Mike Smith, of Palmerston North, asks :-

I read somewhere that enough solar energy falls on Australia in a day to provide the world's energy needs for a year. Is this correct?

Ralph Sims, of Massey University's School of Engineering and Advanced Technology, responded.

Not quite. Each year the world currently consumes around 500,000,000,000,000 Mega Joules (MJ) of primary energy – also called 500 EJ (exa Joules) to make it a smaller number.

Dividing this by 3.6 MJ (as 3.6 MJ is equivalent to 1 kWh of energy) shows that it is equivalent to around 139,000,000,000 kWh/yr – or around 140 tera Watt hours (TWh) per year of energy.

The sunlight continually arriving at the Earth (the irradiance) has an energy value of around 1370 W/m2. Since the sun shines all the time, this energy continually arrives at the Earth’s atmosphere. However, around 30 per cent of this sunlight is reflected back to space and the rest has to be spread over the whole surface area of the globe as the planet revolves. The sunlight is not just warming the flat disc shape of the Earth that is what we would see if looking at it from the sun, but it warms the whole globe of the planet. Therefore, each square metre of the planet’s surface on average receives around 240 W/m2 (though this varies between night and day and with the latitude of any given location. This is equivalent to 240 MW (mega or million Watts) on average for every square kilometre of the Earth’s surface – including the oceans – since there are 1 million square metres in 1 square kilometre .

Australia is not near the equator but it gets a high share of sunshine. So let’s assume it receives 240 MW/km2 on average. The area of Australia is 7,741,220 km2 – which sounds big but is only around 1.5 per cent of the total surface area of the planet (of which over 70 per cent is water and ice).

So the total sunlight the land mass of Australia continually receives (averaged between day and night) is equivalent to around 240 times 7,741,220 = 1,857,892,800 MW or about 1,858 TW (tera Watts).

One year has 8,760 hours ( or 8784 hours in a leap year). So in one year, Australia receives 1,858 TW for 8760 hours which is about 16,276,000 TWh/yr of solar energy which equates to approximately 58,560 EJ/yr. This is equivalent to around 160 EJ/day which is a lot more than the world consumes in a day (500 EJ divided by 365 days equals 1.34 EJ/day) but is much less than the 500 EJ of primary energy that the world consumes in a year.

In addition, it should be noted that solar energy cannot be used directly to provide energy unless it is first converted into useful heat or to electricity. If we assume the technologies to do this (solar water heaters, concentrating solar power, solar photovoltaic panels etc.) are on average 25 per cent efficient at converting the solar energy to useful heat or electricity, then from the total solar energy landing on Australia (160 EJ /day) only around 40 EJ/day would become available as useful energy in the form of electricity or heating. This equates to around 14,600 EJ/yr which is still far more than the 500 EJ of primary energy that the world consumes each year.