Graham,
I have some direct experience building a solar heater for a 500 gallon (~2000 liters ) hot tub. I used one 4x8 vortex pool heater panel and a 12V a Shuflo ag pump. The heater works great. I will be glad to share my learned experience with you. Pool volume, sunlight hours and temperature rise determines the number of panels. The temperature rise is the calculated by the average temp that the pool is exposed to during the coldest month you plan on using your pool. Also, the temperature rise is determined by the desired water temp of the pool. The pump chosen largely depends on how much head (vertical rise between water level and pump) and lift (vertical rise between pump and solar panel. I needed a self priming pump with 6 ft head and 4 ft lift. I also insulated my pipes. Once a pump is selected, power requirements can be determined. Let me know your requirements and I make some suggestions.
Thanks
Marc Medley
