Quote:
Originally Posted by bernard
But 5 kWh of what? Not an electric energy, but what? Thermal energy?
How can I compare 4 kWh of electric energy with 5 kWh of Thermal energy?
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There are several types of energy but the quantity is always measurable in joules (J). 1 joule of electrical energy is exactly as good as 1 joule of chemical or potential or kenetic energy.
Also one type of energy can be converted into the other, the amount of energy involved always stays
exactly the same, you just don't always get 100% of what you
want. An electrical heatingelement converts electricity in heat, and does that almost 100% efficient. Your car-engine uses the chemical energy present in petrol to bring your car in motion, but it's not very efficient, a lot of energy is 'wasted' in the form of heat. (But the total amount of energy stays exactly the same.)
The trick is to get the form of energy you want by using as little energy in total as possible. This is better for the environment and often better for your wallet, although sometimes that takes a few years.
For example, an electricityplant (gas/coal) converts chemical energy into electricity, but it's roughly 35% eficient. A gasfurnace for the central heating of a house is roughly 98% efficient nowadays. So heating your house with natural gas is more efficient than using electrical heating.
But if you look at a heatpump with a SCOP higher than 3 (And they exist with a SCOP of higher than 5) then it is suddenly more economical to use this inefficiently produced electricity for heating your home.
But a short answer to your question: they are all measured in joules, or kWh or BTU or kCal because the amount is the same, it's just another form of energy.