Hi Patricia, we will certainly need to know how big the pool is including the depth. We did cover the subject on the old forum setup but I wouldn't know how to locate the old posts (James?)
For turn key heating at any time including night time if you are lower cost over night electricity then a air to water heat pump is a good idea.
Solar's fine but only tends to work when it's warm/sunny unless you have evacuated tubes. My reason for not using evacuated tubes is when the pool is up to temp you need to do something with the heat still being generated by the tubes so clever plumbing means you can heat a hot water tank for instance. Otherwise some kind of sun blind to shield the tubes to prevent over heating damage.