The amount of heat delivered by a solar water heating system depends primarily on the amount of heat delivered by the sun at a particular place (insolation). In the tropics insolation can be relatively high, e.g. 7 kWh/m2 per day, versus e.g., 3.2 kWh/m2 per day in temperate areas. Even at the same latitude average insolation can vary a great deal from location to location due to differences in local weather patterns and the amount of overcast. Calculators are available for estimating insolation at a site.