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What the Heck is the Difference Between a Kilowatt and a Kilowatt-hour (kW vs. kWh)

So you look at a light bulb and it says “100 Watts.” What the hell does that mean? What the Heck is the Difference Between a Kilowatt and a Kilowatt-hour?

Your power company charges your for “Kilowatt-hours” that you use… What the hell is a Kilowatt-hour?!

Juice ManFor starters, a Kilowatt (kW) is just one thousand watts, just like a megawatt (mW) is one million watts. A “watt” (W) is a measurement of POWER. To be precise it’s using one joule per second. However, since no one has uttered the word “joule” since that physics class they took fifteen years ago, we use watts instead of Joules per second, because they’re printed on light bulbs and people have an idea of what they mean. To put it in perspective, a car engine cruising uses about 25,000 watts, which is about 30 horsepower.

But that’s a car… Mechanical energy. We want to talk about electricity since we’re concerned with solar power. With electricity, POWER is voltage time amperage. Or 1watt = 1volt * 1ampere.

A good way to think about electricity is that it’s a lot like water. Voltage is the pushing, or the pressure, and amperage is the flow. A dammed up lake, although it has a hell of a lot of pressure (voltage), doesn’t flow anywhere (zero amperage) so there’s no power (lots of volts * zero amps = zero watts). On the other hand take a super soaker water gun. Super fast stream (lots of amps) but a tiny little compressed reservoir (not a lot of volts). I wouldn’t go so far as to call that “powerful.” Finally, look at a rushing river. Lots of volts, lots of amps, huge power. So wait, Dave, you got off subject, we’re talking about Watts here…

Oh yah, Watts. Ok, so a Watt is energy burned per second. If you flick on a 100 Watt light bulb it’s eating up 100 Joules of energy every second (interestingly, a standard candle is exactly 1 Watt). So now what’s a watt-hour (wH)? Well, don’t get confused when you see a confusing or seemingly incorrect usage of watt or watt-hour in mass media. The two terms are often interchanged and misused.

“Watts per hour” doesn’t make sense because it’s already a measurement of “joules per second.” Does “Joules per second per hour” make sense? No. A Watt-hour is what your power company uses to charge you, and it’s a way of removing the “per second” from Watts. So now instead of talking about Joules per unit of time, you’re just talking about Joules period…. but we call them Watt-hours because no one knows what the hell a “joule” is.

Think of Watts as the speed you’re running and Watt-hours as how far you’ve actually run. A kilowatt-hour is the amount of energy equivalent to a power of 1 kilowatt running for 1 hour. If you leave a 100 Watt light bulb on for 1 hour, you’ve done gone and used up 100 watt-hours.

But your electric company will bill you by the kilowatt-hour, so you’ll get a bill for .1 kWh, multiplied by your per-kWh rate. That means if you run a 100 watt bulb for an hour a day for 30 days in a month, and you pay $.10/kWh, your bill will be for .1 kWh x 30 days x $.10, or $.30.

Ok, I feel like I’ve made that explanation 100 times longer than it should have been. Hope that helped.

By Dave Llorens

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39 thoughts on “What the Heck is the Difference Between a Kilowatt and a Kilowatt-hour (kW vs. kWh)

  1. Googleuser says:

    Thank you. I was confused because watts is a unit of time and so watt-hour didnt make sense. The long explanation was much appreciated.

  2. “A DAMNED up lake, although it has a HELL of a lot of pressure (voltage)”

    *LOL* I believe “dammed” is the word you seek. It’s plugged up, not condemned for all eternity.

    Still, the mention of Hell seven words later reminds me of that old joke questioning if Satan’s realm is exothermic or endothermic…

  3. This long article will help its readers to understand the difference better. I was really confused with both this Kw and KWH. Your well explained example helped me to clear all my doubts.

  4. Anonymous says:

    I use between 58 and 60 Kwh per day. How many 100 watt solar panels do I need to at least cover 75% of my total power usage?

  5. I have a generator how can i find the information about the power?

  6. Chuck says:

    Okay, so then the bulb is already rated at how many watts it will use in an hour? Meaning if I’m using a 3 watt LED it will only consume 3 watts of energy over 60 minutes worth of time and therefore it’s watt-hour rating is 3?

    I’m desperately trying to understand this so I con’t get out in the back country and run out of power halfway through the evening and am stuck until the sun comes up to kiss my solar cells in the morning.

