Electricity Saving Tips For Homes And Offices

What are Watt, Kilowatt and a unit of electricity

By on March 6, 2014

When we get our electricity bills, it shows that we have used a certain number of units for the given period. When we go to buy appliances, most of them have watts mentioned on them. If you find it difficult to understand the relation between the two, then you are not alone. Electricity bill and its components are confusing to many and with this article we will try to explain what are watt, kilowatt and a unit of electricity.

Confused

Power and Energy/Electricity

Power and Energy/Electricity are two words that are used so much for each other that many feel that they mean the same. Interestingly both of them have a very different meaning. Power is the rate at which electricity is used and energy/electricity is the actual consumption. To give an analogy, power is similar to speed but electricity/energy is the actual distance travelled.

So        Power x Time = Electricity (or energy)

Just like          Speed x Time = Distance Travelled.

Power and its units

Power is always represented in watt (W) or kilowatt (kW). A thousand (1000) watts make one kilowatt. So if any appliance is rated as 1.2 kW then it means that it consumes electricity at a rate of 1200 W. Now as we discussed earlier that power is the rate at which electricity is consumed and not the actual electricity consumed, Watt or Kilowatt just represent the rate at which electricity is consumed per hour. Which means that when you buy a 100 W bulb, it does not consume 100 units of electricity but consumes at a rate of 100 W.

Energy/Electricity and its units

A unit (as mentioned on the electricity bills) is represented in kWH or Kilowatt Hour. This is the actual electricity or energy used. If you use 1000 Watts or 1 Kilowatt of power for 1 hour then you consume 1 unit or 1 Kilowatt-Hour (kWH) of electricity. So the reading on the electricity meter represents the actual electricity used. Just like the odometer on your vehicle that shows the actual distance travelled by the vehicle, electricity meter shows the amount of electricity that is used. So a 100-Watt bulb if kept on for 10 hours will consume:

100 x 10 = 1000 Watt-Hour = 1 Kilowatt-Hour (kWH) = 1 units (on your meter).

Estimating units consumed by any appliance

Now with most of the concepts explained we would like to make it easy for you to calculate how much units does any appliance consume. Most appliances have wattage written on them (either on their container box or somewhere on the appliance). Once you have the wattage, next you need to figure out how many hours a day do you use it. After that you can use the formula below:

Daily Units = (Wattage x Usage hours per day) ÷ 1000

Monthly Units = Units x 30 (or 28,29,31 based on month)

Please note that this formula may not work always. For appliances like Air Conditioner, Water Heater, Cloth Iron (any heating or cooling device) and pumps, this will not work.Infographic3

Source of Information:

http://www.energysavers.gov/your_home/appliances/index.cfm/mytopic=10040

http://en.wikipedia.org/wiki/Watt#Confusion_of_watts.2C_watt-hours.2C_and_watts_per_hour

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There Are 17 Comments

  1. Ganesh says:

    Please clear my confusion

    What is Ah w.r.t domestic electricity ?
    When somebody tells that we have electricity with Ah what does it mean?
    do Ah allows run more equipment with more watt usage?
    what is the maximum watt can be consumed by any equipment without Ah(without any additional transformer or stabilizer for ex immersion coil also I know that watt consumption is depend on item which we purchase but I am speaking about maximum watt item which we can run without ah, I need that maximum watt) for domestic electricity in india?
    How volts affects the watts?
    also finally whats the maximum watt can be consumed by any equipment with Ah

    Kindly don’t give explanation with formulas I am weak in mathematics :)

    • Abhishek Jain says:

      Ah is typically used for batteries and represents the amount of energy it can store. Every battery has certain voltage and VAh is a unit for energy (A 12 V battery of 150 Ah size can store 12×150 VAh energy). Watts is rate at which the energy is used. Now not all of the VAh can be used and thus useful energy (Wh) is roughly 0.8 of the total VAh value. So Wh (or watt hour) the above battery stores = 12x150x0.8 = 1440 (approx). And so if you have a 10 watt bulb you can run it for 1440/10 = 144 hours.

      Sorry I could not explain it to you without formulas.

  2. surinder bansal says:

    i have i ton o genral ac how many unit per hours.

  3. Shalini says:

    Hi i have one 1.5 ton 5 star ac 1480 watt.
    I run it at 25-30 c° 90% of the time and 10% of the time it runs at 16c° for an average of 10 hours a day(6 days in a week in day time) and 1day 15 hours throughout the day.
    What’s going to be the monthly unit and the electricity bill.? We pay rs8 per unit.
    Also tell me how to read the meter.
    Suppose it shows 21 so is it 21 unit or 2.1 unit?

