Tom,
I will give you a very general and basic idea. You need to determine how much water is required for irrigation and your household use during irrigation. Then you need to determine you wells output and subtract that from the need.
So you need to find the flowrate and total volume for each zone of your irrigation. Example: 10 sprinklers that use 1.5 GPM for 20 minutes = 15GPM for 20 minutes = 300 gallons. Then add up all of you zones for total. If you have 6 zones that are the same your irrigation system will need 1800 gallons in 120 minutes or 2 hours. Add up the maximum amount of water you will use in the house while running your irrigation (ie running washing machine, and taking a shower etc) I will make up a number for this example and say 120 gallons per hour in household use or 240 gallons in 2 hours. Adding the irrigation to the house hold and you will need 2040 gallons.
Now you need to determine what your well will dependably produce. Let’s say 10 GPM for this example. Therefore your well will produce 1200 gallons in 2 hours while you are watering. For a starting point on determining the tank size you can subtract the make-up water from the well to the demand or 2040 – 1200 = 760 gallons. This is the minimum amount and that is when the tank is at the lowest level. This does not have any margin for error or little water leaks etc.
I can be pedantic. So I would make a time table to see how much water that I can expect the tank to have in the tank at the end of each zone’s cycle, because it is unlikely that they will have the same flowrate. This should give you an idea of how to size the tank but not the pump.
For the pump, you need to know two things, the maximum flow rate and the pressure required. The irrigation system you already know from above. But the instantaneous flowrate for the house will be different. You need to get an idea of the flowrate when everything is running (ie 2 showers at 1.5 GPM + sink at 1GPM etc.) Then you can determine the pumps flowrate.
The pressure or head can be a little trickier to determine. It can be expressed in psi or feet of head: 1 ft = 0.43 psi or 1 psi = 2.31 ft. First you need to determine the pressure required at the irrigation system and your house. Maybe, 40 psi for each sprinkler head and 40 psi for the house, or about 92 feet. Then you need to know the elevation of the the sprinklers above and house above the tank bottom. Let’s say 30 feet elevation (fairly flat property). Next is the tricky part, it is determining the friction losses and minor losses in the pipe, valves and fittings.
Probably the easiest way for you is to use one of the many online calculators. I just put “friction loss in pipe†into google and a number of calculators came up where you enter the inside diameter of the pipe, the length, and the flowrate. Here is one link
http://www.freecalc.com/fricdia.htm (for this one: use PVC if you are using any type of plastic, they should be close; viscosity is 1; specific gravity is 1). You need to find the pressure loss in each section of pipe at its maximum flowrate.
Minor losses are in fittings and valves. Don’t worry about the house, it should be covered in the pressure required at the house. If you don’t have a bunch of elbows and tees, you can probably neglect the loss or add a couple psi to your final number for saftey. But your sprinkler valves and backflow preventers can add up. You can usually find the expected loss for these on the manufactures website.
You want to find the maximum required pressure, so for each sprinkler head (and your house) you need to add the pressure required, the elevation above the tank, the friction loss of each section of pipe going to the device, and any minor losses from valves. The highest number will determine the required head for the pump. Note: you should talk to a pump supplier for recommendation on a pump.
This is very basic information. It does not account for many possibilities and senarios. After typing this, I remembered a good website with irrigation design information.
http://www.irrigationtutorials.com/