kW, kWh and kilowatt/hour : What do they stand for?

An electricity meter reading kWh

Don’t confuse kW and kWh. It matters!

One of the most common sources of confusion I come across when talking to non-electrical people about solar power are the terms: kW and kWh. What do they stand for? What do they mean? And what’s the difference between the two?

Folks who are really confused also throw the term “kilowatt/hour” into the mix. I’ll deal with that doozy at the end of this post1.

Starting with the absolute basics

Please excuse this post if you think I am explaining the bleeding obvious! But I think it is really important to write a post that describes these key units of electricity in a way that requires zero prior knowledge of anything electricity related.

So here is my humble attempt to answer the question:

What is the difference between a kW and kWh?

Let’s start with what each letter stands for.

k stands for kilo. Which means “one thousand”.

W stands for Watt. Which is a measure of power.

h stands for hour. Which is obviously a measure of time.

So kW means kilowatt which is 1000 Watts. It is a measure of power. Notice that, if you like to keep anal electrical engineers like me happy, the correct way to write it is always with a small k and a capital W. The size of a solar system is defined by its peak power. e.g. a 1kW system can produce 1kW of power on really sunny days.

kWh stands for kilowatt-hour. A kWh is a measure of energy (not power). If your solar panels (for example) continuously output 1kW of power for a whole 60 minutes, you will have produced 1 kWh of energy.  The amount of electricity you use (or generate) is defined in kWhs. e.g. “My solar system produced 4 kWh of electricity today!”

So at the highest level: kW measures power, and kWh measures energy.

Why is the difference between Energy and Power important?

It is very common for people to mistakenly interchange the terms energy and power as if there is no difference. Most people do it all the time without noticing. It drives electrical geeks like me up the wall. Especially when I read it in national newspapers and books!

For example: If someone is talking about their electricity usage and says:

“I used 8kW yesterday”

They probably mean that they used 8 units of electrical energy yesterday, In which case they should have said

“I used 8kWh yesterday”

Yeah, yeah I know what you are thinking: Who cares?

Well it is actually quite important if you are buying a solar system. If someone says they need a solar power system to produce 8kW, they might end up being quoted an 8kW solar power system. Which will cost about $18,000 at today’s prices and produce about 32kWh per day.

If, what they actually meant was that they need one to cover an energy usage of 8kWh per day, then they really need a 2kW solar system which costs about $3,500 at the time of writing!

So please don’t confuse kW and kWh. If you do you may end up with a solar system that is completely the wrong size!

How do kW and kWh relate to batteries?

When buying batteries, you need to think about both the power of the battery and the energy storage capacity of the battery you are looking at.

The energy storage capacity of the battery is measured in kWh. For example the ‘Powerwall 2’ stores 13.5kWh of energy. Its power is 5kW, so it can charge or discharge at that rate. At full power, then, it can fully discharge in under 3 hours.

The more energy (kWh) a battery has the more you can store and the longer it will last, the more power it has the faster you can get that energy in our out of the battery,

In other words the more power it has, the more appliances you can power with it and the more energy it has, the longer you can power those appliances.

For normal household use, you want enough energy to get you through the night and at least 5kW of power for speedy charging.


Top tip for filtering out the worst solar and battery salesmen: Ask them to explain the difference between a kW and kWh. If they get this wrong how on earth are they gonna understand your requirements? A lot of cold calling door knockers will fail this test in my experience.

 The technical bit for those that are interested:

Energy: Energy is measured in Joules. Energy is the capacity of something to do work.

Power: Power is the rate at which energy is used. Power is measured in Watts.

1 Watt is a rate of energy usage of 1 Joule every second, or 1 Joule per second (J/s).

Footnotes

  1. A kilowatt/hour is a nonsensical unit in most contexts. If you find yourself using this unit, double check what you really mean. Which is probably kilowatt-hour. Kilowatt/hour means kilowatts per hour. Kilowatt is a measure of power (units: Joules per second ) so you are really saying joules per second per second, or joules per second squared. Which would be a measure of how quickly power is increasing. An acceleration of power if you like. There are very few circumstances where that is a relevant measure. Especially when talking about solar or batteries.Lots of people get kilowatts/hour confused with kWh. Please don’t let it be you. 99% of the time you’ll mean kWh.
About Finn Peacock

I’m a Chartered Electrical Engineer, Solar and Energy Efficiency nut, dad, and founder of SolarQuotes.com.au. My last “real job” was working for the CSIRO in their renewable energy division.

