What is power and energy?



Every single electrical device requires a certain amount of electrical power to operate. Technically it is expressed as Joules per second, but that is meaningless to most of us! Let’s just say it is a measure of how much power something needs to do its job. Power is measured in Watts. A thousand Watts equals one kilowatt, abbreviated as kW. The power requirement of devices are usually printed on a label, somewhere on the appliance.

A kettle typically requires 2000 Watts or 2 kW. An old-fashioned bulb might need 100 Watts and a LED light bulb only needs 5 Watts.

1000 Watts = 1 kiloWatt = 1 kW


Picture1Only when we switch the device on does it start to consume energy. The longer it is on, the more energy it will consume. The time in use becomes a factor.

By multiplying the time in use with the power requirement we get the ENERGY consumed.

Watts x hours = Watthours

kiloWatts x hours = kiloWatthours or kWh.

kWh is what your electricity meter measures and what you buy from Eskom or your municipality. Should  you have a solar system like me, you will get your kWh from the sun! J Most people talk of electrical units, which are the same as kWh.

We can now calculate the energy consumption of any device, and we can add together the energy consumption of all the appliances to arrive at a single figure which is the total energy consumption for a given 24 hour day.

A simple example:

  • Our kettle requires 2 000 Watts and was used for 5 minutes. The energy consumption was 2000 W x 5/60 hrs = 166 Wh
  • That old fashioned bulb needs 100 Watts and burns through the night for 10 hours. 100 W x 10 hrs = 1000 Wh
  • The TV uses 200 Watts and is on for 4 hours every night. 200 W x 4 hrs =  800 Wh
  • 1 966 Wh

Total energy consumption:  =  166 + 1000 + 800 = 1 966 Wh or  1.966 kWh.

Interesting to see that the 2000 Watt kettle consumed a lot less energy than the 100 Watt light bulb! All because of the time in use.

Now let’s replace the 100 Watt Bulb with a 5 Watt LED light bulb:

  • Energy consumption = 5 W x 10 hrs  = 50 Wh
  • Kettle = 2000 W x 5/60 hrs = 166 Wh
  • TV       = 200 W x 4 hrs  = 800 Wh

Total energy consumption:   = 1 016 Wh

Through replacing the bulb, we have lowered consumption by 950 Wh! Everyday! This demonstrates the benefit of energy efficient appliances..


A 2 kWh system will generate 2 kWh (2000 Watt hours) on a daily basis. This system will be more than adequate to satisfy the energy requirement as calculated above.