Electrical Power and Efficiency

The idea

Power is the rate of using energy. Efficiency is how much of that energy did the job you wanted.

$$P=VIE=Pt$$ $$\text{efficiency}=\frac{E_{\text{useful}}}{E_{\text{total}}}\times 100\%$$

Worked example

A lamp draws $0.50\mathrm{A}$ at $120\mathrm{V}$:

$$P=VI=120\mathrm{V}\times 0.50\mathrm{A}=60\mathrm{W}$$

Left on for 3 hours:

$$E=Pt=60\mathrm{W}\times 3\mathrm{h}=180\mathrm{Wh}=0.18\mathrm{kWh}$$

Where the rest of the energy goes

Device	Useful output	Wasted as	Rough efficiency
Incandescent bulb	Light	Heat	~5%
LED bulb	Light	Heat	~40%
Electric kettle	Heat in water	Heat elsewhere	~80%
Electric motor	Motion	Heat, sound	~85%

Nothing reaches 100%

Energy is conserved, but not all of it stays useful. In an incandescent bulb, 95% of the electricity becomes heat — it is a space heater that glows. That single number is why they have largely disappeared.

Measure this for yourself in Efficiency of a Light Bulb.

Curriculum connection

D2.8

determine the efficiency of various electrical devices that consume or produce electrical energy, and identify the energy transformations in each device

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D2.7

explain the difference between electricity and electrical energy

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