You can actually get up to 15 HP in single phase. Not knowing the actual pumping level, and assuming it drops from 70' to 100', a 15HP can produce up to 175 GPM at 50 PSI. Large single phase motors take a lot of amps to start, like 300 locked rotor amps. But when up to pressure and starting normally with the CSV in the almost closed position, the CSV can reduce the start amps greatly like as much as 50%. But on the very first start or starting after the system is drained, the pump will start without any head against it and will have the high amp start.
Once up and running a 15 HP will draw 72 amps when producing 175 GPM. But with a CSV it will draw much lower amps when using lower flow rates. Some pumps will drop in amps by as much as 50% when using a CSV. We can look at the Horsepower curve for several pumps and determine which would have the best amp drop.
And even though that is a 175 GPM pump, the CSV can make it safely pump as little as 5 GPM. If you use less than 5 GPM the CSV will let the pump cycle on and off. But with a little pressure tank like an 80 gallon size that holds about 23 gallons of water, it will be a slow cycle. When set up properly, when using only 2 GPM, the pump would run for about 3 minutes and be off for 12 minutes, which is a very slow cycle and won't hurt a thing. When using more than 5 GPM, the CSV will keep the pump running constantly and the motor will only draw the amount of amps needed for the flow rate being used.
A 10HP would produce up to 105 GPM from that depth and pressure, would only have cold starting amps of 200, and with a CSV would work down to 5 GPM as described above.
I have systems with as large as 75HP that will do 1000 GPM on the irrigation, and is also tied into the golf course superintendents house and the club house to do as little as 5 GPM when needed.
Having said all that about being able to use as large a pump as you want when using a CSV, there are reasons you may want to consider a smaller pump. You have a lot of stuff dependent on the well water, and you only have one well. That is like putting all your eggs in one basket. Even though the CSV makes pumps very dependable and last much longer, when the well pump does quit, it will be at the most inopportune time. Pumps always quit at 5PM on Friday, or the start of a holiday when you can't get someone out for service.
If you had two wells, you could have a two pump system, which would be more efficient at the lower flow rates and gives you a spare pump when needed. You could put a 5HP in each well, or even better yet something like a 7.5HP and 3HP combination. That way the 3HP is the first to come on and the only pump that will run until you use over about 30 GPM. When more than 30 GPM is used the 7.5HP would come on automatically and supply the extra water needed up to as much as the 100 GPM total available. This way the starting amps would be greatly reduced, the system would be much more efficient at low flow rates, and you have a backup pump when needed. The wells do not need to be close to each other or have any wires between them. The pressure switch setting for the small pump would just be 10 PSI higher than the large pump, and this will make just one pump run for small flows and the two pumps work together when higher flow rates are needed.
All of these numbers are based on the pumping level in the well being at about 100' and supplying the system with 50 PSI. If the pumping level is different, or the pressure needs to be higher, the size of the pumps will change. A well test after drilling would keep us from guessing at these numbers and allow for a much more precise sizing of the pump(s).
