Energy calculation example 1

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Calculations are important. Numbers are not always what they seem to be. The amounts of energy can be very small or very big. This is not related to the effort which is required to use a method or develop a strategy. Consequences are also important, for example for the environment. Investments required for starting also have to be considered.

Example 1: Batteries

Batteries Energy indication: eg 1200 mAh means for one hour (3600 seconds) 1200 mA = 1.2A To know the energy you have to multiply by the Voltage, say this is a 9V battery: P = 9 * 1.2 Joule/second, total energy is E = 9 * 1.2 * 3600 = 38880 J

(You have to calculate in standard units, like A=Ampere, V=Volt, C=Coulomb, then you get J=Joule.)

So the same battery, but running at 1.4V has a total energy of: E = 1.4 * 1.2 * 3600 = 6048 J. Now...these are the calculations, but reality is chemistry inside a battery, so depending on what is inside and the quality, the battery might be able to deliver this energy or less.

Example 2: Solar Cells

Solar cells can be compared to batteries. There are some things you need to know:

  • Exposure

But only if they are fully exposed the Sun. But the Sun is not always in one place. So all fixed solar cells are only totally efficient within a small time range. Directing the solar panel to the Sun costs energy, and besides it requires maintenance of mechanical parts.

Depending on the position on the Earth the solar energy differs a lot. The second factor is the climate: in winter the radiance is less than in summer: the Sun is lower. (Depending on the altitude of the position.)

A site which can tell you the consequences of position on Earth is: [ ]. You can give data like country, city, and positioning of the solar panel. Then get indications for months in kWh/m2/day units.

  • MPPT relation

If you know this you have to make the "load" of the cell optimal. This means the cell has to be connected in a circuit which has the right resistance. Otherwise you don't get the indicated energy out of the cell. There are "intelligent" microcontrolers or chips which can take care of this. More details will be given at another wiki-page [1].

Example 3: Comparison with a banana

If you know the energy in Joule, you can compare:

eg eating a banana means your body gains (on the average banana) 202 kJ.

eg charging my cellphone is 4 hours at 5V with a current of 700 mA, calculating: Energy = 4*3600*5*.7 = 50400 J = 50kJ so with the energy out of a banana I could charge my cell phone 4 the same time delivering the same energy using a dynamo and my muscles for 4 hours (hand crank USB device) is not really comfortable for a human being!

eg a human being of 60 kg running up a flight of stairs, 3 meter high is 1900 J = 2kJ [2] Can you keep running up flights of stairs during 25 * 3.5 seconds = 90 seconds for charging your cell phone? Yes you can, but the dynamo would have an efficiency of 50%, this becomes 180 seconds, 3 minutes...which is still perfectly possible, but slowly you'll get a bit tired of charging your cell phone this way?