Most Royal Fiberglass Pools, Inc. Spas, alone or independent, would require between a 150,000 BTU - 250,000 BTU gas or propane heater size to effectively spot heat the water about 15 degree increase in an hour; (this may vary higher depending on the climate).  By upsizing the heater to 300,000 BTU in the case of a Pool/Spa Combination for small and medium sized Royal Fiberglass Pools, Inc. pool models or to 400,000 BTU on the larger Sized Royal Fiberglass Pools, Inc. pool models will not only decrease the heat time of the spa but give similar degree rise in a 24 hour period for the pool.  This will vary by region and temperature but the use of a solar or hard cover would increase the economy for achieving these results.

If electric heaters are required, keep the spa independent or if it is a pool/spa combination, have isolation for spa circulation only on heat mode and use an 11 KW heater on the maintenance method.  By utilizing a solar blanket and/or a hard insulating cover on the spa, it will help maintain heat.  Wire size and circuit breaker size is important for efficient operation usually requiring a #8 wire and 30 amps or larger circuit breaker.   A 5-10 degree temperature rise an hour can be expected but remember it has to be a realistic starting temperature for this to be effective.