt generator, whereas some house pumps would need around 6000 – 8000 watts or more!...electric pumps are notorious for having quite high starting loads. So, you would need to find out the peak load (in Watts) required to start your pump. If you cannot find the starting load from the product manual or data plate, you could contact the manufacturer of your pump, or engage an electrician to take a reading.
Once you have that all important stating load amount for your pump, you can then choose the right sized generator to run your pump.