Comments

  1. Peter Leeke says:

    I don’t care how simplistic it sounds, your article has been a revelation. AT LAST I understand what it is all about.
    Thanks a heap, much appreciated.
    Peter L

  2. Hi Finn. Good site mate…I am ready to start a new job with Goforsolar in Port Melbourne. Haven’t seen any reviews on them on here yet…Do you know much about them and there quality please. Cheers Mick

  3. you website is such a gem. thank you for sharing great info.

  4. Cam pro eco says:

    It sounds like it would be better to have a system that is not connected to the grid,. After reading your info, very enlightening BTW, it sounds like the authorities and retailers are making it harder for people to reduce their reliance on fossil fuel produced electricity. Sounds like retailers are complaining to government authorities that they are not making enough profit.

  5. Erica Charteris says:

    Awesome website. Thank you so much for the lay persons explanations. Starting to get an understanding of this stuff.

  6. Noel Nielsen says:

    Hey Finn
    I also think your delivery was perfectly acceptable and in no way offensive but can see why Jock “mighta” been annoyed but the facts WERE provided in a humorous way and I thank you for that. After all it was my English teacher at school many years ago told me “English as she’s wrote ain’t as she’s spoke”

  7. Phii Bramley says:

    Im still confused Finn…..im off grid and my regulator, PL60, tells me how much solar energy I have harvested in Amp/Hours….so how do I convert this to kWH?

    • To convert Ah (Ampere-hours) to kW hours, simply multiply by the voltage (because Power = Voltage x Current) and divide by 1000 (because it is *kilo* Watt hours).

      For example:

      If a 12V battery has given you 100Ah, then:

      12V x 100 = 1,200 Watt hours

      1,200 divided by 1000 = 1.2kWh

  8. steven greer says:

    Hi Finn,
    I am buying a 3 bedroom house in the country and it has gas cooktop gas hws,electric oven and all the rest lights fridge etc.(2 people)
    My question is Is it worth installing a solar power system to go help keep the power bill down? We will be on the pension very shortly.Your comments would be much appreciated.
    regars Steve

  9. Tim Forcey says:

    My name is Tim Forcey. You can shorten that and call me Tim if you want, and you probably won’t get in any trouble whatsoever (unless there are other “Tims” around).

    But never ever shorten kilowatt-hours to kilowatts!

  10. Robert Black says:

    Finn I am lost, I really am. Here is the deal I use 30kwh per day on a single phase house which is our current house. We are building a new house which will be 3 phase. ( Most of the house is single phase but i will be spreading circuit loads across all 3. there will be 50 some odd different circuits spread across 3. The only thing I have that is 3 phase is a 30 kw/cooling capacity not electrical capacity air con unit ) I am hooking up a daisy chained Tesla power wall system ( the 7kw versions. ). I wanna stay on the grid ( so hybrid ). I live in melbourne. I dont wont to pay a single bill. I refuse to be told from solar sales people as the ones i have dealt with honestly have no idea what they even sell let alone the tesla system. What on earth do I need? I.e How many watts or kilowatts must I produce from the panels? What size inverter do i need? ( the biggest I can find tesla ready is the fronius Symo 5.0-3-S Hybrid 5kw. I would like to use this brand of inverter as they are tesla ready and have had over 60 years experience in battery charging ) Will i need more than 1 inverter? How many tesla batteries do I need, Can the tesla batteries be daisy chained? Any help dude would be great. Im just lost.

  11. James Lehmann says:

    Im in Canada and we are years behind you in Austrailia in solar tecknowledgy, aiways a interesting read. Thx

  12. G’day Finn and team,

    12 years ago when we built our house I had it connected to 3 phase supply, I no longer have a need for the 3 phase, should I have it changed to single phase, is there any advantage o disadvantage?

    I also have two separate solar systems. A 1.5 kW about 6 yeas old and a 5 kW about 3 yeas old, they are separately connected to two of the three phases.

    On average my system exports twice the amount of power that I import, yet I still get a power bill although most of it is the ridiculous supply charge, or connection fee as some call it.

    I also have a 6kW petrol generator that is connected to my power board via a mains/ emergency switch. because of the high number of blackouts we get.