  7. D says:

    Here’s a better way to look at it:

    1 Watt (W) is a measure of electrical power.
    1 W = a specific rate of energy use/generation during a specific time period (e.g.: 1 Joule (J) per second (S)).
    1 J/S = 1 W/S

    1 Watt-hour (Wh) = a specific amount of energy used/generated in 1 hour (H). Since there are 3600 S/H (i.e. 60 S/min (M) x 60 M/H = 3600 S/H) and there is 1 W/S, we see that 1 Wh = 3600 W/S (i.e. 1 W/S x 3600 S = 3600 W)
    1 kWh = 1000 Wh (i.e 1 kilo (1000) x 1 Wh = 1 kWh)
    1 kWh = 3,600,000 W (i.e. 1 kilo (1000) x 3600 W = 3,600,000 W)

    A “60W” light bulb uses 60 W of energy in 1 hour. That is 60 W x 1 H = 60 Wh. Since we are billed in kWh, we must convert that into kWh as follows: 60/1000 = 0.06 kWh. If a person pays 10 cents (i.e. 0.10) per kWh, then it would cost her/him 0.006$ or 6/10 or 60% of a penny (0.06 x 0.10 = 0.006) to operate the light bulb for 1 hour. Energy cost for 1 month would be 4.32$ (i.e. .06 x .10 x 24 x 30 = 4.32$)

  8. Ian says:

    I THINK (please correct me if I am wrong) but if a solar system had 10 200 watt panels and peak solar sun time averaged 5 hours per day AND the inverter and the panels were able to actually OPERATE at the rated output of 2KW (10x200watt panels) then with 5hours per day of Solar sun the system would produce 10KWh (killowat HOURS) of energy available during that time. ALSO if during that day the power is produced then NO power in EXCESS of the hourly draw of 2KW could be drawn from the system. Is this right please?

  9. Ray says:

    I have an aquarium pump with a 70 watt capacity. I have a solar panel which outputs 17 volts dc and can charge a 12 volt car battery nicely. If I get an inverter, connect it to the battery charged by the solar panel will it run my pump 24 hours. I live in the Philippines so sun is no problem.

  10. suhas says:

    i have 30 incandescent bulbs of 60w each . show me calculation for kilowatts of energy used

  11. Akinwumi says:

    Love dis site very much

  12. Nabil says:

    I think KW and KWH are the same, as the load of 100 KW means it will consumes 100 KW if it runs for an hour… so I don’t agree with the above explanation
    http://www.think-energy.net/KWvsKWH.htm
    plz refer this link

    1. Dave Llorens says:

      Hi Nabil,

      Think of kW like horsepower. You could have 100 kW panel, but if it’s in the dark, it is not going to produce even 1kWh over the course of an hour. But, in perfect conditions, that is what it could produce in an hour. There are some more complications than that as in what it will, in practice, produce, but you get the idea. Think of kW as horsepower of the system. But the car may be idling or off.

  13. MARK says:

    PLEASE EXPLAIN THIS !
    People often quote costs/kW when they are really talking about costs/kWh. Since these sound similar, they must be similar. Unfortunately, they are not.
    WHAT IS THE DIFFERENCE THEN ?

  14. confused says:

    Can someone help me figure something out? In trying to understand how a sample solar estimate came up with the number of panels it would need. Ok, so a home wants to eliminate 587 Kwhs per month.
    -Solar hours per day is 4.9 with the derate factored in.
    -Panel/inverter AC CEC watts is 197.8 with inverter efficiency considered
    -Kwh/mo/panel is 29
    -# of panels required is 20

    Ok for this particular type of panel brand…
    -DC STC WATTS 4500 WATTS
    -DC PTC WATTS 4142 WATTS
    -AC CEC WATTS 3956 WATTS

    Sooo, I’m confused because I thought the goal was to have only enough panels to power your house for the Kwh per one day of what you want to elimate. But it seems the number of panels are for a months worth of usage.

    Can someone please explain this to me? Thanks!

  15. Joy King says:

    Your explanation is wonderful, thank you! And thx to Sam Harriman too for his articulation on how capacity compares to generation….

  16. Sam Harriman says:

    Thanks for explaining this difficult concept Dave. Here’s a good excerpt from an article that John Hynes wrote for Renewable Energy World:

    Capacity versus Energy

    To learn how to compare technologies within a load factor category, one has to understand a number of terms and concepts, like the difference between capacity — measured in kilowatts (kW) or megawatts (MW) — and energy, measured in kilowatts-hours (kWh) or megawatt-hours (MWh). People often quote costs/kW when they are really talking about costs/kWh. Since these sound similar, they must be similar. Unfortunately, they are not.

    Capacity for a power plant (kW or MW) is probably best explained with a highway analogy. A 10-lane highway is able to allow more cars to get from one point to another in a given time period when compared to a three-lane highway. Likewise, a 1,000 MW power plant has the ability to put more energy, or MWh, to the grid in a given time period than a 500 MW power plant. The size of the highway is analogous to the capacity, or MW rating, of the power plant. The number of cars that pass from one point to another on the highway during a given time period is analogous to the energy, or MWh, that the power plant delivers during the same time period.

    So the more lanes on the highway, the more cars that can pass from one point to another in one hour. Therefore, the larger the power plant’s capacity, the more energy the plant can deliver to the grid in one hour.

