In cloud conditions, the panel output can be as low as 80%, meaning you will need a reliable battery system to hold energy stored for those days when the generation via solar isn't enough.
[pdf] To optimize performance, tilt your panels at an angle that is 10-15 degrees less than your latitude. For example, if you live at a latitude of 40 degrees, set your panels at about 25-30 degrees.
[pdf] Where temperatures below about 95 °C (200 °F) are sufficient, as for space heating, flat-plate collectors of the nonconcentrating type are generally used. Because of the relatively high heat losses through the glazing, flat plate collectors will not reach temperatures much above 200 °C (400 °F) even when the heat transfer fluid is stagnant. Such temperatures are too low for
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