A HDD uses something like 10W. A SSD uses something like 2-3W. What does that mean for your battery? Like others have said, maybe a few more minutes per charge. But a SSD has many more benefits than just using less energy, it also loads your OS and apps a lot more quickly.
If your primary goal is battery life, small screen, no gpu, efficient cpu are your friend. HDDs just don't use enough power where switching to a SSD is going to be a meaningful gain. Your money is much better spent buying a spare battery.