    I’d like to go off grid. Am I right in thinking that with the generator as backup I don’t really need to calculate battery storage for the occasional 3 o 4 day low solar period, and could allow for say only two days battery supply.

    Thanks for the great work,

    Bob

    • Hi Bob,

      I don’t see any advantage in taking 2 phases off your house. I’d leave all three.

      And yes – a generator lets you have much fewer batteries – if you find you are using your generator too much – then you can add more batteries later.

      Hope That Helps,

      Finn

  13. The use of kWh as energy unit is as clumsy as saying the length of a trip is 100 km/h day. The proper energy unit in the international system is the Joule (J) and 1 kWh is 3.6 MJ. Alas energy companies are unable to speak properly of energy.

  14. Hi,
    a usage of 8kWh per day as you say can be done with a 2kW solar system.
    Do you multiply 2kW x 4 ( meaning 4hr of sun per day? )
    please clarify this.
    Thank you.
    Regards
    Lothar

    • yes – you get about 4 ‘sun hours’ per day averaged over the year in Australia – a bit more up north – a bit less in Melbourne and Tassie. So 4 kWh owe kW installed.

  15. I am setting up of the grid as there is no mains where i am so what kva invertor would i need when i on two oanels one 12vokt battery to run a small house that only using piwer for a five start rated frigde a tv and led lights threw out and a water pump ? The system i have has a gen back up of 2800kva max and the solar can be added to as miney permits . So the wuestion is is my daily usage is up to 16kwh what kva do I need ?

  16. Leslie Rajaratnam says:

    Hi Mr:- Finn Peacock, My daily usage is 2kwh, i have 3 phase system, only air conditioner is in 3 phases, pool and others are in balanced 3 phase, i have an off peak meter with heat pump, cooking on gas converted, ready to install in 2 weeks, solar of 5kw with 7.5kw 3 phase inverter, best in market, how much i will save in quarterly payments, no one in home, from 9am to 4pm, pool in auto timer in morning, before 11am and after 4pm, that cause pump run out of water, burnt the seals of motor, happen when i am not there, so auto timer i cancelled and i start by myself, after come home at 4pm, no battery requested for solar system, so what is your advice to save money, washing done at the same time, when i am back, due, to unbalance load stoppage, and drain plug blocked,in sink, and wet my carpet, please advice me sir, dishwasher, i disconnected, drier none in home, fridge is 4 yrs old, with 5 star, no old tvs, 56inch, led, use 1hr/per day, whole home is compact fluorescent lamps(CFL), how far i go sir, ready to retire, please advice, me thanks.

    • Ronald Brakels says:

      Hello Leslie, Ronald here. If you have gotten your electricity use down to 2 kilowatt-hours a day then you have already done very well. I can’t think of anything at the moment to further reduce your electricity use. But if you are going to retire soon then you are likely to be home during the day which should let use more solar electricity and even less grid electricity.

      • Leslie Rajaratnam says:

        What happens when i don’t use, will you please explain, thank you sir.

        • Leslie Rajaratnam says:

          I need air conditioner to run in nights, not in the morning, when i am along, please, advice me, sir.

          • Ronald Brakels says:

            You could install a battery system to store solar energy during the day to use at night, but at the moment they are too expensive to pay for themselves for normal households.

            There are air conditioners that can use solar electricity during the day to make ice to cool a house at night, but they don’t make financial sense either.

            I am afraid there is not anything I can really think of to help you, apart from suggesting you use solar power in the afternoon to cool the house down so you won’t have to use quite so much grid electricity to cool it at night.

        • Ronald Brakels says:

          If you mean what will happen to solar electricity you don’t use, it will be sent into the grid and you will receive a small feed-in tariff for it. In most states it is usually only about 6 cents a kilowatt-hour. While this isn’t a lot, it will still help to reduce your electricity bills.

    • Bob Gitsham says:

      Hi you don’t say what size your pool is. but most common pumps for domestic pools are around 0.75kw and generally are recommended to run 4 to 6 hours a day. To make best use of solar power you should set timer to run in late morning and again in early afternoon.

      It is unfortunate you damaged the pump by running out of water. but don’t explain how this could happen. I’ve never had this experience in 12 years. you could fit o loss of pressure switch that would stop the pump if it ever happened again

  17. John Jones says:

    We find there is very little direct sunlight in our part of Tasmania. Does cloud cover still produce power in solar panels? Should more panels be purchased to compensate for lack of direct sunlight?

