Vanessa, of Balclutha Primary School, asks :-

How much energy is in the Sun compared to a single light bulb?

Bob Lloyd, a physicist at the University of Otago, responded.

This is an interesting question and one that can be directly investigated, even by high school students. In fact we have a simple experiment in one of our second year laboratory sessions at Otago University Physics Department, which measures the energy falling on a square aluminum panel exposed to clear sky sunlight, by finding the temperature that the panel rises to over a set time period. The panel is painted black and thermal insulation is put around the back of the panel to reduce heat loss. The results of such an experiment give around 1000 Joules of energy falling per square meter of surface area each second. This amounts to a power level of 1000 Watts per square metre.

If we allow for the effect of the atmosphere, this value increases to around 1365 Watts per square meter emitted by the sun at a point just outside the earth’s atmosphere. Now if we know how far the sun is away from the earth we can calculate the total power emitted from the sun as the area of the sphere, of radius the earth sun distance, multiplied by the 1365 Watts per square metre. The earth sun distance is about 150 million km and so the area of the sphere is about 280,000 million million million square metres. The result suggests the total power radiated by the sun is 382.2 million million million million Watts. Compare this with a typical lightbulb which emits 100 Watts.

So the sun emits the same power as about 4 million million million million light bulbs.