The thermal losses on the surface of a pool is significant and can drastically cool down the pool. This will get worst when even a slight cool breeze is blowing over the surface of the pool. The more the thermal loss the more panels you would need to try and maintain the desired temperature. It is therefore highly recommended to always use a thermal pool blanket to cover the pool when not in use. Even for indoor swimming pools this is important as the evaporation from the surface of the pool will cause heat loss and high levels of humidity inside the building.
Please note that the use of the correct type of pool thermal blanket will also substantially reduce evaporative water loss, filtration pump time and chemical use and therefore we would highly recommend it even if you are not interested in a heated pool.