Efficiency - Power
useful energy = efficiency x total energy
efficiency
= useful energy
total energy
total energy = useful energy
efficiency
(Power is energy per second, the equations are the same but energy is replaced by power)
1. A 60W television transfers 30W to light and sound. What is the efficiency of the television?
2. A radio transfers 20W. The radio turns 5W into sound is. What is the efficiency of the
radio?
3. A computer transfers 120W. 80W of heat is given out. What is the efficiency of the
computer? How much is wasted?
4. An electric fire has a power rating of 2 kW. 200 W is wasted. What is the efficiency of the
electric fire?
5. A kettle is rated at 1.2 kW. 1.1 kW is transferred to the water. Calculate the efficiency of the
kettle.
6. A tungsten filament light bulb of 100W is only 1% efficient. How much power is wasted
and how much is useful?
7. A compact fluorescent lamp (CFL) which is equivalent to the 100W tungsten is rated at
20W. It is 20% efficient. How much is power is wasted and how much is turned into light?
How efficient is it compared to the tungsten filament bulb?
8. A LED bulb equivalent to the 100W tungsten is 7W. How much more efficient is it?
9. Official figures comparing bulbs for the same amount of light per Watt are;
Tungsten 14, CFL 55 and LED 123. How do the efficiencies compare?
10. A 40W electric fan is 60% efficient. How much power is wasted as heat and sound?
And finally
11. A wind farm over a year could have produced 17,586,000 MW hours of energy running at
full capacity. In reality, 3,881,900 MW hours was generated so what was the efficiency?