If you're using your water pump so much that you're worried about its energy usage, you should probably think of things differently.
How big will your tank be? Let's call it 50 gallons. My pump can do 2gpm easily. If I run my whole tank empty, that will only take about 30 minutes. Lets say my pump uses 8 amps. We're looking at a max of 4AH @12v to use all my water. Thats only 48 watt hours.
A single golf cart battery holds nearly 1000 useable watt hours.
You'd have to have a lot of water in storage and a lot of usage demand for the electrical side to be the limiting factor.
Trust me, your water pump isn't the issue.
Patina enthusiast and professional busman