# Units of electricity used

1. Oct 6, 2009

### Gringo123

How can I work out how many units of electricty are consumed in problems such as the following?

During one day, a 2.5 kW appliance was used for 8 hours. How many Units of electricity were used?

I know that the answer is 20 but I have no idea how that figure is arrived at. Also, in what unit is the electricity measured. What exactly does "20 units of electricy" mean?

Thanks a lot

2. Oct 6, 2009

### mgb_phys

kW is a measure of power = the rate of energy used (kW = 1000J/s)
Electricity is sold in units of kWh = one kilowatt for one hour ( = 1000J/s * 3600s = 3,600,000J )
So a 2.5 kW appliance used for 8 hours = 2.5kW * 8h = 20kWh

3. Oct 6, 2009

### Gringo123

Thanks a lot for your help.
So a 2.5 kW appliance means an appliance that consumes 2.5kW of energy if it runs for 1hour, is that right? Sorry, I'm a real beginner!

4. Oct 6, 2009

### mgb_phys

Not quite, 2.5kW is a unit of power, it is using 2.5 kJ (2500J) of energy per second.
So after one hour it has used = 2500 J/s * 3600s = 9MJ = 2.5kWh of electricity.

It's a bit strange using power (which is energy/time) multiplied by time to get back to energy - but Joules gives large numbers and kW and hours are more convenient for everday use it's easier.

It's a bit like if we measured distance by having a unit of car speed (mph) and instead of saying it was 30miles to town we said it was 30mph-hours. So after one hour at 30mph you have gone 30 miles.

5. Oct 16, 2009

### Gringo123

Thanks a lot for that! It's a lot clearer now.