kW / kWh: What’s the Difference? Power & Energy Explained

hi I'm Gary and welcome back to my channel Gary does solar now not everyone has an electrical or Electronics background and when you enter the world of solar there are so many new terms to learn and it's easy to get confused by it all I mean what is the difference between kilowatts and kilowatt hours to help out then I thought I'd make this short video to demystify these terms in just a few minutes and even if you already have a good understanding of the relationship between power and energy it's worth sticking around anyway as I cover some other stuff shall we get stuck in [Music] kilowatts and kilowatt hours thin you'll see kilowatts written on many electrical appliances you own and you'll see kilowatt hours on your monthly electricity bill but what do these terms mean let's start with kilowatts kilowatts is a measure of power and power is the rate at which work is done power is measured in a unit called warts named after James Watt who developed the concept of horsepower a kilowatt is one thousand watts in the same way that a kilogram is a thousand grams or a kilobyte of memory is a thousand bytes kilowatts are generally used in preference to Watts when talking about energy in the home as many larger appliances use more than a thousand watts of power looking at kilowatt hours then this is a measure of energy which means the amount of work done and these two terms are related energy is equal to power multiplied by time and because of the hours in kilowatt hours time is generally expressed in hours as opposed to minutes or seconds so we can think of energy as simply a level of power being applied over a certain number of hours it's easier to see this relationship between power and energy with an example here are two people about to race each other lifting heavy boxes using a rope the first person will pull with a certain amount of power but the second person will pull with double that amount of power okay let's start the race on your marks get set go as you might imagine then the second person wins this Race easily it only took them three seconds while the first person took six seconds but here's the thing although the second person put in twice the amount of power both people still expelled the same amount of energy this is because energy is equal to power multiplied by time if you double the power you half the time with this in mind let's see how power and energy work with electrical appliances let's take a kettle holding two liters of water and having a power rating of three kilowatts if the water is cold and we switch on the kettle we can expect it to take around four minutes to boil in order to work out the amount of energy required to boil those two liters of water we must convert the time into hours there are 60 minutes in an hour so four minutes is 1 15 of an hour sixty divided by four now we can work out the energy required using our little formula from earlier we multiply the power three kilowatts by the time taken in hours 1 15 to get 0.2 kilowatt hours let's now consider a kettle that is only two kilowatts the time taken to boil is longer now six minutes which when we convert to hours is one tenth of an hour the energy then is one tenth of the power which works out as 0.2 kilowatt hours we shouldn't be surprised that the energy is the same as for the three kilowatt Kettle because that is the energy required to boil two liters of water no matter what the power of the kettle is let's go a bit further now and look at the amount of energy each Kettle uses over a year let's assume an average of six boils a day that works out to 2190 boils over the course of a year we can work out the annual energy requirement to cover that which is that number multiplied by 0.2 to get 438 kilowatt hours we can now work out the cost in the UK a kilowatt hour of electricity is soon to be around 40 Pence but you can substitute this value for the cost of the country you live in this gives a total cost of just over 172 pounds for the year that's quite a lot of money for just one Appliance and maybe here is where an obvious saving can be made if you're only making two or three cups of coffee or tea every time you boil the kettle then by not filling up every time could save you nearly 100 pounds a year okay let's look at another common electrical Appliance then the electric light bulb and actually we'll look at two types of bulb an LED bulb which is the most common type of bulb in use today and an older type of bulb called an incandescent bulb which many of you still might be using in some of the rooms of your home an LED bulb typically has a power rating of 14 watts which will produce the same amount of light as a hundred watt incandescent bulb if we assume each bulb is used for an average of let's say eight hours a day that works out at 2920 hours per year with this we can work out the total amount of energy required for each bulb over the year remembering that energy is power multiplied by time for the LED bulb this is 14 watts times 2920 which works out to 40.9 kilowatt hours we do the same for the incandescent bulb 100 Watts times 2920 which is a whopping 292 kilowatt hours we can then work out the cost of powering each bulb over the year as follows 40.9 kilowatt hours times 40p equals 16 pounds 36 for the LED bulb and 292 kilowatt hours times 40p equals 116 pounds 80 Pence for the incandescent bulb crikey that's for just one bulb it's worth checking around your house then just to make sure you've replaced all of your incandescent bulbs with LED ones okay let's take a look now at the power requirements for a variety of consumer appliances in your home we've already seen that a kettle runs at two to three kilowatts a hair dryer is roughly equivalent a microwave is typically less than one kilowatt but the clothes dryer comes in at a whopping four to seven kilowatts at 40 Pence per kilowatt hour that's almost three pounds per hour to operate that machine there are a few ways to find out how much your own appliances use the first is a device like this one which you can buy online for about 20 to 30 pounds let's say you want to know just how much power your clothes dryer actually uses the device is very easy to use just unplug the appliance you want to test plug in the monitoring device and then plug the appliance back in on top next check the devices in the correct mode by repeatedly pressing the function button until the screen shows kilowatts you're ready now to perform the test simply switch on the appliance and read the power value from the screen in this example the power is nearly 5 kilowatts now for some appliances it might be difficult to access the socket where they're plugged in or those appliances might be directly connected to the grid supply for example an oven or a hob you can still measure the power consumption of those appliances however if you have an in-home display unit like this one this device is normally supplied to you whenever you get a smart meter fitted if you don't have an in-home display either because you don't have a smart meter or like me you've simply lost it you can use one of these older style units instead these units come with what is called a CT or current Transformer clamp that fits around the incoming supply line in the electricity meter box and sends readings wirelessly every 10 seconds or so to the display unit getting hold of one of these units can be tricky though as they are not easily available new but if you look on websites like eBay or Facebook Marketplace