It is said that Mankind uses about 15TW of energy continuously, with most of that coming from fossil fuels. However I think there is another just as valid way of looking at how much energy we use that gives a very different answer. This is relevant when people say in a discussion about reducing fossil fuel use “But what we really have is an energy problem” as if it is somehow a more profound assessment of our situation. I consider it misleading for the following reasons:

Consider heating your house on a cold but sunny day. You could heat it with electricity from the grid, by having large windows letting the sun in, by having solar panels power a heat pump, or by burning some dried out weeds that you have lying around. All of these would work but they would get counted very differently. Using the grid gets counted as energy in the 15 TW figure, so might solar panels (who knows if they are not connected to the grid). But compare solar panels connected to a heat pump, to having the sun stream in through glass windows. This is pretty much the same thing, however one may get counted as using energy, the other definitely will not.

Now compare solar panels to the weeds. Solar panels take energy from the sun, convert it to electricity which then heats the house. So does growing weedy plants, chopping them down and burning them, but with the energy stored chemically rather than electrically. The amount of sunlight hitting the weeds over their lifetime would be huge, of which only a small fraction is made use off when you cut them down and burn them. So how much energy do you “use” when you heat your house this way?

1. Nothing because it wasn’t electricity
2. The amount of heat that was given off in the fire, or 
3. The total amount of energy hitting those plants that came from the sun over their lifetime, even though only a small %, say 1% actually ending up as heat in your house.

I would argue that #3 makes the most sense, because If you then go and replace that land with weeds on them with solar panels, you are still using all that energy one way or another, as electricity rather than chemically. (Ecologically of course it is different, but not necessarily even better to have plants if they are intensively farmed and you consider the ocean dead zones etc from fertilizer runoff.) So, where does this lead? 

It is said that about 50% of all land based photosynthesis has been co-opted by humans in some way, this includes land directly to make crops, indirectly to make grass that is eaten by cows to make milk, meat, and land used to grow cotton etc. As I am going to make a rough estimate, lets go with the 50% figure. Now world energy consumption is quoted as 15 TW average. (wikipedia) This figure includes sources of energy such as fossil fuels, nuclear, and renewables, but does not include the energy needed to power those plants the support humanity. So what is that energy? The land surface area of earth is 148,940,000 km^2 or 1.514 m^2. Assuming plants cover 1/2 the land as a very rough guess (deserts, high mountains excluded etc) and humans use half of this, then we use 3.7513 m^2 of land. So what is our real energy usage that we would need if the sun went out? The average insolation of the sun in space is about 1366 W/m^2, but about 1000 W/m^2 at sea level when you take into account losses through the atmosphere. The average through all the day/night is ¼ of this because of geometry. (The surface area of a sphere is 4*pi*r^2, the radius facing the sun is pi*r^2). So without clouds the average would be 250W/m^2. As a rough estimate taking cloud cover to be 50% we get 125W/m^2 continuously. This is approximately the average amount of energy hitting the land that we use day and night. 

So how much energy do we get if we use this figure, and the land area calculated before?
125 W/m^2 *3.7513m = 4.715W or 4700 TW

Compared to 15 TW this is over 300* the amount of energy in the widely quoted 15 TW figure. Now this is deliberately not accurate because I have not even included any of the sea. How do you calculate the energy needed for our fishing? If you had to include the sun that powered the plankton at the bottom of the food chain that has to exist for fish to survive it would make the 4700TW significantly larger.

That troubling 1%

To me this means that mankind is already over 99% solar powered, we just don't realize it. To me that also means that 15 TW is not large at all, but small and when people talk about mankind's supposedly large energy and unsustainable energy usage, I don't agree at all as they are generally not talking about the energy needed to grow our food/cotton/fish etc. If 4700TW is large, then 15 TW is not. We definitely are using a large amount of the land area of the planet, but I’m not sure our direct energy usage of 15 TW or so is a problem, or even a meaningful number. The technology and resources (fossil fuels) we use to provide this energy definitely causes problems but this is not the same thing. Pointing out that it will be difficult to replace the source of the 15 TW is also valid, but saying the 15 TW itself is too large is not so valid without making some pretty sweeping judgments about what will be or is already technologically possible. If someone says that we really have an energy problem then what energy are they talking about? The 15 TW or the 4700 TW? Saying that 4700 TW is too much is just a roundabout way of saying that our current land usage is too much or is being used unsustainably.