Cover first
The biggest heat loss is evaporation from the surface, especially at night. A cover at night keeps the day's warmth. Before you think about heating, think about a cover.
Heat pump
The most common solution. It works like an air conditioner in reverse: it takes heat from the air and gives it to the water, delivering several units of heat for each unit of electricity. It goes after the filter, outdoors, where air flows freely. In April and October, while the air is still mild, it runs very efficiently.
Solar collectors
Black pipes or panels on a roof, through which pool water flows. They use no electricity, but need an area of about half the pool and plenty of sun. A good option for houses with a free south-facing roof.
Which option for which pool
Small pool or spa: a small heat pump, quick results. Large family pool: a cover plus a heat pump with enough output. Holiday lets: a heat pump so the pool opens from Easter. Dark liners help a little too.