you can generally pick them up for around 20 to 30 pounds both types of device tell you how much power your home is consuming at that very moment and so to measure the power for any particular device just do the following first make sure the appliance is currently switched off and then wait until the reading on the unit is stable that is the display showing the same value for around 20 seconds note down that values we'll need it in a moment then switch on the appliance and wait another 10 seconds or so and you should see the reading on the display go higher note down this new value as well now the difference in these two values will be the power consumed by the appliance you're measuring in our example here the clothes dryer is consuming just under five kilowatts of power now it might be that other appliances switch on and off while you're making your measurement for example a fridge or a freezer so you may need to repeat this exercise two or three times to get an accurate answer finally for certain appliances it's more complicated to monitor their power consumption a dishwasher or washing machine will consume varying amounts of power depending on what part of the cycle it's in and a fridge or a freezer is only consuming power at certain times of the day to find out the power consumption of these types of appliances we'll need to take a slightly different approach using the first monitoring device we talked about earlier we'll first need to put that device into a different mode press the function button a few times until the display shows kilowatt hours now if the display already shows some numbers from a previous time it was used just use a pen or a cocktail stick to press a little reset button in the middle and this will reset all the numbers back to zero you're now ready to start measuring the energy used by your Appliance over a few days and note this time we're measuring energy in kilowatt hours instead of power kilowatts just use your Appliance as you normally would over the next few days or even weeks and the monitoring device will count the days and also the kilowatt hours of electricity consumed during that time in our example here the energy consumed by this fridge freezer over one month is just over 15 kilowatt hours which works out to almost half a kilowatt hour every day just on a side if you're getting a lot out of my videos please could you hit the like button so that YouTube will recommend the videos to others and if you subscribe you'll always be the first to see what new content I'm working on thank you okay now that we have a good understanding about power and energy let's have a look at how all this relates to your own solar and Battery setup for this we'll use an example solar array of 15 panels each panel can produce 400 watts of power in direct sunlight so 15 panels will produce 6 kilowatts actually we usually refer to this value as kilowatts Peak as this is Peak output unless sunny days the output will be less this array is connected to a string inverter which is in turn connected to the main circuit in the house as is the National Grid Supply in our example we'll have three appliances connected to the main circuit a kettle a toaster and a washing machine on a bright sunny day in summer the array might generate six kilowatts of power but not all six kilowatts will make it into the main circuit and this is because the inverter here is rated for a maximum of five kilowatts the other kilowatt of power is simply lost so what appliances can we power with the five kilowatts available we can switch the kettle on which requires three kilowatts and the remaining two kilowatts of power that we're generating from the inverter is simply exported out to the grid we can also switch on the toaster which requires two kilowatts that's a total power consumption of five kilowatts now which matches the output of the inverter and as a result no power is exported to the grid anymore but what if we now switch on the washing machine all three appliances will work but as there's not enough power coming from the inverter the extra two and a half kilowatts required will have to come from the grid okay let's switch all the appliances off again and add in a battery to the installation note that battery sizes are measured in kilowatt hours not kilowatts this is because batteries store energy not power this particular battery can store up to eight kilowatt hours of energy it's also a DC coupled battery which means it connects directly to a special kind of inverter called a hybrid inverter let's say that the solar array is only producing two kilowatts of power now because it is cloudy if we're not using any of the appliances at the moment all two kilowatts of that power will go into the battery charging it up let's suppose now that the solar production increases to four kilowatts we can expect all four kilowatts to go into the weight what we can actually only charge this particular battery at a rate of Two and a Half kilowatts and you'll find that different batteries have different charging rates at a rate of Two and a Half kilowatts charge if the battery is empty remembering that energy equals power multiplied by time we can work out it will take just over three hours to charge okay later when the sun has gone down we have no generation from the array if we switch on the kettle we can see that the power to the kettle is being supplied by the battery but not all the power it needs only two and a half kilowatts this is because this battery also has a limit to the rate of discharge in this case two and a half kilowatts the remaining half kilowatt of power is provided by the grid you can check the data sheet for your own battery to see what the charge and discharge rates are now I mentioned earlier that the battery we're using here is a DC coupled battery one of the great benefits of this type of battery is this let's suppose the solar generation is at its peak of six kilowatts again as before the inverter can only output a maximum of five kilowatts into the main circuit the remaining one kilowatt was lost before but now with a DC coupled battery that one kilowatt can be used to charge the battery instead okay let's now change this DC coupled battery for an AC coupled one this particular battery is a Tesla powerwall which can hold 13 and a half kilowatt hours worth of energy let's switch on all three appliances the power requirement then is seven and a half kilowatts but let's say there is only three kilowatts of solar generation at that time the inverter is able to supply three kilowatts to the main circuit but there's still four and a half kilowatts to find luckily the Tesla powerwall has a discharge rate of 5 kilowatts so can easily meet that requirement if the power wall is full we can calculate how long it will last by rearranging our Energy Formula time equals energy divided by power so 13.5 divided by 4.5 is exactly three hours not bad at all thanks for staying with me hopefully now you've not only got a good grasp of power and energy but also how these relate to your own solar sector just before we go if you'd like to support the work on this channel and you live in the UK here's my referral code for octopus energy I'm going to be making a video all about them shortly as I think they're leading the way for us all to have a clean energy future until then thank you for watching [Music]

Channel: Gary Does Solar

Views: 25,537

Keywords: kw, kwh, energy, power, solar

Id: IzBHs4878BY

Length: 15min 48sec (948 seconds)

Published: Sun Jan 08 2023