jcaldeira, I just noticed your question concerning how much the drop in water temperature is over night with the drums of storage water inside my greenhouse. The solar heat up of the water barrels inside the greenhouse, and the drop in the water's temperature over night varies daily and also from season to season. However, to answer your question I took the water temperature yesterday at dusk, and again the following morning. Note that in the northern hemisphere during January the sun is very low on the horizon during the day, so January's solar production is the lowest of the year. Anyway, the water's temperature at sunset was 61.5 F, the next morning it was 57.5 F, a drop of 4 degrees. This translates to a release of 4 free BTU's of heat into the greenhouse for every one pound of water inside the barrel. Each 55-gallon drum contains 551 pounds of water. There are 100 drums (acting as benches) inside the greenhouse Therefore, 100 barrels of water inside the greenhouse last night gave of 4 X 551 X 100 = 220,400 free BTU of heat. Note, I had the greenhouse propane heaters set at 55- F, so the water would not go much lower. One could get 10 or 15 times more free BTU's by setting the propane heaters at 35 F night temperatures, but I want the extra winter growth, from the trees by maintaining the higher temperature range. The amount of released solar heat will increase in February, March, and April. From May through August the water storage actually helps cool the greenhouse during the hot summer days by absorbing the daytime heat. - Millet