The concept of the solar heating system gathering free heat from the sun is attractive and does work. However in this climate it is unreliable at maintaining a constant temperature in the pool. The usual heat input from a correctly sized solar panel system is 2-4°C (35-39°F) above the natural temperature of the unheated water; the higher rise occurring at the hottest part of the season. The panels are controlled by different controllers set to measure the temperature of the water and the temperature of the air. When the air is sufficiently warmer than the water in the pool, water flow through the panels starts Standard pool solar collectors require a bank of panels equivalent to approximately 76% of the surface area of the pool. Ideally the bank should be angled at 15 to 45 degrees to the horizontal and be facing south. The distance between the filter pump and the panels must be taken into consideration when designing and siting such a system. Solar panels are most effective when combined with a fossil fuel heater.