    Need your help asap. Thank you in Advance bro.

    • Abhishek Jain says:

      Hi Shalini,

      If the AC is rightly sized (not undersized or oversized), it should consume about 1 to 1.1 unit in an hour when you run it at 25 to 30. At 16 it will go up to 1.3 or 1.4 units an hour (both assuming temperature outside is not more than 35-36).

      Meter always shows the absolute number and there are no decimal places in units. And when calculation happens it is previous reading minus current reading in units.

  4. Rusi says:

    How to calculate the total electricity load of a typical flat .
    How to figure out whether the meter required would be 3 phase or Single phase.

    • Abhishek Jain says:

      Hi,

      If you can provide a list of appliances that you have, we can help you calculate the load. To choose between single and 3 phase, you can read this: http://www.bijlibachao.com/electricity-bill/what-are-single-phase-and-three-phase-connections-and-how-to-choose-between-the-two.html

  5. surya says:

    good article. SIr, a 1.5 ton AC would require approximately 5KW power,isnt it? If Iam correct then how many units does it consume for 1-hour.If im wrong,please help me in converting ton to KW

    • Abhishek Jain says:

      Hi Surya,

      Typically 1.5 ton ACs require 1.4-2 kW of power depending on BEE star rating. 5KW is a lot. Although there are several factors on which electricity consumption depends as listed on this page: http://www.bijlibachao.com/air-conditioners/things-that-impact-electricity-consumption-in-air-conditioners.html …. but to do a very high level calculation we assume that AC compressor is on for 70% time. So in 1 hour a 2 kW AC would consume 2 * 0.7 = 1.4 units of electricity (typically at temperature of 22 on thermostat and outside temperature of 34).

      Also ton is not directly proportional to power consumption. It is proportional to cooling capacity kW.

      • pratibha says:

        how much kilowatt meter should be connect to1 ton a.c or for 1.5 ton a.c 3 star for home.(single a.c)

      • Gurpreet says:

        hello there ,i brought onida AC in 2007 jispe likha hai 4500Kcal/h ..
        bhaut bijli khaata hai ,, kyaa kru ???
        Ac bhi lagwaaya aur chlaa bhiu nahi sakte :-\

        • Abhishek Jain says:

          sir aap 25-26 par AC chalao, saath mein pankha chalao. Aur yeah bhi dekho ki room band hai aur bahar ki hawa andar nahin aa rahi hai. Try applying some insulation techniques that we have mentioned on this link: http://www.bijlibachao.com/save-electricity/insulation …. garmi ka ghar mein aana kam kar do AC ka bill waise hi kam ho jayega :)

  6. Deepak says:

    You have written…

    “For appliances like Air Conditioner, Water Heater, … , this (formula) will not work.”

    Why ? Is it because, e.g. for Window AC’s if the specs says 1130 Watts, does it means with the compressor running ? So actual units consumed may be less as the AC is cut-off on reaching the desired temprature?

    Even then, your formula should atleast give approximation ? Atleast it can be helpful while comparing between two products?

    • Abhishek Jain says:

      That is right. In ACs, Water Heaters and Electric Iron there is a thermostat that switches off the appliance when the appropriate temperature is reached. In ACs, the compressor goes on and off and the peak watts mentioned on the specs is when the compressor is on. The units consumed in AC depend on several factors as mentioned in this post: http://www.bijlibachao.com/air-conditioners/things-that-impact-electricity-consumption-in-air-conditioners.html

      As an assumption while doing payback calculation we take actual unit consumption to be 0.7 x (AC watts). So 1130 x 0.7. But that is very high level approximation. Temperature setting on thermostat plays a very important role. In some tests that we have done on our own AC, we observed that a 1.5 ton window AC which is rated at 1990 watts consumed:

      1.4 units at 22 degrees without ceiling fan which is about 0.7 times 1990
      1 unit at 26 degrees with ceiling fan on (giving same comfort as AC at 22 degrees)
      0.8 unit at 26 degrees without ceiling fan.

      If you keep it at 18 degrees it will be more than 0.7 times the power. Also it will depend on the size of the room. If you put a 1.5 ton AC in small room, it will cut off quickly and will also consume more electricity. If you put 1.5 ton AC in a huge room, it will run the compressor continuously. So an approximation itself will be highly approximate :).

  7. Vinod says:

    Great help…. Thanksss

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