Before I tell you how, let me explain a few simple principles. First, water expands when heated, which is why your car's radiator overflow bottle fills up when your cars engine warms up. So if I take a black hose (with one end closed) fill it with water, lay it in the sun, water will come out the other end...It's really that simple and here's how to fill your water tank (without an electric pump).
This process is slow but sure and you will pump hot water, but as it sits in a tank it will cool down....so here is what you do. Depending on the depth of your water table decide how much flexible black pipe you'll need...the more pipe laying on the ground, in the sunlight, will determine the volume produced. Lets assume your water is down the well sitting at 30' (called the Water Table). I would purchase 140' of that flexible black pipe and fit each end with a check valve...making sure the check valves are attached so one end allows water in (goes down the well into the water) and the other allows water to exit. The exit valve should be put on after the pipe is lowered down the well (over 30') and filled with water so as to remove as much air as is possible. I would use 1 1/4" pipe and check valves as they tend to pump more water also. Make sure you place a hose clamp tightly on the pipe about 35' (assuming Water Table is 30' and tie a rope to it and to a strong fixed object so the hose doesn't fall into the well.
There can be a number of different ways to accomplish this task. Say you have a pipe down the well already with a good check valve at the bottom it may be possible to attach the down the well end to the outlet of the pump. This will accomplish the same thing as long as the lines are full of water and the black hose/pipe is topside.
Now coil the pipe or lay it in the sunlight and water will exit the pipe as it heats and as it cools at night it will draw more in and pump more out the next day and so on. It's up to you to attach a hose of any size to your tank from here. You'll be surprised at the amount of water you can accumulate as clouds pass. You can use this same principle to heat a swimming pool or spa as well.