Electrical Power, Energy and the Cost of Electricity
Power being used now
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Current from the mains, I = P/V
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Energy per day
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Energy for the period, E = Pt
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Cost for the period
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Tap an appliance to switch it on or off, and set how long it is used each day.
Power is the rate of using energy: P = E/t = VI = I2R = V2/R.
An appliance's rating (e.g. 2000 W 240 V) is the power it uses when it is connected to its rated voltage.
The heating element keeps the same resistance, R = V2/P = 38.4 Ω for the 1500 W 240 V kettle. So on a lower supply voltage it uses less power: at 120 V only a quarter, 375 W.
Electricity in Singapore is paid for by the kilowatt-hour. 1 kWh is the energy a 1 kW appliance uses in 1 hour, = 3.6 × 106 J.