we've seen a few times now that energy can be transferred from one form into another form however it's not always transferred into the form that we intend because most devices aren't 100 efficient so in today's video we're going to take a look at what efficiency is and see how to calculate it both in terms of energy and power let's start by considering a lamp its input energy is going to be electrical energy wireless output will be split between light energy and heat energy as the purpose of a lamp is to provide light we consider the light energy as useful energy output whereas the heat energy can be classed as a waste energy or non-useful energy output the efficiency of a device like this is just the proportion of the energy supplied so the input energy that's actually transferred into the useful energy output so transferred into light energy when it comes to your exams you'll need to be able to calculate the efficiency of a device as a decimal or a percentage by using the equation which is efficiency equals useful energy output divided by the total energy input or if you're using power then its efficiency equals useful power output divided by total power input to put this into practice let's imagine that we have two lamps and we supply 300 joules to each of them the one on the left has an old incandescent bulb which converts 45 joules of that 300 into light energy meanwhile the other one has a modern led bulb which converts 225 joules into light how many times more efficient is the led lamp first of all we need to calculate the efficiency of each bulb for the incandescent bulb we would do 45 joules which was the useful output divided by 300 joules which was the total input this gives an efficiency of 0.15 meanwhile for the led we do 225 divided by 300 to get an efficiency of 0.75 then to find out how many times more efficient the led lamp is we just divide its efficiency of 0.75 by the efficiency of the incandescent bulb 0.15 which gives us 5. so the led lamp is 5 times more efficient if we instead wanted these efficiency figures in percentage terms we would just multiply them by 100 giving us 15 percent and 75 percent and we could then do 75 divided by 15 to give us 5 again when you're calculating the efficiency one of the common mistakes you need to watch out for is to get the division the wrong way around if you ever do this you should be able to notice it though because you'll get a value bigger than one or bigger than 100 percent in our example here you'd get an efficiency rating of 1.33 or 133 percent which is impossible because it implies that you've got more energy out than you put in in the first place and if you remember the conservation of energy principle energy can only be transferred never created or destroyed let's try another example but with power this time this particular microwave is 70 efficient and has a total power input of 800 watts calculate the useful power output first we need to take our efficiency equation and rearrange it to get useful power output by itself we also need to convert our efficiency into a decimal by dividing 70 by 100 to give 0.7 finally we can plug our values into the equation which would be 0.7 times 800 watts to give us a useful power output or 560 watts now no device is 100 efficient because all devices produce some form of waste energy most often in the form of thermal energy an exception to this rule though is devices whose function it is to produce heat in the first place for example if electric heaters produce lots of thermal waste energy then it isn't really waste and instead would count as useful output anyway that's all for today so if you enjoyed it then please do let us know down below and we'll see you next time