    Now that we understand the difference between capacity and energy, we can begin to compare the costs of one power plant to another.

    Read the whole article here: http://www.renewableenergyworld.com/rea/news/article/2009/10/how-to-compare-power-generation-choices?cmpid=WNL-Friday-October30-2009

  17. joe says:

    if i have a hair dryer rated at 1200 watts, and i use it for 20 min a day how many kilowatt-hours is this per day?

  18. Penny says:

    Hi! My dad has a grinding machine.
    Amperes: 180
    Phases: 3/380 V
    HP 75
    Price of electricity per kwh: 2.84 rupees .
    He wants to know how much electricity he pay for 1 hour? Can anyone help me,plz?

  19. SHAN says:

    THIS IS THE ANSWER TO #4
    ok. so kWh= kilowatts per hour used. a kilowat is 1000 watts, and a watt is a measurment of power.

  20. matt says:

    how come power bills come as kwh instead of just kw?

  21. Jon says:

    I have a 3500sq foot of living space above ground and a 1500sq foot of living space in the basement finished. I average about 3500kwh a month. Does this sound about right, I have 3 gas furnaces heating the home, 3 air conditioners during the summer cooling the home, and of course all the lights and tv’s throughout the house. In the mid of last summer my bill was Almost $600.00 for the month, is there a solar panel/wind turbine system out there big enough that can run my house using this much electricity and if so, how much would a system like this cost?

  22. Raveesh says:

    why ups neutral cable is bigger(Guage) when compared to raw power neutral. please help me out to know this.

  23. Dan Hahn says:

    DJ,

    How much did you pay for electricity last month? If you feel you’re using about as much electricity as your neighbors, I’d compare power bills. If they are similar, perhaps you could jointly approach the power company with a request for clarification. Otherwise, I’d definitely look into a home energy audit. Even if you aren’t hemorrhaging electricity (which I think you might be), your auditor will probably be able to show you how to save more in power expenses than the audit costs. Either way, you save.

    Good luck and please give us an update. I’m sure you’re not the only one with a power bill like this.

  24. DJ says:

    Dan –
    No, my current energy used since the last meter reading is 3640 kWh – the statement shows my daily averages as: Current Month 117; Last Month 126 and Last year as 133 – granted, I am in Arizona and therefore have two a/c/heatpump units on the home, we use these to “zone” the usage. However, we have solar panels to heat our water so I am still baffled at the amount I am using. I can tell from your reaction this is still as extreme as I was thinking. Do you think it would be worth my while to find an energy auditor to check my home out or do you think SRP (the power co) should do some explaining?

  25. Dan Hahn says:

    DJ,

    That is a whole bunch of electricity per month. Are you propagating anything in your basement? If not (and you aren’t operating a theme park or live in a gigantic mansion), you might want to check your electric bill– again. Perhaps that was the amount you used in the past year.

  26. DJ says:

    My power bill says I am using 3640 kwh per month, but the national average I found on the web is only 780 Kilowatts per month for a 2500 sq foot house..why am I so far over?

  27. Tracies says:

    How would I determine what size generator that can power my whole house, water well and central?

  28. Tom says:

    Eddy check this link out.

    Seems like your 120 KW turbine would produce about 210240 KWh a year which equates to about £21,000 worth of electricity (probably about $21,000 dollars to taking into account the differences in energy prices over here).

  29. EDDY says:

    M realy confused!!! Can some1 tell me if a turbine has an output of 120 KW, what does it mean? does it mean, that it ll produce 120 KW of power per hr?

  30. confused says:

    If I have a machine that says 4.37kW/24 hours, does that mean its kilowatt or power rating is 4.37/24=0.182kW, ie, it consumes 0.182kW of power every hour? Thanks in advance

  31. Daniel says:

    Two questions, So if an appliance has a surge rating of 5000 watts and a surge time of 5 seconds (5/3600 of an hour) is the power used 6.94 Wh [.00694 kWh]?

    Are kWh or kW usage at a peak time more important when evaluating a solar or wind system capacity?

  32. Nick Walker says:

    Can you convert a kilowatt to a kilowatt hour?

    1. The are two different things. In the article it provides a good metaphor for thinking about the difference between the two.

      A kilowatt hour is is one kilowatt of power for one hour of time.

  33. Joan says:

    If I have a machine rated 480W. does that mean it burns 0.48kW per hour?

  34. Joan says:

    I have an oxygen generator which is rated 480Watts. 4.8 Amps. So if I run it for an hour am I using .48kW ? Trying to figure out how much electricity it uses in a 24 hour period. All help gratefully accepted. Thank you

  35. Dan Hahn says:

    Dave,

    so let’s say I put in a 100 watt bulb, leave it on for two hours. My electric company here in Oregon is gonna charge me .20KwH? So that’s like two pennies? (.2 * $.09/KwH = $0.018)

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