Solar PV vs bio fuels
Our member Blincoln has written a very interesting and thought provoking comparison on our forum:
You get to learn a lot of things about energy when you start to read about (and drive) Tesla vehicles. One thing is how much energy a car requires in order to move itself forward at a reasonable speed; for example, my Tesla probably needs somewhere around 20 kW to keep a steady 90 km/h on flat grounds. At the same time, my electric car has an efficiency of 90%; compared to a maximum of 40% efficiency in a diesel car. That begs the question then; how much energy do you burn in order to drive at 90 km/h using diesel fuel?
How do we generate the energy we need, in a renewable manner? There are many alternatives, such as solar PV, hydro or wind for electric cars. You can also grow energy crops to create bio diesel for diesel cars. So, which way is the most efficient between electricity and bio diesel, in terms of energy production area?
Let's calculate using a heavy car which requires 2 kWh/10 km, or 0.7 liters of diesel per 10 km, and that we wish to drive 18 000 km every year on average.
First out, through solar PV: A pretty mediocre solar PV panel has an efficiency rating of turning 17% of the total sunlight hitting it into usable electricity (while a really good one's rating is about 21-22%). On a sunny day anywhere in the world the sunshine provides roughly 1 kW/m² at the most if you have a surface angled directly towards the sun. This means we will get at the most 170 W per m² of solar PV surface. In southern Sweden, the yearly production per W of solar PV results in about 1000 Wh. An interesting side note is that this almost entirely depends on the weather throughout the year, not whether you are near or far away from the equator. On Crete, you will receive almost 3000 Wh/W per year. Yes, that is three times as much, all due to the weather generally being more favourable on Crete than in Sweden.
The yearly production of a mediocre solar panel in southern Sweden is thereby around 170 kWh per km². From 25 m² we will receive 25x0.17 = 4250 kWh per year. If we are using this electricity to charge an electric car we will have an efficiency of around 85% with the battery, which results in 3600 kWh usable electricity for the car, which is enough for 18 000 km per year.
You can create bio diesel from growing oil plants: On Wikipedia you can find the interesting page Table of biofuel crop yields which lists how many liters of bio diesel you get per year, per m² from different kinds of plants. In Sweden, you can easily grow rapeseed for example. How much will we need of that in order to drive said 18 000 km per year?
We will need 1800x0.7 = 1260 liters of bio fuel. Rapeseed, according to the Wikipedia article, will provide 1190 liters/10 000 m²/year, which means we will need a grand total of 10 500 m² of crop growing area.
To drive a heavy weight electric car, powered by solar PV in southern Sweden needs 25 m². A comparable car running on bio diesel needs 10 500 m².
It is 420 times more efficient to drive an electric car powered by solar PV.
Really, it is perhaps not so strange that it is more efficient to convert solar energy into electricity stored in a battery which then powers an electric motor than it is to let plants use the same solar energy to create dextrose through photosynthesis, which in turn through further chemical reactions turns to rapeseed oil in the plants which then a farmer has to harvest on his lands, transport to a factory where the rapeseed oil is extracted, and then in turn transport the oil to a refinery where bio diesel is created, only to again transport the diesel to a fueling station where it is filled up in a vehicle and then burnt in an engine which makes more waste heat than actual power to the wheels. But, the fact it is such a massive difference should be an eye opened for many people.
Most people should be able to find 25 m² of free roof space somewhere nearby. 10 000 m² of arable land is a bit harder. If we look at the total area needed for all of Sweden's transports, we are talking about half the area of the island of Öland, compared to about a tenth of Sweden's entire surface. Almost twice as much total arable land as we have in the country today. 
) att min solcellsanläggning toppar runt 18,5kW, men bara när det är växlande molnighet. Är det klarblå himmel så ligger toppen bara runt 14-15kW. Anledningen är uppenbarligen att panelerna kyls ner när solen går bakom moln. När den kommer fram igen får man en jätteboost som varar i några minuter. Man kan nörda in sig hur mycket som helst i det här ämnet.
Tack för att du delade med dig av dina beräkningar, Blincoln!