Updated June 2017

How much solar energy is available depends on locality, cloud cover and season. This article explains its measurement, and how to know exactly for free.


This map shows average yearly global solar input. Solar power is totally feasible in all areas where input exceeds about 2-2.5 kWh/m2. Pic: courtesy of SolarGIS.

Right now (2017) 7000 times more solar is available than is needed to supply global needs. Around midday, much of the world has 800-1000 watts of solar energy for every flat square metre. Currently, affordable solar panels are 14% to 20.5% efficient. That is about 120 to 140 watts per square metre. But, as this article explains, not all is available for use

How much solar energy is available – reality can be less than expected

Technically, solar energy is shown as above (in watts per square metre). The solar industry, however, simplifies this for non-technical readers. It likens the sun’s energy to daily rainfall. Apart from calling it ‘insolation’ it expresses it similarly.

Solar is best visualised as sunlight being measured (much as rain in a rain-gauge) via a standard-size ‘light bucket’.When ‘full’ that amount of energy is equivalent to 1000 watts per square metre. The industry calls each ‘bucket full’ one Peak Sun Hour. This is usually abbreviated to 1 PSH.

In areas like central Australian or Arizona etc that bucket typically fills 5-6 times a day much of the year. That 5-6 PSH is thus 6000-7000 watt hours. During a Melbourne or Madrid mid-winter, however, it will be far less. There, solar input may be as low as 1-1.5 PSH.

For exact and full data,  first establish your geographic coordinates (via Google or a good map). Open https://eosweb.larc.nasa.gov/cgi-bin/sse/[email protected]  Enter that data.

This bring up more information than you need.  to know. Open the table ‘Radiation on an Equator-pointed Tilted Surface’. (By ‘Equator’ NASA means solar modules facing South in the northern hemisphere and North in the southern hemisphere.)

The data you now see is based on a running ten-year averaged basis. It is thus corrected for local and global changes. As you will see,  exactly-correct orientation is unnecessary. Five to ten degree differences make only minor change. Furthermore, you can tilt the modules to obtain more input in winter – at the expense of summer. And vice versa. Some people thus have them adjustable. 

How much solar energy is available –  the energy that’s actually usable

Whilst a lot of sunlight comes in, solar modules are far from efficient. Their electrical output is likely to be 12%-21.5% of that PSH. It  increases as solar module technology evolves, but is already close to the theoretical limit. However, solar capacity becomes cheaper year by year. The common limit now is space for it.

How much solar energy is available  how and where to go from here?

That above shows how you can establish your most probable solar input. You can thus see whether it is worth going ahead with your solar energy plans. How and where to go from here is covered in my two globally-selling solar books.

As with my articles, my books are in plain down-to-earth English. And, as I am an engineer, they are technically correct. My bio is at: https://www.successfulsolarbooks.com/collyn-rivers/

Full details of solar for homes and properties – see Solar Success. That for boats, cabins and RVs is in Solar That Really Works! and Caravan & Motorhome Electrics. See also Our all solar house

If you are interested also in RVs – see our associated caravanandmotorhomebooks.com