Efficiency

When transferring energy from one form to another, there will be some energy lost during travel.
Like when you charge your phone using electrical energy and it goes into your phone's battery for chemical energy, it loses some of its energy due to heat. 

There is an equation for calculating useful energy:

Efficiency = Usefull energy output / Total energy input.

But if you are using Power:

Efficiency = Useful power output / Total power input.

Let's take two lamps, one powered by iridescent bulbs and one powered by LEDs, and let's say the iridescent lamp can convert 45 joules to light energy, while the LED can convert 225 joules into light lets figure out how many times more efficiently the LED lamp is compared to the iridescent bulb.

First let's give 300 joules of power to each bulb, and figure out which one is more efficient with the 300 joules. 

So we divide the 45 joules from the iridescent bulb by 300, then we get 0.15, and then we can calculate the LED bulb which is 0.75.
Then we divide the 0.75 by the 0.15 giving us 5, meaning the LED bulb is 5 times more efficient than the iridescent bulb. 
If you mess up the flip of the order of division, you will notice immediately because you will get a value bigger than 100% or a value bigger than 1.

Most of the time, you would want this in either decimal or percentage terms, so to get these numbers, you would need to multiply it by 100.
So, if you wanted to convert 0.15 and 0.75, you would need to multiply it by 100 to get 15% and 75%, then once again we can do 75% / 15% to get 5 again so that we know fully that the LED bulb is 5 times more energy efficient than the iridescent bulb. 
If we were to do the equation wrong, instead of dividing the 225 / 300.
We instead do 300 / 225, we would get 1.33 or 133%, which is impossible as you can't go over 100% in efficiency and not over 1 in decimals because this would be making joules of power which is incorrect because matter cannot be created or destroyed but it can be transferred.

Let's take a new subject, a microwave, with an efficiency of 70% and a total output of 800 watts. Once the equation is: Usefull power output / Total power input, it is 70% / 100% which is 0.7. So to get the useful power output we have to once again multiply the 0.7 by the power input of 800 watts.

So we multiply 0.7 * 800, and we get 560watts, as you know all devices leave some sort of waste when they are using energy, like when you use a electronic device some of the chemical energy and lose some energy in the form of heat.
But for things like electronic heaters, the "wasted energy" is then turned intoT