What is in a kilo-Watt hour or kWh?

It is important to know how to determine amount of electricity used or is going to be consumed. We first have to understand how a unit of electricity is measured:

If I am paid 10k per hour of work,  after 4 hours I will have earned 40k (rate*hr).  If I work for only 30 minutes  I will earn 5k (10k*0.5hr).
Since hourly rate is known, I can estimate in advance how much money I am going to be paid if I know the time I will spend on the job.

Similarly,  amount of electrical energy used is determined by multiplying the rate, given in seconds, by the number of seconds  the appliance was switched on.

All electrical appliances have a rate of energy consumption given in joules per second or watts (W).

1,000W or 1kW means 1000 joules are used per second.

600W means 600 joules per second, 10W means 10 joules per second and so on.

If 1,000 joules are used per second, then in 3,600 seconds or an hour 3,600,000 joules are used.  But this is a bulky number when quantifying amount of energy used.

Just like we don’t say 1,000,000 millimetres when quantifying distance. Instead we convert to km and  say 1km.

In the same way, we use 1kWh to mean 3,600,000 joules of energy. 1 unit of electrical energy is equal to 1 kWh. 20 units is the same as 20kWh.

1 unit of electricity=1kWh=3,600,000 joules

0.5kWh is same as 1,800,000 joules, 0.25kWh means  900,000 joules and so on

20kWh is same as 72,000,000 joules.

To find amount of electrical energy in joules, we multiply rate of energy flow given in Watts by the time given in seconds.

To find the same amount of energy in kWh we still use rate and time, but in bigger units, namely kiloWatts and hours. We multiply rate of energy, given in kW by the time, given in hours. For example to find electrical energy in kWh consumed by a 700W rice cooker that has been on for 30 minutes, we change W to kW and minutes to hours and multiply:

0.7kW*0.5h = 0.35kWh

In Uganda a unit is about Shs1,000 (26 US cents). Cooking rice for half an hour will therefore cost 1000*0.35 = Shs350.

Using this rice cooker costs less than all the other available forms of cooking fuels like charcoal, LPG gas and firewood. Whenever power is available especially from clean sources like hydro, solar, wind and geothermal, use these energy saving appliances. Keep charcoal / gas for backup purposes.