What is the cheapest heater to heat a 500 gallon outdoor spa?

The cheapest heater for a 500-gallon outdoor spa is typically an electric heater, as they are often more affordable upfront and efficient for smaller volumes of water. A 5.5 kW or 11 kW electric heater can effectively heat the spa, depending on your location and heating needs. For cost savings, consider using a solar cover to retain heat and reduce heating time and energy consumption. Additionally, look for models with good energy efficiency ratings to minimize long-term operational costs.