Never over pump your water well. Water wells all have GPM ( gallon per minute out put.) If you have a water well that makes 10 gpm, and you put in a 15 gpm pump, you will be over pumping your well. And cause damage to it. This will cause liquefied sand and sediment to sand in your well. After a while the sand will be so bad in the casing is no longer room for water so well is ruined and sanded in. Your better off to use the correct gpm out pump or less. Never use too large of water well pump. Most over the counter well pumps are cookie cutter sizes. When you blow the well this will give you a idea of what GPM out of water. Once you know the GPM out put, then you will know what pump to use. NEVER BUY TO BIG OF A PUMP. If you have a high out put well, then larger pumps are ok. Just because you buy a 20gpm pump, does not mean your well pump 20gpm. Matching the right GMP pump is very important. I have had customer buy the well pump, before i even finished the well drilling. How can anyone buy a pump, without 1st know the GMP out put. Save yourself a lot of trouble. Only buy your well pump after you know your well water out put gpm! Our water well drilling and troubleshooting book will help. Click here for books.