Updated December 2015
How sun input is measured for solar energy system purposes is much as for rainfall. It is by how much is captured in a day rather than minute by minute. By recording solar input over time, peaks and dips average out as the sun (apparently) sweeps the sky, and is obscured at times by cloud. Irradiation maps show the result in so-called Peak Sun Hours (PSH) – a solar industry unit – of which each is much the same as one hour of ‘strong’ midday sun. Technically, 1 PSH is 1000 milliwatts per square metre per hour. (In reality such irradiation is more likely to be 800 milliwatts per square metre per hour. The maps are available from met offices in most parts of the world.
This map shows where solar is economically realistic (i.e. most areas that are between latitudes +38 degrees and – 38 degrees). It is feasible outside such areas (coloured green) but needs a lot of solar capacity. Pic: original source currently unknown.
At present (early 2105) solar panels are typically 14%-20.5% efficient. This results in that captured being 110-160 watts/square metre.
Full information on every aspect of how sun input is measured and every aspect of solar in homes and properties is in my book Solar Success and also (for caravans and motorhomes) Solar That Really Works!.
(Re: How sun input is measured see also How much solar energy is available.)