Dang it, I was afraid the power company would want to charge you an arm and a leg.

So definitely on to Plan B. Sounds like a well beside the pond is probably your best option.

Fixing the windmill is certainly one option. Is the windmill tower still there? If so, that is one of the expensive components of pumping with a windmill. Is the windmill rotor and gearbox still there? That is the other expensive component. (Sometimes you can get a good used one at a farm auction.)

If the major windmill components are all gone, are the pump rods still there? If so, there is a slight chance you can pump the well just by reciprocating the pump rods. (This only works on very shallow wells.) It probably won't pump, but you can still pull the rods and get an estimate on the depth of the well.

If there are no well components left, then you can still measure the depth with a drop weight. I like using a big, heavy window sash weight. Sometimes old wells will get a crust built up in the well bore at the usual static water level. You want a big weight, so you can hammer through some moderate obstructions and get to the total depth.

Once you get your measurement of total depth, also measure the well casing diameter. Finally, note the material. It may be old and still made of PVC. If it is very old, then it is probably either straight steel pipe or a fluted galvanized pipe. IMO, the fluted galvanized pipe is the most likely to be rotted out.

You can START sizing your pump knowing the depth of the well and the diameter of the casing. However, you really also need to know the static water level, the producing water level, and the flow rate of the well.

You can readily figure out the static water level by lowering a cup into the well at known intervals until you finally recover water. The actual pumping parameters are by far the most difficult items to determine. IMO, actually testing the well is probably your best option.

Do you own a portable generator? If so, then you can buy a cheap submersible pump that runs on 110V AC and start testing your well. If not, then there are cheap 12V submersible pumps available at Amazon. (I have used a cheap Chinese 12V sump pump for several years at my farm to load water to irrigate my trees. Piece of crap still runs.)

Those types of pumps always have a trade off between lifting height and flow rate. Therefore, you need to size your pump to lift from its setting depth plus a little extra margin allowance. Then get the largest flow rate you can.

I would then test that system for a week at least, to determine how much water your well is capable of producing over the long term. Once you have determined that your well is capable of an acceptable minimum rate, THEN you can start sizing your expensive pump and solar system.

I think you will be disappointed with the flow rates of the submersible pumps. If your depth to water is less than 25' (usually), then you can utilize a surface jet pump. These are far more efficient, and you are more likely to be able to move enough water to keep your pond full.

If your existing well is shot, then you obviously need to drill a new one. However, I think it might be cheaper over the long term to build a "sump" for your well head. If your "producing" water level was at 25' depth, you might be able to dig an 8' deep sump, so you were only lifting water from 17'. It is much more efficient to "push" water, rather than "pull" water. In that case, you might be able to easily operate a large flow rate jet pump on your solar system and move the maximum water available from the well perforations.

If you do reach the point of drilling a new well, discuss all of these items with your drilling contractor. He will have excellent local knowledge of the most efficient ways to pump for a given aquifer depth and flow rate.

Sorry for the long discussion, it was the only way to throw out many of the items you need to evaluate to complete your supplemental water project.