Solar pool heaters work better with variable speed pumps because the optimum flow rate can be achieved to maximize solar performance. A single speed pump is typically fine to use with solar pool heating, but if you want maximum pool heating, a variable speed pump can both save you a tremendous amount of money and provide the proper flow rate.
Most solar pool heating panel manufacturers recommend a flow rate of 4 gallons per minute per panel. If you have seven panels, you need a flow rate of 28 gallons per minute. Anything less and the efficiency drops off quite rapidly. Anything more and you are wasting money on your pumping costs, pumping water at a rate higher than required for solar pool heating to be efficient.
When we have a solar pool heater installed, we insist that every job gets a flow meter installed (at least temporarily). The pump can be programmed to operate at the proper speed when solar pool heating is likely to be required. With an automatic controller, the pump can be set to go to optimum speed whenever solar pool heating is required, but reduce the speed when heating is not active.
There are lots of other variable speed pump resources on my Solar Southwest Florida blog. I have also assembled some Pentair variable speed pump marketing videos below. Enjoy!