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It's almost certainly insolation (sol, not sul). UV/IR rejecting film on the windows might help, given mild enough winters that blocking it year round is fine. Check out https://youtu.be/uhbDfi7Ee7k

If you can find someone willing to do it, dumping the heat (pumped out as air conditioning) directly into the pool would be quite efficient relative to heating the pool separately. Have it dump to the ambient outdoor air only as overflow when the pool's thermostat is satisfied (upper 80s or whatever).