    • Ronald Brakels says:

      Hello John, I’m afraid cloud cover really reduces the output of solar panels and so Tasmania is the worst place in Australia for solar power. Light cloud may result in panels producing about 30% as much as under clear skies while heavy cloud can reduce it to 10% or less.

      Installing extra panels can make up for the lack of sunshine. You can have panels that total to 133% of your inverter capacity and still receive Small-scale Technology Certificates (STCs) for them that reduce the cost of rooftop solar. I wrote about this here:

      https://www.solarquotes.com.au/blog/oversizing-solar-arrays/

      Getting a larger inverter and installing a much larger solar system overall is an option, but note that the larger your rooftop solar system, the less electricity you will use yourself and the more will be sent into the grid for a low feed-in tariff and this will increase its payback time. So from a purely dollars and cents perspective it doesn’t make sense to make your rooftop solar system too large. But if you are doing it for environmental reasons, then go for it. But note that, unlike in South Australia, if you want a solar inverter larger than 5 kilowatts in Tasmania, it unfortunately complicates matters.

  18. Hi Finn – just perusing your great website and beginners guide to solar – full of excellent & pertinent information. Has got me thinking – is it feasible to install a small 3kW system and not connect it through your fusebox/metering system? For example, install a hybrid system, but run new wiring from the inverter to say new power points in the laundry and bathroom and use the washing machine/dryer etc during the day. Lights might be the only thing needed in these areas overnight and could run off the grid. Perhaps different coloured power points could indicate solar.

    Similarly, new individual wiring could be run to free-standing wall heating units to warm the house during the day via solar to limit heating expenses.

    Is something like this legal and feasible? I own an old turn-of-the-century house that is hard to heat in winter, especially the upstairs bedrooms – maybe this is all I need?
    Cheers
    Daryl

    • Ronald Brakels says:

      Hello Daryl, Ronald here. What you suggest is possible, and provided it is never connected to the grid it is legal. But economically speaking it doesn’t make a lot of sense, as you would have to pay extra to get a hybrid inverter installed and you wouldn’t be able to send surplus solar electricity into the grid for a feed-in tariff. In addition, because the output of solar panels varies a lot during the due to the position of the sun and because of clouds, you would need batteries to have a reliable off-grid supply of electricity and they are still too expensive to pay for themselves.

      So if you are able, it makes much more economic sense to install a grid connected solar system.

  19. wow. thanks you so much. thanks you. i was about to make a mistake. you save me.

  20. Hi Finn
    I have a tesla 14k system and would not have purchased it if I had known about the evergen 17 k unit in time.
    I was wondering though if I could still purchase the evergen unit and create extra storage from my 5.2 k solar? I also have solar hot water which I started with but there doesn’t seem to be a need anymore with the battery being fully charged up each day by noon?

    • Ronald Brakels says:

      Hello Ray.

      Extra storage capacity will only be of use if you regularly use more than the 13+ kilowatt-hours you can get out of the Powerwall 2 at night.

      But if you did want the extra storage, the Evergen Equinox is DC coupled, so while you could technically get it, this would involve scrapping your current inverter. Assuming your Powerwall 2 is AC coupled, it should continue to work, although I don’t know if you would have problems balancing their use. One might end up doing most of the work.

      Economically it won’t pay for itself for an on-grid installation.

  21. Jack Hendrix says:

    BTW, what do you mean by “anal electrical engineer”? “Anal”???

    • Ronald Brakels says:

      Jack, I’ll give you two definitions and you can decide which one is most likely to be correct:

      (1) An anal engineer specializes in building artificial sphincters.

      (2) It is a term from Freudian Psychology that is short for “anal retentive” and, “is used to describe a person who pays such attention to detail that it becomes an obsession and may be an annoyance to others.”

  22. Thanks a ton buddy…helped me a lot to understand basics.

Trackbacks

  1. […] that appears to think renewable energy is the devil’s spawn, you also get a miserable 7c per kWh for any solar energy you export to the grid, compared to saving 25c per kWh if you use the energy […]

  2. […] should get an average of 3.6kWh per day per kW installed average. So that works out at  14.4 kWh per day. His last reading was […]

  3. […] when selling into serves such as Reposit,  power (kW) is more valuable than energy ( kWh) [click here if you think kW and kWh are the same]  So if you want to use these services in the future, Tesla may allow you to make more […]

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